Patent Publication Number: US-8977064-B2

Title: Rotational transcoding for JPEG or other coding standards

Description:
TECHNICAL FIELD 
     This disclosure relates to image processing, and more particularly, to techniques for rotating encoded images. 
     BACKGROUND 
     Digital cameras are commonly incorporated into a wide variety of devices. In this disclosure, a digital camera device refers to any device that can capture one or more digital images, including devices that can capture still images and devices that can capture sequences of images to record video. By way of example, digital camera devices may comprise stand-alone digital cameras or digital video camcorders, camera-equipped wireless communication device handsets such as mobile, cellular or satellite radio telephones, camera-equipped personal digital assistants (PDAs), computer devices that include cameras such as so-called “web-cams,” or any devices with digital imaging or video capabilities. 
     JPEG is a commonly used method of compression for images. The name JPEG stands for Joint Photographic Experts Group, the name of the committee that created the standard. The JPEG encoding process includes several steps. First, the pixels of an image may be converted from red green and blue (RGB) to YCbCr, which includes one luma component (Y), representing brightness, and two chroma components (Cb and Cr), representing color. 
     Next, the resolution of the chroma data may be reduced, e.g., usually by a factor 2, to reflect the fact that the human eye is less sensitive to fine color details than to fine brightness details. Then, the image is split into blocks of 8×8 pixels, and for each block, each of the Y, Cb, and Cr data undergoes a discrete cosine transform (DCT). The amplitudes of the frequency components are then quantized. Since human vision is more sensitive to variations in color or brightness over large areas, than to the strength of high-frequency brightness variations, the magnitude of the high-frequency components may be stored with less accuracy than the low-frequency components. The quality setting of the encoder may be adjusted to control the extent of resolution for each frequency component. If an excessively low quality setting is used, the high-frequency components may be discarded altogether. 
     After the DCT process, the resulting data for all 8×8 blocks is further compressed, e.g., using differential coding techniques, and entropy coding techniques such as zig-zag scanning, Huffman coding, run-length coding, variable length coding (VLC) methodologies, application of coded block patterns (CBPs), or the like. The decoding process reverses the different JPEG coding steps to reproduce an array of pixel values that can be used to drive a display. 
     In many imaging applications, rotation of an image is desirable. For example, if an image is captured by a camera that is rotated 90 degrees, it may be desirable to rotate the image data so that the image is not rotated when the image is output on a display. Conventionally, an encoded JPEG image is rotated by decoding the image, rotating the decoded image in the pixel domain, and then re-encoding the rotated image. This conventional technique is very memory intensive because it requires temporary storage of one or more copies of an uncompressed image and storage of the compressed image before and after the rotation. 
     SUMMARY 
     This disclosure describes techniques for rotating an encoded image, such as an image encoded according to a JPEG standard. The encoded image may comprise a plurality of minimum coded units (MCUs). In one example, the MCUs may comprise blocks of discrete cosine transform (DCT) coefficients coded from  8  by  8  blocks of pixels via discrete cosine transformation. In accordance with this disclosure, an image is rotated without fully decoding the MCUs from the DCT domain to a pixel domain. In particular, the image may be rotated in the DCT domain, and this disclosure proposes processes that facilitate such rotation in the DCT domain. In some cases, differential decoding may be applied with respect to a low frequency (DC) component of each of the MCUs, but in this case the high frequency (AC) components of the MCUs remain in the DCT domain when the data is rotated. 
     According to this disclosure, first, the MCUs may be reordered so that the MCUs are positioned in a rotated order relative to the original encoded image. Then, the data within each of the MCUs may be rotated. In this way, a rotated version of the encoded image is generated without decoding all of the DCT coefficients back into a pixel format. Techniques for indexing the MCUs are also described, which can simplify the reordering of the MCUs. Furthermore, additional techniques are described, such as using the image rotation process as an opportunity to apply file size constraints to the encoded image. In this case, the original image may be coded without the file size constraints, and the rotated image may be coded with the file size constraints. In general, the image rotation process described herein may provide an opportunity to perform a number of additional coding steps, such as application of file size constraints. 
     In one example, this disclosure provides a method for rotating an encoded image comprising reordering MCUs of the encoded image according to a specified rotation of the encoded image, rotating image data within the MCUs according to the specified rotation, and generating a rotated version of the encoded image comprising the reordered MCUs and the rotated image data within the MCUs. 
     In another example, this disclosure provides an apparatus comprising memory that stores an encoded image, and a rotational transcoder that receives the encoded image from the memory, reorders MCUs of the encoded image according to a specified rotation of the encoded image, rotates image data within the MCUs according to the specified rotation, and generates a rotated version of the encoded image comprising the reordered MCUs and the rotated image data within the MCUs. 
     In another example, this disclosure provides a device comprising means for reordering MCUs of an encoded image according to a specified rotation of the encoded image, means for rotating image data within the MCUs according to the specified rotation, and means for generating a rotated version of the encoded image comprising the reordered MCUs and the rotated image data within the MCUs. 
     These and other techniques described herein may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the software may be executed in a processor, such as a general purpose microprocessor, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate array (FPGA) or other type of processor. Such software may be initially stored in a computer-readable medium and loaded and executed in the processor to facilitate the rotation of an encoded image. 
