Abstract:
A stereoscopic video temporal frame offset measurement system uses an improved signature curve generator to generate a robust signature curve for each of the left and right image sequences of a stereo video signal. The resulting signature curves are cross-correlated over a specified correlation range to generate cross-correlation coefficients. The cross-correlation coefficients are compared to a high threshold, and a temporal frame offset is generated and displayed for each cross-correlation coefficient that exceeds the high threshold. Each new temporal frame offset result is highlighted when displayed, otherwise the last result is displayed in a background fashion.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to three-dimensional (3D) television (TV) video processing, and more particularly to the measurement of stereoscopic video temporal frame offset. 
         [0002]    Stereoscopic three-dimensional television (3DTV) video is a sequence of stereoscopic frame-pairs. One of the frames in each frame-pair is intended for viewing by the Left Eye only, and the other frame is intended for viewing by the Right Eye only. In this manner the binocular vision of the frame-pair creates the stereoscopic illusion of depth along with the traditional image height and width. These frame-pairs may be separate video streams of the Left Eye (L) image frames and the Right Eye (R) image frames. Each stream is generally taken from a 3D camera system that is really two cameras with separate video outputs to produce a serial digital dual link. It is desirable that the two cameras are synchronized so that the L and R frame-pairs are captured at the same time. 
         [0003]    These dual-link outputs may be sent as separate compressed video streams to a location where the image pairs are combined with one of several methods to produce a single stream for distribution. Each stream may be compressed with a separate coder/decoder (CODEC), one for the L image sequence and one for the R image sequence. However each CODEC often has an undetermined frame delay or processing latency. As a result the decompressed output sequence of L and R frames may no longer be pair-wise, frame synchronous, i.e., there may be one or more frames of temporal miss-alignment or temporal frame offset. 
         [0004]    U.S. Pat. No. 6,751,360, issued Jun. 15, 2004 to Jiuhuai Lu and assigned to Tektronix, Inc. of Beaverton, Oreg. and incorporated herein by reference, describes a fast temporal alignment estimation method for temporally aligning a distorted video signal with a corresponding source video signal. A temporal signal curve (SC) is created for each of the video signals, and the resulting SCs are cross-correlated with each other to determine a match between corresponding frames of the two video signals. The maximum cross-correlation result is an indication of the amount of temporal displacement between corresponding frames of the two video signals. However the resulting SC, as shown by the SC graph of  FIG. 1   a , has a large offset. The large offset does not allow robust determination of the frame offsets from the SCs and requires a large bit-size for digital integer implementation in hardware (HW).. 
         [0005]    What is desired is a method, without any a priori knowledge of video content, of measuring the frame offset or L to R temporal miss-alignment, which measurement provides a robust indication of any uncorrected temporal frame offset between the L and R image sequences. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    Accordingly the present invention provides a stereoscopic video temporal frame offset measurement technique that is robust, and does not require any a priori knowledge of video content. The stereoscopic video temporal frame offset measurement system uses an improved signature curve generator to generate a robust signature curve for each of the left and right image sequences of a stereo video signal. The resulting signature curves are cross-correlated over a specified correlation range to generate cross-correlation coefficients. The cross-correlation coefficients are compared to a high threshold, and a temporal frame offset is generated and displayed for each cross-correlation coefficient that exceeds the high threshold. Each new, reliable temporal frame offset result is highlighted when displayed, otherwise the last result is displayed in a background fashion. 
         [0007]    The objects, advantage and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0008]      FIG. 1  provides a block diagram view of an improved signature curve generator for the measurement of stereoscopic video temporal frame offset according to the present invention. 
           [0009]      FIG. 2  provides a block diagram view of the system for stereoscopic view temporal frame offset measurement according to the present invention. 
           [0010]      FIG. 3  provides a block diagram view of the signature curve generator having reduced hardware requirements according to the present invention. 
