Method and system for determining the legibility of text in an image

Legibility of text in an image of a page is determined by comparing a measure of the text in the page image with a measure of the page image itself. In one aspect, a measure of the text in the page image may be the height of a line of text, while the measure derived from the page image may be the height of the page image. A text-to-page height ratio is determined and compared to one or more thresholds for determining legibility. In another aspect of the invention, a measure of the text in a page image is obtained by measuring the word density in the page image, while the measure derived from the page image comprises compressing the page image and determining the size of the compressed image file. Legibility is then determined by comparing the measure of word density with the compressed image file size.

FIELD OF THE INVENTION

The present invention is directed to processing of digital images, and more particularly to processing and displaying images of content having text therein.

BACKGROUND OF THE INVENTION

As the use of computers and computer-based networks continues to expand, content providers are preparing and distributing more and more content in electronic form. This content includes traditional media such as books, magazines, newspapers, newsletters, manuals, guides, references, articles, reports, documents, etc., that exist in print, as well as electronic media in which the aforesaid content exits in digital form. The Internet, in particular, has facilitated the wider publication of digital content through downloading and display of images of content. As data transmission speeds increase, more and more images of pages of content are becoming available online. Page images allow the reader to see the page as it would appear in print. Furthermore, graphics, such as charts, drawings, pictures, etc., and the layout of such graphics in a page, are not lost when the page of content is provided as a digital image.

Despite the great appeal of providing digital images of content, the cost of storing images of content remains a concern for many content providers. To minimize storage costs, content providers desire to minimize the size of files used to store the images. Digital images may be represented at a variety of resolutions, typically denoted by the number of pixels in the image in both the horizontal and vertical directions. Typically, though not always, higher resolution images have a larger file size and require a greater amount of memory for storage. The cost of storing images of content can greatly multiply when one considers the number of images it takes to capture and store large volumes of media, such as books, magazines, etc.

While reducing the size and resolution of images often reduces the requirements for storing the images, low resolution images eventually reach a point where, if too small, the image is difficult for readers to perceive when displayed. This problem is further exacerbated when the images represent pages of content containing text that readers desire to read. If the text in an image of content is not legible, the value of the image significantly decreases. Content providers wishing to provide page images with text that can be read must ensure that the images have sufficient resolution to provide legible text when displayed.

The legibility of text in a digital image is largely a matter of human perception. Content providers that have a significant amount of digital images of content face the difficulty of determining whether a given image of content has sufficient resolution to be perceived as legible by most readers. One solution is to employ human readers to visually inspect images of content to determine whether the images are legible. For large repositories of content, however, a process of human review can become inordinately time-consuming and expensive. What is needed is a method and system that can be implemented in a computer to process images of content and determine whether text in an image is likely to be legible to readers. The present invention addresses this need and other shortcomings in the prior art.

SUMMARY OF THE INVENTION

Embodiments of the invention discussed herein are directed to a computer-implemented method and system for determining the legibility of text in an image of a page. In accordance with a method embodiment of the invention, the method includes first obtaining an image of a page of content having text therein. Text recognition is then performed on the text in the page image from which a measure of the text is produced. The measure of the text is compared to a measure derived from the page image itself, and based on the comparison of the text and page image measures, the legibility of the text in the page image is determined.

In one aspect, the text recognition may comprise identifying a spatial dimension of the text in the page image. In this aspect, the measure of the text in the page image is a measure of the text in the identified spatial dimension, e.g., the height of a line of text, while the measure derived from the page image itself may be a measure of the height of the page image in the same spatial dimension. The text height is divided by the page image height to produce a text-to-page height ratio that is compared to a threshold. If the text-to-page height ratio does not satisfy the threshold, the text is determined not legible. Conversely, if the text-to-page height ratio does satisfy the threshold, the text in the page image is determined legible.

Text recognition may be performed on text in a single line of the page image. Alternatively, the text recognition may be performed on multiple lines of text in the page image. Each of the multiple lines of text may produce a text-to-page height ratio that is compared to a threshold. The legibility of the text in the page image may then be determined based on a percentage of the text-to-page height ratios that satisfy the threshold.

Multiple thresholds for determining legibility may be used in the invention. For example, if a text-to-page height ratio satisfies both a first and a second threshold, the text may be determined legible. If the text-to-page height ratio satisfies the first threshold but not the second threshold, the text may be determined possibly legible. Otherwise, if the text-to-page height ratio does not satisfy the first threshold, the text is determined not legible. Page images having text determined to be legible may be stored in a memory for display. If a page image is determined to be not legible, the page image may be replaced with a higher resolution image of the page.