     Accordingly, this disclosure also contemplates a computer-readable medium comprising instructions that upon execution cause a processor to reorder MCUs of an encoded image according to a specified rotation of the encoded image, rotate image data within the MCUs according to the specified rotation, and generate a rotated version of the encoded image comprising the reordered MCUs and the rotated image data within the MCUs. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  a block diagram illustrating an exemplary imaging device capable of implementing the techniques of this disclosure. 
         FIGS. 2 and 3  are block diagrams illustrating components that may be used to rotate an image. 
         FIGS. 4 and 5  are block diagrams illustrating components that may be used to rotate an image consistent with the techniques of this disclosure. 
         FIGS. 6 and 7  are flow diagrams illustrating techniques for rotating an image consistent with this disclosure. 
         FIG. 8  is a conceptual diagram showing an image that is indexed according to MCUs. 
         FIG. 9  is another flow diagram illustrating a technique for rotating an image consistent with this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes techniques for rotating an encoded image. As an example, the encoded image may comprise an image encoded according to a Joint Photographic Experts Group (JPEG) standard, although the techniques of this disclosure may be applicable to other image compression standards. The techniques of this disclosure can be used to rotate still images, or encoded images that comprise frames of a video sequence. Rotation is often needed in digital camera devices where image data is generated in the same scan-line order regardless of whether the user orients the camera device in a landscape or a portrait orientation. For at least one of these orientations, a ninety (90) degree, one hundred eighty (180) degree, or two hundred seventy (270) degree rotation may be desired. 
     An encoded image may comprise a plurality of minimum coded units (MCUs). MCUs may comprise blocks of encoded data, such as blocks of data encoded via a discrete cosine transform (DCT) process. In this case, the MCUs may comprise blocks of DCT coefficients coded from blocks of pixels of an image. This disclosure provides techniques that facilitate image rotation without the need to decode such MCUs from the DCT domain back to the pixel domain. For example, the image may be rotated in the DCT domain, and this disclosure proposes effective processes that can facilitate such rotation. In some cases, if differential coding is used with respect to a low frequency (so called “DC”) component of a given MCU, that differential coding on the DC component may be decoded. In this case, however, the AC coefficients are not decoded to the pixel domain, and image rotation occurs with the AC coefficients remaining in the DCT domain. 
     In accordance with this disclosure, the MCUs may be reordered according to a specified rotation so that the MCUs are positioned in a rotated order relative to the original encoded image. In order to facilitate this reordering of MCUs, this disclosure may make use of an indexing scheme. For example, in accordance with the JPEG standard, a restart marker value associated with an encoded image may be set to one. This may result in each DCT coded MCU forming an independently decodable unit of a JPEG image. Each MCU can be indexed, and the MCUs can be fetched in a reordered manner based on the indexing, so that the MCUs are positioned in a rotated order relative to the original encoded image. Then, the data within each of the MCUs is rotated according to the specified rotation. In this way, an encoded image that includes MCUs coded via DCT techniques can be rotated without decoding some or all of the DCT coefficients back to a pixel format. The described techniques can significantly reduce memory requirements associated with image rotation. Furthermore, additional techniques are disclosed herein, such as using the image rotation process as an opportunity to apply file size constraints to the encoded image. In this case, the original encoded image may be coded without file size constraints, and the rotated image may be coded with the file size constraints. The application of file size constraints may yield compression of the rotated image relative to the original encoded image. 
       FIG. 1  is a block diagram illustrating an exemplary digital camera device  2  that implements techniques of this disclosure. By way of example, digital camera device  2  may comprise a stand-alone digital camera, a digital video camcorder, a camera-equipped wireless communication device handset, such a cellular or satellite radio telephone, a camera-equipped personal digital assistant (PDA), a computer device equipped with a digital camera, web-cam or the like, or any other device with imaging or video capabilities. The techniques of this disclosure are most applicable to wireless applications, such as in camera-equipped wireless communication device handsets, where memory is limited and power consumption is a concern. 
     As shown in  FIG. 1 , device  2  comprises a camera  12  that captures an image. The captured image may comprise a still photo, or an image frame of a video sequence. In the latter case, the techniques of this disclosure may be used successively for each image frame of the video sequence. JPEG coder  14  may comprise an encoder or a combined encoder-decoder (CODEC). JPEG coder  14  may encode the captured image in real time according to the JPEG compression standard. In this way, JPEG coder  14  generates an encoded image based on the captured image, wherein the encoded image is compressed according to the JPEG standard. 
     Memory  16  stores the encoded image. If desired, display  20  can display a decoded version of the image to a user. JPEG coder  14  may also facilitate decoding of the encoded image so that pixel data can be presented by display  20 . Display  20  may comprise any type of display, such as a cathode ray tube (CRT), liquid crystal display (LCD), plasma display, or any display capable of presenting images to a user. In some examples, display  20  may comprise a viewfinder display of device  2 . A data communication bus  15  facilitates data transfer between the different components of device  2 . In some cases, however, camera  12  may be coupled to JPEG encoder  14  via a dedicated bus, and JPEG encoder  14  may perform so called “in-line” encoding as images are received. In this case, the rotational transcoding described herein may also be viewed as an in-line process that executes automatically anytime image rotation is needed. Such rotation may be determined to be needed based on user input or possibly based on the orientation of camera  12  during image capture. 