           [0011]      FIG. 4  provides a block diagram view of an input filter for further reducing the hardware requirement according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring now to  FIG. 1 , a sequence of luminance images, Y f (n), is input to a temporal signature curve (C f ) generator  10  to produce a frame-to-frame cross-variance output as described in the above-mentioned U.S. Pat. No. 6,752,360. The luminance sequence is input to a high-pass filter (HPF)  12  to produce a sequence of high-pass filtered luminance images, Yh f (n), which in turn is input to a frame/field delay circuit  14  that provides an output sequence of luminance images, Yh f-1 (n), that are delayed by one frame/field. The two resulting sequences of luminance images are input to a multiply-accumulate circuit  16  effecting an un-normalized cross-correlation, also known as a co-variance measure, to produce the SC, C f , according to the prior art, and which is graphed as  FIG. 1   a . As described previously, at this point the SC has a large offset so that variations are not readily discernible. Therefore SC is further processed by a differentiator  18  that provides a normalized first-past difference, Cd f , of the frame-to-frame cross-correlation, C f , a graph of which is shown in  FIG. 1   b . For the purpose of comparison, Cd f  is normalized by the number of frames, N, as shown in  FIG. 1   b . It is apparent by comparing the two graphs of  FIGS. 1   a  and  1   b  that now variations are readily apparent, i.e., the dynamic range of the SC is significantly reduced and the AC content is increased. 
         [0013]    Referring now to  FIG. 2 , using the improved SC generator of  FIG. 1 , luminance component pixels, YL f (n) and YR f (n), of a stereoscopic image pair are applied to a dual SC generator  20  to produce the first-past differences, CLd f  and CRd f . Each pixel, indexed by n, of the input and output frame/field of the frame/field delay  14  is then cross-correlated, without normalization to the individual standard deviations, by the multiply-accumulate circuit  16  to produce the covariance measure of the frame difference of each input frame, indexed by f, and the previous frame, indexed by f−1. This is done separately on the left and right input frame sequences, YL f (n) in the left sequence frame/field delay  14 L and multiply-accumulate circuit  16 L, and YR f (n) in the right sequence frame/field delay  14 R and multiply-accumulate circuit  16 R. The resulting covariance values for each left frame, indexed by f, from the left multiply-accumulate circuit  16 L are then differentiated by differentiator  18 L to produce signature curve values, CLd f . Likewise the resulting co-variance values for each right frame, indexed by f, from the right multiply-accumulate circuit  16 R are differentiated by the differentiator  18 R to produce signature curve values, CLd f . This signature curve process is preferably implemented in hardware to allow processing in real time. 
         [0014]    The improved temporal signal curves of the L and R image sequences, as discreet values at the sequence frame-rate, are shown superimposed in the graph shown in  FIG. 2 . As indicated in this instance as an example, at frame  12  the right channel skipped a frame so the signature curves are not synchronous, i.e., after the frame skip there is a one frame offset between the two channels. 
         [0015]    The respective first-past differences are input to respective first-in/first-out (FIFO) buffers  22 L,  22 R having a length equal to twice a specified correlation range R. The outputs from the FIFO buffers  22  are available to a typical microprocessor and memory system  24 . The microprocessor and memory system  24  computes a sliding correlation, as illustrated in  FIG. 2 . The microprocessor  24  reads past samples of CLd and CRd from the FIFO buffers  22  within the real-time frame processing hardware after a sufficient record of frame-rate samples have been acquired. The record length, as indicated above, is 2×R, where R is the correlation computation range and offset detection range, as shown in the equation of  FIG. 2 . 
         [0016]    The root mean square (rms) values over the correlation range R of the respective CLd and CRd are used in the denominator of the shown equation to create a normalized correlation coefficient ranging from −1 to +1. Therefore there is the possibility that, within the given computation range R, all the frames are still frames, i.e., are identical, so that the rms value of that section of the signature curve approaches zero. To avoid this situation, the microprocessor  24  tests the rms values against a minimum value, and disregards the set of frames over the correlation range R where the rms values are less than the minimum value. 