In another aspect of the invention, the text recognition performed on the text in the page image may comprise identifying and counting words in the page image. A measure of the text in the page image is obtained by calculating a measure of the density of the words in the page image based on the number of words and the spatial size of the page image. Furthermore, in this aspect of the invention, a measure derived from the page image may comprise compressing the page image to form a compressed image file and determining the file size of the compressed image file. The legibility of the text in the page image is then determined by comparing the measure of word density in the page image with the file size of the compressed image file. Page images, for example, having a large compressed file size and a high word density may be determined legible, while page images having a high word density with a small compressed file size may be determined not legible. Page images having a low word density and a small compressed file size may be determined possibly legible. Multiple legibility tests may be applied to images determined to be possibly legible to determine whether, in fact, the text in the page images are legible.

Further embodiments of the invention described herein provide a computer-implemented method for displaying a page image based on a determined legibility of text in the page image using methods substantially as described above. Embodiments of the invention also describe a multistage method for automated determination of legibility of text in an image of a page using multiple tests of legibility.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to a method and system that determines the legibility of text in an image of a page of content. The legibility of text in a page image is determined by comparing a measure of the text in the page image with a measure derived from the page image itself. In some embodiments, a test of legibility is performed by identifying a spatial dimension of a page image and comparing measures of text height and page image height in the identified spatial dimension. In other embodiments, a test of legibility is performed by calculating a measure of word density in the page image and comparing the word density to a file size of the page image that has been compressed using an image compression algorithm. The following detailed description provides several exemplary implementations of the invention. Although specific flow diagrams and system configurations are illustrated, it should be understood that the examples provided are not exhaustive and do not limit the invention to the precise forms disclosed. Persons having ordinary skill in the field of computers and digital imaging will recognize process steps described herein that may be interchangeable with other steps, or combinations of steps, and still achieve the benefits and advantages of the present invention.

To provide a context for describing embodiments of the present invention,FIG. 1illustrates a functional block diagram of a computing system10that may be used to implement the present invention. The computing system10includes a computing device11having a processor12in communication with a variety of computing elements, including a network interface14, an input/output interface16, and a memory18. The network interface14enables the computing device11to communicate data, control signals, data requests, and other information via a computer network (LAN, WAN, Internet, etc.). For instance, the computing device11may receive a file containing a digital image of content from a computer network via the network interface14.

The input/output interface16enables the computing device11to communicate with various local input and output devices. An input device20in communication with the input/output interface16may include computing elements that provide input signals to the computing device11, such as a digital camera, a scanner, a keyboard, mouse, external memory, disk drive, etc. Input devices comprising cameras and scanners, for example, may be used to provide digital images of content to the computing device11for processing and legibility analysis in accordance with the present invention.

An output device22in communication with the input/output interface16may include typical output devices, such as a computer display (e.g., CRT or LCD screen), a television, printer, facsimile machine, copy machine, etc. As to the present invention, the output device22may be used to display images with text that the computing device11determines to be legible.

The processor12is configured to operate in accordance with computer program instructions stored in a memory, such as the memory18. Program instructions may also be embodied in a hardware format, such as a programmed digital signal processor. Furthermore, the memory18may be configured to store digital images of content for processing transmission, and display in accordance with the present invention. Those having ordinary skill in the art of computers will recognize a wide selection of commercially-available components that can be used to construct a system such as the computing system10illustrated inFIG. 1.

FIG. 2illustrates a Web environment in which the present invention may be used. Content providers having digital images of content may distribute those images over the Internet for display by remote computing devices. For example,FIG. 2illustrates the display of a computing device that is operating a browser program30. At the top of the browser display are various controls for operating the browser. Depicted underneath the controls is a Web page in which a digital image of content32is displayed. The Web page may include additional controls34for browsing through the pages of content32being displayed. Although the controls34, as shown, indicate that a reader of the page image32may “zoom in” or “zoom out”, content providers may prefer, at least initially, to display the page image32at its native (i.e., full) resolution.

For purposes of reference,FIG. 3depicts the entire page image32that is shown in the browser display30ofFIG. 2. Lower resolution images will produce a smaller display image and higher resolution images will produce a larger display image. Scrolling through the Web page in the browser display allows the reader to access the bottom portion of the page image not shown inFIG. 2.