     Device  2  may also include many other components. For example, if device  2  is a wireless communication device, such as mobile telephone handset, device  2  may include various components such as an antenna, transmitter, receiver, and modulator to facilitate such wireless communications. These additional components, however, are omitted from  FIG. 1  for simplicity and ease of illustration. 
     In accordance with this disclosure, device  2  includes a rotational transcoder  18 , which facilitates image rotation in accordance with this disclosure. To do so, rotational transcoder  18  reorders MCUs of the encoded image and rotates image data within the MCUs such that the reordering of the MCUs and rotation of the image data generates a rotated version of the encoded image. After generating the rotated version of the encoded image, rotational transcoder  18  may output the rotated version of the encoded image to memory  16 . The MCUs may comprise blocks of DCT coefficients coded according to a JPEG standard. 
     Rotational transcoder  18  may fetch the encoded image from memory  16 , wherein the encoded image is coded via one or more entropy coding techniques. Rotational transcoder  18  entropy decodes the encoded image to generate the MCUs prior to reordering the MCUs, and entropy encodes the rotated version of the encoded image according to the one or more entropy coding techniques prior to outputting the rotated version of the encoded image. In this way, rotational transcoder  18  may perform a partial decode of the encoded image to remove entropy coding, but does not decode the DCT of the encoded image. Instead, rotational transcoder  18  rotates the encoded image substantially in the DCT domain, and then re-applies entropy coding to the rotated image. If differential coding is used on low frequency (DC) component of the DCT coefficients, rotational transcoder  18  may decode the DC component of the DCT coefficients so that differential coding can be removed. In this case, however, the AC components of the DCT coefficients associated with the given MCU may remain in the DCT domain for the image rotation process. 
     In some cases, in processing the reordered MCUs, rotational transcoder  18  may apply file size control parameters. In this case, the rotated version of the encoded image complies with the file size control parameters. For example, the rotated version of the encoded image may be compressed relative to the originally encoded image so as to comply with the file size control parameters. The application of file size control parameters typically requires a second coding process following an initial JPEG coding process. The techniques of this disclosure can provide a change to apply file size control parameters as part of the rotational transcoding. This can eliminate a separate coding process that would otherwise be needed to apply file size control parameters. Instead, this disclosure may use a single coding process (following an initial JPEG encoding process) to both perform rotation of an image and to apply file size control parameters to the image. 
     Rotational transcoder  18  may recorder the MCUs by assigning a restart marker value of 1 to the encoded image, indexing the MCUs based on the restart marker value, and reordering the MCUs based on the indexing. The restart marker value, according to JPEG, refers to a value designed to make JPEG files more resilient. By defining a restart marker value, sets of MCUs can be independently decoded, and therefore if errors occur in one set of MCUs, such errors will not propagate to other sets of MCUs. By assigning a restart marker value of 1 to the encoded image, each DCT coded MCU forms an independently decodable unit of a JPEG image. Each MCU can be indexed, and rotational transcoder  18  fetches the MCUs in a reordered manner based on the indexing, so that the MCUs are positioned in a rotated order relative to the original encoded image. Then, rotational transcoder  18  rotates data within each of the MCUs. A rotated version of the encoded image comprises the reordered MCUs with rotated data for each of the MCUs. 
     Rotational transcoder  18  may perform two or more operations in rotating image data within the MCUs. For example, rotational transcoder  18  may perform a vertical flip operation on coefficient values of the MCUs, and a transpose operation on the vertically flipped coefficient values of the MCUs. This can result in ninety (90) degree clockwise image rotation. In another example, rotational transcoder  18  may perform a vertical flip operation on coefficient values of the MCUs, and a horizontal flip operation on the vertically flipped coefficient values of the MCUs. This can result in a one hundred eighty (180) degree clockwise image rotation. 
     In another example, rotational transcoder  18  may perform a horizontal flip operation on coefficient values of the MCUs, and a vertical flip operation on the horizontally flipped coefficient values of the MCUs. This can result in one hundred eighty (180) degree counter clockwise image rotation. In another example, rotational transcoder  18  may perform a transpose operation on coefficient values of the MCUs, and a vertical flip operation on the transposed coefficient values of the MCUs. This can result in a two hundred seventy (270) degree clockwise image rotation. Rotational transcoder  18  may perform the desired amount of image rotation based on a specified level of rotation, which may be determined automatically based on image capture conditions associated with camera  12 , or may be received from a user as input via an input unit (not shown) of device  2 . 
       FIG. 2  is a block diagram illustrating components that may be used to rotate an image. As shown in  FIG. 2 , a camera  22 A captures an image, and readout  24 A of camera  22 A is provided to a JPEG encoder  26 A that encodes the image according to the JPEG standard. A memory  28 A stores the encoded JPEG file  30 A of the image. A JPEG decoder  32 A then decodes the JPEG file and stores a decoded file  34 A in memory  28 A. Rotate and copy unit  36 A performs rotate and copy operations on decoded file  34 A to generate a rotated version  38 A of the decoded file, which is stored in memory  28 A. JPEG encoder  26 A then encodes the rotated version  38 A of the decoded file and writes an encoded JPEG file of the rotated image  40 A in memory  28 A. 
     According to the process executed by the components of  FIG. 2 , an image is JPEG encoded, decoded, rotated, and then re-encoded. Unfortunately, the process executed by the components of  FIG. 2  is very memory intensive. In particular, memory  28 A is required to store JPEG file  30 A, and two decoded versions  34 A and  38 A of the image. In addition, memory  28 A also stores JPEG file  40 A. Furthermore, the multiple encoding and decoding steps can add undesirable latency to the process of rotating an image. The techniques of this disclosure can reduce such memory requirements associated with image rotation according to the process executed by the components of  FIG. 2 , and can avoid the need to store uncompressed decoded versions of the image in pixel format. 