         [0017]    The equation shown in  FIG. 2  computes a correlation coefficient, Xdc(s,f), over the range R for several frame offsets, s, to determine the best estimate of the offset between the two signature curves, CLd(f) and CRd(f). In the example shown, the range of offsets, s, is from −3 to +4 frames, and the correlation range, R, is 8. However a longer correlation range (&gt;8) may be used to get more accuracy in the computation of Xdc(s,f). Also common methods of interpolation or sample up-conversion of CLd(f) and CRd(f) may be used prior to correlation to achieve higher time resolution to fractions of a frame/field time. The value of Xdc for each offset, s, is tested to see if it is above a certain threshold, typically above 0.98. If the value of Xdc is above the threshold, then a reliable indication of the L to R frame offset has been found and the value of the corresponding offset, s, is displayed on an instrument display screen  26 . In the given example the value of LR Frame Offset is +1, which value is displayed. For each calculated LR Frame Offset, the resulting offset, s, is highlighted, if found to be reliable, to indicate that an update has occurred. If the readout is not updated, then the readout displayed is shown in gray or otherwise not highlighted to indicate that the last good value is shown without a recent update. Every time a new frame, f, is added to the FIFO buffers  22 , the oldest value is discarded and the computation of Xdc(s,f) is repeated and, if a value above the threshold is found, the readout is again updated with this reliable value and highlighted on the display  26 . 
         [0018]    It may also be preferable to graph the resulting updated frame offset, s, over time to create a record of the L to R image sequence temporal offset over long periods. This allows time-stamping or alarming of intervals where the temporal misalignment exceeds a pre-determined threshold for an unacceptably long period. 
         [0019]    Although hardware implementation is preferable, the complete process may be implemented and tested in software (SW). From such testing, the threshold value and correlation range of the example shown were determined, although other values may be used. 
         [0020]    To reduce the required HW, the YL(n) and YR(n) pixels from the L and R image sequences may be decimated by two and interleaved into a single sample stream, as illustrated in  FIG. 3 . The pixels from the respective image sequences are input to respective low pass filters (LPF)  32 L,  32 R of a low pass filter and interleave circuit  30 , which is part of an input filter circuit  28 . The outputs from the respective LPFs  32  are input to respective decimators  34 L,  34 R, with one of the outputs from the decimators being delayed one pixel interval by a delay circuit  36 . The outputs from the un-delayed decimator  34 R and the delay circuit  36  are input to a multiplexer  38  to produce a single pixel stream having interleaved L and R image sequence pixels. The resulting single pixel stream is then input to a high pass filter (HPF)  12 ′ to remove the DC or luminance-mean component before input to the frame/field delay  14 . The input and output from the frame/field delay  14  are input to a modified multiply-accumulate circuit  16 ′ to effect a cross-correlation, producing respective CL f  and CR f  streams that are input to the respective differentiators  18  to produce the same SC outputs as shown in  FIG. 2 . The single multiply-accumulate circuit  16 ′ is now effectively a cross-covariance detector, since the result is not normalized by the frame rms values. 
         [0021]    Due to the interleaving in  FIG. 3 , the separate HPF filtered L and R luminance pixels are independently accumulated for the cross-covariance between temporally adjacent frames or fields, then differentiated and normalized into the desired SC values for input to the microprocessor  24  shown in  FIG. 2 . 
         [0022]    Further simplification of the HW may be done by combining the LPF, interleaving and HPF filtering  30 ,  12 ′ as shown in  FIG. 4 . The impulse responses of the LPFs  32  and HPF  12 ′ may be combined, or convolved, into a single impulse response. The result then may be decomposed into a two-phase implementation and selection multiplexer, creating the desired total horizontal frequency response for each luminance component YL(n) and YR(n). The output is the interleaved and decimated luminance values with the combined LPF and HPF horizontal frequency response which is then input to the single frame/field delay  14 . The upper portion of  FIG. 4  shows a detailed block diagram of the combined polyphase interleaver finite impulse response (FIR) filter  40 , while the bottom view shows how the convolved LPF and HPF impulse responses are decomposed into separate, simplified, processing paths in the combined polyphase interleaver FIR filter. 
         [0023]    Thus the present invention provides a stereoscopic video time frame offset measurement system using an improved signature curve generator to produce signature curves for left and right image sequences, which signature curves are then correlated with each other to determine time frame offsets between the two image sequences.