Embodiments of the invention described herein determine whether text in page images, such as the page image32shown inFIG. 3, is likely to be legible to readers when displayed in a computer display, such as shown inFIG. 2. Generally speaking, if text in a page image is determined to be not legible, content providers may seek to obtain a higher resolution image of the page to ensure the legibility of the page image when displayed.

FIG. 4depicts an exemplary method40for determining the legibility of text in a page image according to one embodiment of the present invention. The method40involves assessing the legibility of text in a page image based on a comparison of text height and page height. The method40begins at block42by obtaining a page image at a base resolution. One possible base resolution may be 640×480 pixels. Other base resolutions may be larger or smaller, as desired. A base resolution page image may be obtained by scanning a page of content at the base resolution. Alternatively, the base resolution page image may be derived from an original high resolution image of a page that was previously obtained.

At block44, the method40performs text recognition on text in the base resolution page image. Those of ordinary skill in the art will recognize a wide variety of algorithms and techniques capable of recognizing text in an image. For purposes of the present invention, it is not necessary that the text recognition actually recognize the individual components of words or symbols or interpret the meaning of those words or symbols, as achieved by many optical character recognition (OCR) routines. An OCR-like process may be used in which limited information such as the location and size of text in the page image is ascertained. For example, the boxes shown around text inFIG. 10pictorially illustrate an OCR-like process that has determined the location and size of text in a page image.

It should also be understood that, in the context of the present invention, the term “text” includes all forms of letters, characters, symbols, numbers, formulas, etc., that may be used to represent information in a page image. The term “page” broadly refers to any space of two dimensions or greater in which text may reside. For simplicity, the text recognition performed in the method40need only be performed on part of a single line of text in the page image. As will be discussed below in reference toFIGS. 6A and 6B, embodiments of the invention may alternatively perform text recognition on multiple lines of text in a page image.

At block46, the method40determines the height of the base resolution page image. The height of the page image may be determined using any suitable units of measure, such as inches, centimeters, numbers of pixels, etc., that extend over a spatial dimension. At block48, the method40determines the height of the text that was recognized by the text recognition of block44. The height of the text is determined in the same spatial dimension as the determined height of the page image, preferably using the same units of measure.

The method40divides the text height by the page image height to determine a text-to-page height ratio, as indicated at block50. The text-to-page height ratio, in this embodiment of the invention, is used as a measure of the legibility of the text in the page image. Generally speaking, the greater the text-to-page height ratio, the more likely the text in the page image is legible to readers.

In decision block52, the text-to-page height ratio of the base resolution page image is compared to a threshold ratio. If the text-to-page height ratio is greater than or equal to the threshold, the text in the base resolution page image is determined to be legible and the page image is stored, as indicated at block54, for display to readers. Minimum text-to-page height ratios may be determined as needed for different displays, for example, using empirical studies of the legibility of page images at varying resolutions. A sampling of readers may be polled to determine text-to-page height ratios corresponding to text that the readers find to be legible. Recognized text meeting a threshold minimum text-to-page height ratio is considered to be legible.

If, at decision block52, the text-to-page height ratio does not exceed the threshold ratio, the text in the base resolution page image is determined to be not legible. In view of such result, a content provider may wish to obtain another image of the page at a resolution higher than the base resolution, as indicated at block56. The higher resolution page image may be assumed to be legible and stored, as shown at block58, for display to readers. Alternatively, a legibility test, such as the one provided by the method blocks44,46,48,50, and52, may be repeated for the page image at successively higher resolutions until an image of the page is obtained at a resolution that displays legible text to readers. In instances where higher resolution page images are required, the higher resolution images may be obtained by rescanning the page of content at a higher resolution. In instances where the base resolution page image was obtained from an original high resolution page image, the method40may use the original high resolution page image to again derive an image of the page, but at a resolution higher than the earlier base resolution.

The method40thus determines the legibility of text in a page image by comparing a measure of the text in the page image (here, a measure of text height) with a measure derived from the page image itself (here, a measure of the height of the page image).FIG. 5depicts the page image shown inFIG. 3in which a legibility test is conducted in accordance with the type of method as described inFIG. 4. The height of the page image32is referenced by Yp. The height of text in the page image is illustrated by references Y1, Y2, . . . Y20, for each of the lines of text shown in the page image32. As noted earlier, the method40shown inFIG. 3involves recognition and measurement of text in at least one line of the page image32. The line of text that is recognized and measured by the method40may be chosen at random in the page image, or it may be a line at a selected position in the page image. It should be noted that the text recognition performed in the method40does not need to determine the length of any given line of text; it need only determine the height of text. Other embodiments of the invention may compare the width of a character of text with the width of the page image. Again, these are both measurements of text and of the page in a given spatial dimension.