       FIG. 3  is another block diagram illustrating components that may be used to rotate an image. As shown in  FIG. 3 , a camera  22 B captures an image, and provides the readout  24 B of camera  22 B to a memory  28 B without performing any JPEG encoding on file  34 B. Rotate and copy unit  36 B performs rotate and copy operations on file  34 B to generate a rotated version  38 B of the file, which is stored in memory  28 B. JPEG encoder  26 B then encodes the rotated version  38 B of the decoded file and stores the encoded JPEG file of the rotated image  40 B in memory  28 B. 
     The process performed by the components of  FIG. 3  is less memory intensive and less computationally intensive than the process executed by the components of  FIG. 2 . In particular, in the process performed by the components of  FIG. 3 , one JPEG encoding step  26 B and one JPEG decoding step  32 B are eliminated relative to the process executed by the components of  FIG. 2 . In addition, there is no need to store JPEG file  30 B in memory  28 B. Eliminated units of  FIG. 3  relative to  FIG. 2  are shown in dotted lines and labeled collectively as elements  50 . Nevertheless, the process executed by the components of  FIG. 3  is still very memory intensive, requiring storage of uncompressed versions  34 B and  38 B of the image file. The techniques of this disclosure can reduce memory requirements associated with image rotation relative to the processes executed by the respective components shown in  FIGS. 2 and 3 . 
       FIG. 4  is a block diagram illustrating components that may be used to rotate an image in accordance with this disclosure. As shown in  FIG. 3 , a camera  22 C captures an image, and readout  24 C of camera  22 C is provided to a JPEG encoder  26 C. As shown, JPEG encoder  26 C applies a restart marker interval of one. By setting the restart marker value to one, each MCU in encoded JPEG file  30 C is a separately decodable unit. In this case, the entropy coding may only apply to a given MCU and may not extend between MCUs, as each MCU is itself independently decodable. Encoded JPEG file  30 C is stored in memory  28 C. 
     In accordance with this disclosure, rotational transcoder  60 C performs rotational transcoding with respect to JPEG file  30 C. In particular, rotational transcoder  60 C reorders the MCUs of JPEG file  30 C. Then, rotational transcoder  60 C rotates image data within the MCUs. The reordering of MCUs and rotation of image data within the MCUs by rotational transcoder  60 C generates a rotated version of the encoded image, which can be output to memory  28 C and stored in memory as a JPEG file of a rotated image  40 C. 
     Rotational transcoder  60 C may fetch the encoded JPEG file  30 C from memory  28 C, wherein the encoded image is coded via one or more entropy coding techniques. Rotational transcoder  60 C may entropy decode the encoded image to generate the MCUs prior to reordering and processing the MCUs, and entropy encode the rotated version of the encoded image according to the one or more entropy coding techniques prior to outputting the rotated version of the encoded image. In this way, rotational transcoder  60 C may perform a partial decode of the encoded image to remove entropy coding, but does not fully decode the DCT coded information of the encoded image. Instead, rotational transcoder  60 C rotates the encoded image substantially in the DCT domain, and then re-applies entropy coding to the rotated image. In some cases, if differential coding is used on the low frequency (DC) component of the DCT coefficients, rotation transcoder  60 C may decode the DC component of the DCT coefficients so to remove such differential coding. In this case, however, the AC components of the DCT coefficients may remain in the DCT domain. 
     The process executed by the components of  FIG. 4  is much less memory intensive than the processes executed by the components of  FIGS. 2 and 3 . In the process executed by the components of  FIG. 4 , the need to store decoded versions  34 C and  38 C of the image in memory  28 C can be eliminated and rotate and copy unit  36 C is replaced by the rotational transcoder  60 C. The additional step associated with JPEG encoder  26 C′ can also be eliminated. The eliminated units of  FIG. 4  relative to those of  FIG. 2  are shown in dotted lines, and are labeled collectively as elements  52 . 
       FIG. 5  is another block diagram illustrating a process for rotating an image in accordance with this disclosure. As shown in  FIG. 5 , a camera  22 D captures an image, and readout  24 D of camera  22 D is provided to a JPEG encoder  26 D. Like in  FIG. 4 , JPEG encoder  26 D of  FIG. 5  applies a restart marker interval of one. Again, by setting the restart marker value to one, each MCU in encoded JPEG file  30 D is a separately decodable unit. Encoded JPEG file  30 D is stored in memory  28 D. In  FIG. 5 , JPEG encoder  26 D generates file size control parameters and forwards these file size control parameters for application by rotational transcoder  60 D. The file size control parameters may comprise one or more parameters that can be used to limit the size of the encoded JPEG file. Such parameters may result in the reduction or elimination of high frequency AC components of the DCT coefficients in order to compress the data. In general, the image rotation process described herein may provide an opportunity to perform a number of additional coding steps, such as application of file size control parameters, or other types of parameters. 
     Rotational transcoder  60 D performs rotational transcoding with respect to JPEG file  30 D. Furthermore, in the example of  FIG. 5 , rotational transcoder  60 D applies the file size control parameters so that the rotated version of the JPEG file (file  40 D) is encoded with size limits and/or constraints. Again, in the rotational transcoding, rotational transcoder  60 D reorders the MCUs of JPEG file  30 D, and rotates image data within the MCUs. The reordering of MCUs and rotation of image data within the MCUs may be based on a specified level rotation, e.g., received as user input or generated automatically based on the orientation of camera  22 D. During the rotational transcoding, the file size parameters can be applied by rotational transcoder  60 D to ensure that the encoded JPEG file  40 D falls within specified size limits. Again, such parameters may cause rotational transcoder  60 D to reduce or eliminate one or more high frequency AC components of the DCT coefficients in order to compress the data under the file size constraints specified by the parameters. The rotated version of the encoded image is output to memory  28 D and stored in memory as a JPEG file of a rotated image with size limits  40 D. 
     As outlined above, rotational transcoder  60 D may fetch the encoded JPEG file  30 D from memory  28 D, wherein the encoded image is coded via one or more entropy coding techniques. Rotational transcoder  60 D may include entropy decoding components to entropy decode the encoded image to generate the MCUs prior to reordering the MCUs. In addition, rotational transcoder  60 D may include entropy encoding components to entropy encode the rotated version of the encoded image according to the one or more entropy coding techniques, e.g., prior to outputting the rotated version of the encoded image. In this way, rotational transcoder  60 D may perform a partial decode of the encoded image to remove entropy coding, but does not decode the DCT of the encoded image. Instead, rotational transcoder  60 D rotates the encoded image substantially in the DCT domain, and then re-applies any entropy coding to the rotated image. In some case, however, if differential coding is used on the low frequency (DC) coefficient of the DCT coefficients, rotation transcoder  60 D may decode the DC coefficient so that such differential coding can removed. In this case, the high frequency (AC) coefficients of the DCT coefficients may remain in the DCT domain. 
     The application of file size parameters may reduce or eliminate so-called high frequency DCT coefficient values within the MCUs. The high frequency values are sometimes called AC values, in contrast to the low frequency “DC” value that represents average intensity of the entire MCU. According to  FIG. 5 , rotational transcoder  60 D not only rotates the image, but may also be used to apply file size parameters in order to generate a JPEG file  40 D that has size limits mandated by the file size parameters. In general, the image rotation process described herein may provide an opportunity to perform a number of additional coding steps, such as application of file size control parameters, or other types of parameters that may be conventionally applied via a second coding pass following an initial JPEG encoding pass. 
     Like the process executed by the components of  FIG. 4 , the process executed by the components of  FIG. 5  is much less memory intensive than the processes executed by the components of  FIGS. 2 and 3 . In the process executed by the components of  FIG. 5 , the need to store decoded versions  34 D and  38 D of the image in memory  28 D can be avoided, and rotate and copy unit  36 D may be replaced by the rotational transcoder  60 D. The additional step performed by JPEG encoder  26 D′ can also be eliminated. The eliminated components of  FIG. 5  relative to those of  FIG. 2  are shown in dotted lines, and are labeled collectively as elements  54 . 
       FIG. 6  is a flow diagram illustrating a technique consistent with this disclosure.  FIG. 6  will be described from the perspective of device  2  of  FIG. 1 . As shown in  FIG. 6 , rotational transcoder  18  of device  2  partially decodes image data associated with an encoded file to generate MCUs ( 61 ). As an example, the encoded file may comprise a JPEG file that includes both DCT encoding and entropy encoding. The partial decoding process ( 61 ) may include entropy decoding but does not fully decode the DCT coefficients back to a pixel domain. 
     Rotational transcoder  18  reorders the MCUs ( 62 ). For example, the MCUs may be reordered such that the MCUs are presented in a rotated manner relative to the original image. Next, rotational transcoder  18  processes the image data within each of the MCUs in order to rotate the image data within each of the MCUs ( 63 ). Thus, rotational transcoder first reorders the MCUs ( 62 ), and then rotates the image data within each of the MCUs ( 63 ). The steps of ( 62 ) and ( 63 ) results in the generation of a rotated version of the image. That is to say, rotational transcoder  18  generates a rotated version of the image comprising the reordered MCUs and the rotated image data within the MCUs ( 64 ). The steps of ( 62 ) and ( 63 ), however, could be reversed in some embodiments. 
     After generating the rotated version of the image comprising the reordered MCUs and the rotated image data within the MCUs ( 64 ), rotational transcoder  18  re-encodes the MCUs ( 65 ). This re-encoding process of step ( 65 ) is the opposite of the partial decode process of step ( 61 ). Thus, in the example where step ( 61 ) involves entropy decoding, step ( 65 ) may involve entropy encoding the MCUs. Rotational transcoder  18  then outputs a rotated version of the image data, e.g., a rotated JPEG file, to memory  16  ( 66 ). 
       FIG. 7  is another flow diagram illustrating an exemplary rotational transcoding process consistent with this disclosure.  FIG. 7  will be described from the perspective of exemplary device  2  of  FIG. 1 . As shown in  FIG. 7 , memory  16  of device  2  may originally store a JPEG bitstream of an image  80 , which may have been encoded by JPEG encoder  14 . Rotational transcoder  18  uses restart (RST) markers to index MCUs of the JPEG bitstream ( 71 ). In particular, by selecting a restart marker value of 1, each of the MCUs may comprise a separately decodable unit. Image  82  illustrates an example of an image separated into MCUs with restart markers of 1. 
     Next, rotational transcoder  18  fetches the MCU bitstream in a rotational order ( 72 ). This reorders the MCUs. For example, as shown in image  84 , the MCUs can be fetched in an order that results in rotation of the MCUs relative to those of image  82 . For example, for 90 degree rotation, the first MCU fetched may be the MCU in the lower left hand corner of image  84 , and the last MCU fetched may be the MCU in the upper right hand corner of image  84 . The MCUs may be fetched in the order shown in by the arrows illustrated within image  84 . 
     Rotational transcoder  18  Huffman decodes and inverse zig-zag scans each of the MCUs ( 73 ). This process of Huffman decoding and inverse zig-zag scanning may be referred to herein as entropy decoding. Next, rotational transcoder  18  may decode only the DC coefficient of each MCU and apply DC differential decoding to each MCU ( 74 ). The DC differential decoding of step  74  may remove any differential coding that was applied to the DC coefficient of the DCT coefficients. To do so may require at least a partial decode of the DC coefficient of each MCU, but in this case, the AC coefficients remain coded in the DCT domain. 
     Rotational transcoder  18  rotates the quantized DCT coefficients ( 75 ). In the example of MCU fetching shown in image  84 , this rotation may be 90 degree rotation. However, 180 degree rotation or 270 degree rotation may also be performed. Exemplary matrix transformation equations that can be applied to the MCUs for 90, 180 or 270 degree rotation are listed next to step ( 75 ) in  FIG. 7 . In particular, 90 degree rotation may be achieved by applying:
 