Although determining the legibility of text can be performed on only a single line of text, content providers may desire to analyze multiple lines of text for legibility to avoid circumstances in which the measurement of text in a given line may skew the overall legibility determination. For example, in the page image32shown inFIG. 5, different lines of text may result in different measurements of text height. The text height Y1and Y2, for example, is greater than the text height Y3, which is greater than the text height Y4, which is greater than the text height Y5-Y19. The text height of the last line Y20is the smallest, as it represents the height of a footnote in the page image32. If only a single line is analyzed and it happens to be the last line with height Y20, the entire page image32may be deemed not legible, even though the larger size of text of lines1-19may, in fact, be legible. Recognizing and evaluating text in multiple lines in an image helps overcome the foregoing risk in single line implementations of the invention.

In some circumstances, depending on the importance of legibility, a content provider may wish to identify the smallest text in a page and ensure that the page image is provided at a high enough resolution that the smallest text meets the legibility threshold. In other circumstances, the content provider may determine that the text in a page image is sufficiently legible if a certain percentage of lines of text in the page image are determined legible, as will be seen next inFIGS. 6A and 6B.

FIGS. 6A and 6Bdepict another example of a method60for determining the legibility of text in a page image. In the method60, multiple lines of text in the page image are evaluated for legibility. The method60begins at block62by obtaining a page image at a base resolution. At block64, text recognition is performed on the base resolution page image and at block66, the height of the base resolution page image is determined. The process of obtaining a page image, performing text recognition on the page image, and determining the height of the page image may be performed in a manner similar to the processes described inFIG. 4.

At block68, the height of a line of text is determined, and at block70, the text height is divided by the page image height to determine a text-to-page height ratio. Again, these processes may be performed in a manner similar to those processes described inFIG. 4.

At decision block72, the text-to-page height ratio is compared to a threshold for legibility, and if the ratio is greater than or equal to the threshold, the method60stores a “YES” count in memory, as indicated at block74. The “YES” count signifies text whose text-to-page height ratio equaled or exceeded the threshold.

At decision block76, the method60determines whether additional lines of text that were recognized in the page image remain for evaluation. The method60may be configured to evaluate some or all of the lines of text in a page image, as desired. If additional text lines are determined to remain for evaluation, the method60returns from decision block76to block68, where it determines the height of text for a new line in the page image. The process of determining a text-to-page height ratio for each new line of text, and its comparison to a threshold ratio, is repeated until no further text lines remain for evaluation.

If at decision block72, the method60determines that the text-to-page height ratio for a given line of text is not greater than the threshold, the method60may store a “NO” count in memory, signifying that the line of text is determined not legible.

When there remain no additional text lines for legibility determination, the method60proceeds to block80, as shown inFIG. 6B, where the method calculates the percentage of “YES” counts in the total number of counts stored in memory. At decision block82, the percentage of “YES” counts is compared to a threshold percentage. If the percentage of “YES” counts for the base resolution page image is greater or equal to than the threshold percentage, the text in the page image is determined to be legible. A threshold percentage set at 80, for example, indicates that text in a page image will be determined legible if eighty percent of the text lines evaluated have a text-to-page height ratio equaling or exceeding the threshold ratio. If the base resolution page image is determined to be legible, the page image may be stored, as indicated at block84, for display to readers.

If, at decision block82, the percentage of “YES” counts is not greater than the threshold percentage, the text in the base resolution page image is determined to be not legible. At that point, the method60may proceed to block86where a new image of the page is obtained at a resolution that is higher than the base resolution. As with the method40inFIG. 4, the method60may assume that the higher resolution page image is legible and store the page image at block88for display to readers. Alternatively, the higher resolution page image may be subjected to a legibility test, such as the one provided in method60, to confirm whether the text therein is legible. The method60may be repeated for successively higher resolution page images until the legibility criteria set for the page image is met.

It should be recognized that the threshold used in decision block82to determine whether a sufficient percentage of text lines in the page image is legible is different than the threshold used in decision block72to determine whether the text-to-page height ratio of a given line reflects legible text. For example, in reference toFIG. 5, if the first four lines of the page image32are determined legible but none of the remaining 16 lines are legible, the page image32may be determined as a whole not legible. At a higher resolution, all the text lines in the page image32may be determined legible, except for the last line (footnote). In that circumstance, the percentage of text lines determined to be legible (for which “YES” counts are recorded) may exceed the threshold percentage, and the page image as a whole is determined to be legible. A threshold of 100% for decision block82would require that all of the text lines evaluated in a page image be determined legible.