 X   ij   90 =(−1) i   X   ji  
 
180 degree rotation may be achieved by applying:
 
 X   ij   180 =(−1) i+j   X   ij  
 
270 degree rotation may be achieved by applying:
 
 X   ij   270 =(−1) j   X   ji  
 
     Once the coefficients of the MCUs are rotated, rotational transcoder  18  zig-zag scans and Huffman encodes the MCUs ( 76 ). This process of zig-zag scanning and Huffman encoding may be referred to herein as entropy encoding. Rotational transcoder  18  may output a JPEG bitstream of the rotated image  86  to memory  16 . 
     In order to rotate the DCT coefficients of each of the MCUs (following the reordering of the MCUs), rotational transcoder  18  may perform one or more of the following computations. For example, let
 
 X=CxC   T  
 
where C is the DCT kernel, x represents a matrix of pixel values and X represents a matrix of DCT values. Examples of C and x are shown below:
 
     
       
         
           
             C 
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                         c 
                         21 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         20 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         20 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         21 
                       
                     
                   
                   
                     
                       c 
                       21 
                     
                   
                   
                     
                       c 
                       20 
                     
                   
                 
                 
                   
                     
                       c 
                       30 
                     
                   
                   
                     
                       c 
                       31 
                     
                   
                   
                     
                       c 
                       32 
                     
                   
                   
                     
                       c 
                       33 
                     
                   
                   
                     
                       - 
                       
                         c 
                         33 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         32 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         31 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         30 
                       
                     
                   
                 
                 
                   
                     
                       c 
                       40 
                     
                   
                   
                     
                       c 
                       41 
                     
                   
                   
                     
                       c 
                       41 
                     
                   
                   
                     
                       c 
                       40 
                     
                   
                   
                     
                       c 
                       40 
                     
                   
                   
                     
                       c 
                       41 
                     
                   
                   
                     
                       c 
                       41 
                     
                   
                   
                     
                       c 
                       40 
                     
                   
                 
                 
                   
                     
                       c 
                       50 
                     
                   
                   
                     
                       c 
                       51 
                     
                   
                   
                     
                       c 
                       52 
                     
                   
                   
                     
                       c 
                       53 
                     
                   
                   
                     
                       - 
                       
                         c 
                         53 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         52 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         51 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         50 
                       
                     
                   
                 
                 
                   
                     
                       c 
                       60 
                     
                   
                   
                     
                       c 
                       61 
                     
                   
                   
                     
                       - 
                       
                         c 
                         61 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         60 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         60 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         61 
                       
                     
                   
                   
                     
                       c 
                       61 
                     
                   
                   
                     
                       c 
                       60 
                     
                   
                 
                 
                   
                     
                       c 
                       70 
                     
                   
                   
                     
                       c 
                       71 
                     
                   
                   
                     
                       c 
                       72 
                     
                   
                   
                     
                       c 
                       73 
                     
                   
                   
                     
                       - 
                       
                         c 
                         73 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         72 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         71 
                       
                     
                   
                   
                     
                       - 
                       
                         c 
                         70 
                       
                     
                   
                 
               
               ] 
             
           
         
       
       
         
           
             x 
             = 
             
               [ 
               
                 
                   
                     
                       x 
                       00 
                     
                   
                   
                     
                       x 
                       01 
                     
                   
                   
                     
                       x 
                       02 
                     
                   
                   
                     
                       x 
                       03 
                     
                   
                   
                     
                       x 
                       04 
                     
                   
                   
                     
                       x 
                       05 
                     
                   
                   
                     
                       x 
                       06 
                     
                   
                   
                     
                       x 
                       07 
                     
                   
                 
                 
                   
                     
                       x 
                       10 
                     
                   
                   
                     
                       x 
                       11 
                     
                   
                   
                     
                       x 
                       12 
                     
                   
                   
                     
                       x 
                       13 
                     
                   
                   
                     
                       x 
                       14 
                     
                   
                   
                     
                       x 
                       15 
                     
                   
                   
                     
                       x 
                       16 
                     
                   
                   
                     
                       x 
                       17 
                     
                   
                 
                 
                   
                     
                       x 
                       20 
                     
                   
                   
                     
                       x 
                       21 
                     
                   
                   
                     
                       x 
                       22 
                     
                   
                   
                     
                       x 
                       23 
                     
                   
                   
                     
                       x 
                       24 
                     
                   
                   
                     
                       x 
                       25 
                     
                   
                   
                     
                       x 
                       26 
                     
                   
                   
                     
                       x 
                       27 
                     
                   
                 
                 
                   
                     
                       x 
                       30 
                     
                   
                   
                     
                       x 
                       31 
                     
                   
                   
                     
                       x 
                       32 
                     
                   
                   
                     
                       x 
                       33 
                     
                   
                   
                     
                       x 
                       34 
                     
                   
                   
                     
                       x 
                       35 
                     
                   
                   
                     
                       x 
                       36 
                     
                   
                   
                     
                       x 
                       37 
                     
                   
                 
                 
                   
                     
                       x 
                       40 
                     
                   
                   
                     
                       x 
                       41 
                     
                   
                   
                     
                       x 
                       42 
                     
                   
                   
                     
                       x 
                       43 
                     
                   
                   
                     
                       x 
                       44 
                     
                   
                   
                     
                       x 
                       45 
                     
                   
                   
                     
                       x 
                       46 
                     
                   
                   
                     
                       x 
                       47 
                     
                   
                 
                 
                   
                     
                       x 
                       50 
                     
                   
                   
                     
                       x 
                       51 
                     
                   
                   
                     
                       x 
                       52 
                     
                   
                   
                     
                       x 
                       53 
                     
                   
                   
                     
                       x 
                       54 
                     
                   
                   
                     
                       x 
                       55 
                     
                   
                   
                     
                       x 
                       56 
                     
                   
                   
                     
                       x 
                       57 
                     
                   
                 
                 
                   
                     
                       x 
                       60 
                     
                   
                   
                     
                       x 
                       61 
                     
                   
                   
                     
                       x 
                       62 
                     
                   
                   
                     
                       x 
                       63 
                     
                   
                   
                     
                       x 
                       64 
                     
                   
                   
                     
                       x 
                       65 
                     
                   
                   
                     
                       x 
                       66 
                     
                   
                   
                     
                       x 
                       67 
                     
                   
                 
                 
                   
                     
                       x 
                       70 
                     
                   
                   
                     
                       x 
                       71 
                     
                   
                   
                     
                       x 
                       72 
                     
                   
                   
                     
                       x 
                       73 
                     
                   
                   
                     
                       x 
                       74 
                     
                   
                   
                     
                       x 
                       75 
                     
                   
                   
                     
                       x 
                       76 
                     
                   
                   
                     
                       x 
                       77 
                     
                   
                 
               
               ] 
             
           
         
       
     
     If pixel matrix (x) is transposed in the pixel domain, the corresponding DCT matrix (X) is transposed according to the following operation:
 
 X   T =( CxC   T ) T   =Cx   T   C   T  
 
     A spatial domain vertical flip operation may be given as follows: 
             x   =     [           x   00           x   01           x   02           x   03           x   04           x   05           x   06           x   07               x   10           x   11           x   12           x   13           x   14           x   15           x   16           x   17               x   20           x   21           x   22           x   23           x   24           x   25           x   26           x   27               x   30           x   31           x   32           x   33           x   34           x   35           x   36           x   37               x   40           x   41           x   42           x   43           x   44           x   45           x   46           x   47               x   50           x   51           x   52           x   53           x   54           x   55           x   56           x   57               x   60           x   61           x   62           x   63           x   64           x   65           x   66           x   67               x   70           x   71           x   72           x   73           x   74           x   75           x   76           x   77           ]                     x   ~     =     [           x   70           x   71           x   72           x   73           x   74           x   75           x   76           x   77               x   60           x   61           x   62           x   63           x   64           x   65           x   66           x   67               x   50           x   51           x   52           x   53           x   54           x   55           x   56           x   57               x   40           x   41           x   42           x   43           x   44           x   45           x   46           x   47               x   30           x   31           x   32           x   33           x   34           x   35           x   36           x   37               x   20           x   21           x   22           x   23           x   24           x   25           x   26           x   27               x   10           x   11           x   12           x   13           x   14           x   15           x   16           x   17               x   00           x   01           x   02           x   03           x   04           x   05           x   06           x   07           ]       ,     
     ⁢     where   ⁢           ⁢   x           
represents an original matrix of pixel values of an MCU and
 
 {tilde over (x)} 
 
represents a matrix that is vertically flipped.
 
     The equivalent operation for a vertical flip operation in the DCT domain may be given by:
 
 {tilde over (X)}   ij =(−1) i   X   ij  
 
     Similarly, an operation for a horizontal flip operation in the DCT domain may be given by:
 
 {circumflex over (X)}   ij =(−1) j   X   ij  
 
     In the spatial domain, a 90 degree clockwise rotation may be divided into two operations: a vertical flip operation followed by a transpose operation. A transpose operation refers to a matrix operation T where the any matrix A has the property (A T ) T =A. The DCT operations for a 90 degree rotation in the DCT domain may be given a combination of vertical flip followed by transpose, which may be collectively represented by:
 
 X   ij   90   ={tilde over (X)}   T =(−1) i   X   ji  
 
     180 degree clockwise rotation may also be divided into two operations: a vertical flip and a horizontal flip. Therefore, the DCT operations for a 180 degree rotation in the DCT domain may be given by:
 
 X   ij   180 =(−1) i+j   X   ij  
 
     270 degree clockwise rotation may also be divided into two operations: a transpose followed by a vertical flip. Therefore, the DCT operations for a 270 degree rotation in the DCT domain may be given by:
 