Turning now toFIG. 7, another method for determining the legibility of text in a page image in accordance with the invention is provided. The method90begins with a set of processes that may be performed in a manner similar to processes described in the method40shown inFIG. 4. Specifically, the method90begins at block92by obtaining an image of a page at a base resolution. Text recognition is performed on the base resolution page image, as indicated at block94. At block96, the height of the base resolution page image is determined, and at block98, the height of a line of text recognized in the page image is determined. At block100, a text-to-page height ratio for the text line being evaluated is determined by dividing the text height by the page image height.

In contrast to the method40(FIG. 4) and method60(FIGS. 6A and 6B), the method90uses multiple threshold ratios to assess the legibility of text in a page image. Specifically, at decision block102, the text-to-page height ratio of the text line being evaluated is compared to a first threshold. If the text-to-page height ratio does not exceed the first threshold, the method90determines that the line of text, and therefore the page image, is not legible. Similar to the method40(FIG. 4), a single line of text is evaluated in the method90. Alternatively, the method90may be implemented on multiple lines of text image using procedures described in the method60ofFIGS. 6A and 6B. If the line is determined not legible, the method90proceeds to obtain a new image of the page at a resolution higher than the base resolution. The method90stores the higher resolution page image for display to readers, as indicated at block104.

If the text-to-page height ratio of the base resolution page image is greater than or equal to the first threshold, the text-to-page height ratio is compared to a second threshold at decision block106. The second threshold in decision block106generally sets a higher standard for legibility than the first threshold in decision block102. If the text-to-page height ratio satisfies the first threshold (block102) but does not satisfy the second threshold (block106), the text of the line being evaluated is determined to be possibly legible. For page images with possibly legible text, the method90may subject the page images to one or more additional legibility tests, as indicated at block110. For example, page images determined to be possibly legible may be subjected to a legibility test as set forth in the method120shown inFIG. 9. If the page images are determined legible by the additional legibility test, the page images may then be deemed legible and stored for display to readers. Page images determined not legible or possibly legible by the additional legibility test may be deemed not legible and processed accordingly (e.g., as indicated in block104). Alternatively, further legibility tests may be employed, if desired.

In circumstances where the text-to-page height ratio satisfies both the first threshold and the second threshold, the text in the base resolution page image is determined to be legible and the page image is stored, as indicated at block108, for display to readers.

FIG. 8is a graph that pictorially describes a legibility determination as used in the method90ofFIG. 7. InFIG. 8, text having a text-to-page height ratio that does not exceed the first threshold is determined not legible, as shown at reference numeral112. Text having a text-to-page height ratio exceeding the first threshold but not the second threshold is determined to be possibly legible, as indicated at reference numeral114. If the text-to-page height ratio of text in the page image equals or exceeds both the first and second thresholds, the text in the page image is determined to be legible, as indicated at reference numeral116. Using one or more additional legibility tests in subsequent stages of the analysis for “possibly legible” pages is helpful in identifying those page images that are, in fact, legible under at least one legibility test. The goal of certain embodiments o the invention is to obtain and store page images having a resolution not greater than absolutely required for legibility, which helps reduce the overall storage space, and consequently storage costs, for storing page images.

As noted previously, embodiments of the present invention determine the legibility of text in a page image by obtaining and comparing a measure of text in the page image with a measure derived from the page image itself. In the foregoing methods40,60, and90, the measure of text in the page image was a spatial measurement (e.g., height) of a line of text in the page image. The measure derived from the page image itself was the height of the page image in the same spatial dimension as the height of the line of text.

Described next inFIG. 9is another embodiment of the present invention in which the legibility of text is determined by comparing a measure of text in the page image with a measure derived from the page image itself. In this embodiment, the measure of text in the page image is a determined density of words in the page image, while the measure derived from the page image itself is a file size of the page image when the page image is compressed by a compression routine.

FIG. 9illustrates an exemplary method120that begins at block122by obtaining a page image at a base resolution. At block124, text recognition is performed on the text in the page image. The processes performed in blocks122and124may be performed in a manner similar to those processes described inFIGS. 4,6A, and7, except preferably all of the text in the page image is recognized in block124.

At block126, the method120determines the spatial size of the base resolution page image. As will be understood from the description that follows, the size of the page image is used in calculating the density of words in the page image. The size of the page image may be determined from the number of pixels in the horizontal and vertical directions. The page size may also be determined in other units of measure, such as inches or centimeters, in the horizontal and vertical directions.