 X   ij   270 =(−1) j   X   ji  
 
     In some cases, image rotation in the DCT domain could be embedded or combined with DCT quantization and zig-zag scan operations. Based on the desired rotation, signs of the quantization parameters could be changed accordingly, as set forth by the equations above. Similarly, if a transpose operation is needed, the transpose operation could be combined with DCT transform operations or combined into a zig-zag scan table applied during the rotational transcoding. Accordingly, the rotational operations performed by rotational transcoder  18  need not be applied as separate operations, but could be combined with the entropy decoding or entropy encoding steps discussed herein. 
       FIG. 8  is a conceptual diagram showing an image that is indexed according to MCUs. In particular, by assigning a restart marker value of a JPEG file to 1, each MCU of the JPEG file may comprise a separately decodable unit. This means that any entropy coding may need to be performed within each MCU, and entropy coding does not extend between the MCUs. Furthermore, each MCU may be automatically indexed in a raster order as shown in  FIG. 8 . Once this indexing is performed, rotational transcoder  18  (FIG.  1 ) can fetch MCUs from memory  16  according to a specified rotation. For example, for 90 degree rotation, rotational transcoder  18  may fetch MCUs in the following order according to the index numbers shown in  FIG. 7  ( 85 ,  73 ,  61 ,  49 ,  37 ,  25 ,  13 ,  1 ,  86 ,  74 ,  62 , and so forth). For 180 degree rotation, rotational transcoder  18  may fetch MCUs in a reverse order ( 96 ,  95 ,  94 ,  93 ,  92 ,  91 ,  90 ,  89 ,  88 ,  87 ,  86 ,  85 ,  84 ,  83 , and so forth). For 270 degree rotation, rotational transcoder  18  may fetch MCUs in the following order according to the index numbers shown in  FIG. 7  ( 12 ,  24 ,  36 ,  48 ,  60 ,  72 ,  84 ,  96 ,  11 ,  23 ,  35 , and so forth). In this way, indexing facilitates memory fetches to process data in a manner that allows for image rotation. Upon fetching a given MCU, the data within that MCU is then rotated as addressed herein. The reordering and rotation of data within the reordered MCUs collectively generates a rotated version of the image, e.g., without requiring the MCUs to be fully decoded back to the pixel domain. 
       FIG. 9  is another flow diagram illustrating a technique for rotating an image consistent with this disclosure.  FIG. 9  will be described from the perspective of device  2  of  FIG. 1 . As shown in  FIG. 9 , rotational transcoder  18  of device  2  partially decodes image data associated with an encoded file to generate MCUs ( 91 ). As an example, the encoded file may comprise a JPEG file that includes both DCT encoding and entropy encoding. The partial decoding process ( 91 ) may include entropy decoding but does not fully decode the DCT coefficients back to a pixel domain. As noted, in some cases the low frequency DC coefficient may be decoded to remove differential coding, while the high frequency AC coefficients remain in the DCT domain. 
     Rotational transcoder  18  reorders the MCUs ( 92 ). For example, the MCUs may be reordered such that the MCUs are presented in a rotated manner relative to the original image. Next, rotational transcoder  18  processes the image data within each of the MCUs in order to apply file size parameters and to rotate the image data within each of the MCUs ( 93 ). Thus, rotational transcoder first reorders the MCUs ( 92 ), and then rotates the image data within each of the MCUs ( 93 ). The steps of ( 92 ) and ( 93 ) results in the generation of a rotated version of the image that complies with file size constraints. That is to say, rotational transcoder  18  generates a rotated (possibly compressed) version of the image comprising the reordered MCUs and the rotated image data within the MCUs ( 94 ). The rotated image data may be compressed based on the file size constraints. For example, the file size parameters may specify file size limits, and rotational transcoder  18  may compress data (e.g., by reducing or eliminating values associated with high frequency DCT components) so that the rotated image does not exceed the file size constraints. The steps of ( 92 ) and ( 93 ) could be reversed in some embodiments. 
     After generating the rotated (and possibly compressed) version of the image comprising the reordered MCUs and the rotated image data within the MCUs ( 94 ), rotational transcoder  18  re-encodes the MCUs ( 95 ). This re-encoding process of step ( 95 ) is the opposite of the partial decode process of step ( 91 ). Thus, in the example where step ( 91 ) involves entropy decoding, step ( 95 ) may involve entropy encoding the MCUs. Rotational transcoder  18  then outputs a rotated version of the image data, e.g., a rotated JPEG file, to memory  16  ( 96 ). 
     The techniques described herein may be implemented in hardware, software, firmware or any combination thereof. Any of the described units, modules or components may be implemented together in an integrated logic device or separately as discrete but interoperable logic devices. If implemented in software, the techniques may be realized at least in part by a computer-readable medium comprising instructions that, when executed, performs one or more of the techniques described above. The computer-readable medium may form part of a computer program product, which may include packaging materials. The computer-readable medium may comprise random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), FLASH memory, magnetic or optical data storage media, and the like. The techniques additionally, or alternatively, may be realized at least in part by a computer-readable communication medium that carries or communicates instructions or data structures and that can be accessed, read, and/or executed by a computer. 
     The instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated software modules, hardware modules, or any combination thereof. 
     If implemented in hardware or a combination of hardware and software, the techniques described herein may be embodied in an apparatus, device or integrated circuit, which may comprise rotational transcoder  18  shown in  FIG. 1 , or possibly a combination of components shown in  FIG. 1 . An integrated circuit, for example, can be configured to perform one or more of the techniques described herein. In that case, an integrated circuit may be configured to reorder MCUs of an encoded image, process the reordered MCUs to rotate image data within the MCUs to generate a rotated version of the encoded image, and output the rotated version of the encoded image. 
     These and other examples are within the scope of the following claims.