At block128, the method120determines the number of words in the page image recognized by the text recognition performed in block124. As with earlier described methods of the invention, the text recognition performed in block124does not necessarily require recognizing individual elements of words, nor does it require that the words themselves be recognized (e.g., in terms of spelling or semantic meaning). Text recognition routines that merely identify features of words, such as position and size, may be used to determine the number of words in a page image. At block130, the density of words in the page image is calculated based on the number of words in the page image and the spatial size of the page image.

At block132, the method120compresses the base resolution page image using a compression algorithm to create a file containing the compressed image. If the base resolution page image has already been compressed as a result of an earlier performed compression process, it may not be necessary for the method120to again compress the page image as indicated at block132. Compression algorithms for use in block132are known in the art. For example, compression algorithms that use a cosine transformation or discrete wavelet transformation may be used. Furthermore, the compressed image files may be created in accordance with known formats, such as JPEG, JPEG2000, or Tagged Image File Format, for example.

At block134, the method120derives a measure of the base resolution page image by determining the file size of the compressed page image. The word density of the base resolution page image is then compared to the file size of the compressed image to determine whether the base resolution page image is legible, as indicated at block136. The basis for determining legibility is discussed below. If, as a result of this comparison, the base resolution page image is determined not legible, the method120may optionally take action to obtain a new image of the page at a resolution that is higher than the base resolution, as indicated at block138. The method120may subject the new higher resolution page image to a legibility test, such as the one provided by the method120, or may assume that the higher resolution page image is legible and store it for display to readers.

The legibility test employed by the method120uses a principle of image compression in which the amount of contrast between foreground and background elements in an image is related to the file size of the image when compressed. Typically, an image file with a high amount of contrast between foreground and background elements (e.g., black on white) contains more information and results in a greater file size when compressed. Images having the same spatial size and a low amount of contrast (e.g., gray on light gray) contain less information and result in smaller file sizes when compressed.

The legibility of text in a page image can be highly affected by the contrast between the foreground text and the background of the page image. Text having a higher contrast compared to the background tends to be more legible. Page images having text that blends into the background and has low contrast tends may be more difficult to read at lower resolutions. The method120shown inFIG. 9uses the foregoing principles to evaluate the legibility of text in a page image.

FIG. 10illustrates a sample page image140in which the method120is shown to operate. As indicated at reference numerals142and144, text on the page image140is recognized (as illustrated by boxes surrounding each of the recognized words). The process of determining the number of words in the page image140can be accomplished by counting the number of “boxes” identified by the text recognition. Reference numeral146illustrates a graphic element, e.g., as an illustration on the page image140, that does not represent text in the image, and thus is not part of the word density determination. The width and height of the image140, referenced by Xpand Yp, is used to determine the page size of the image140. As noted earlier, the word density of the page image140can be determined from the number of words in the page image divided by the spatial size of the page image.

FIGS. 11 and 12provide sample graphs illustrating the comparison of word density of a page image to the file size of a compressed image file for purposes of determining the legibility of the text in the page image. InFIG. 11, a sample graph is shown with a single threshold line150separating page images determined to be not legible from those that are legible. The shape of the threshold line150is exemplary only and may be vastly different in actual implementations of the invention, depending on the manner in which word density is calculated and the image compression routine that is used. Regardless,FIG. 11illustrates the principle that page images having a high word density but small compressed file size probably have low contrast in the page image, and thus are likely not legible (at least at the current resolution of the page image). In circumstances where the file size of the compressed page image is greater, the page image is more likely to contain text with high contrast, and thus is more likely to contain legible text, even where the word density of the page image may be lower.

FIG. 12illustrates a graph in which multiple threshold lines152and154are used to determine the legibility of text in a page image. Again, the shape of the threshold lines152and154may vary in actual implementations of the invention. In this example, page images having a compressed file size exceeding a certain threshold152may be determined legible, regardless of the word density in the page image. For page images having a word density exceeding a second threshold154but having a compressed file size lower than the first threshold152, the page images are determined not legible. Page images having a word density lower than the second threshold154and a compressed file size lower than the first threshold152may be determined possibly legible. As with block110of the method90shown inFIG. 7, page images determined to be possibly legible may be subject to one or more additional legibility tests, such as a legibility test used in the methods40,60, or90, previously described.

While several embodiments of the invention have been illustrated and described above, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. The scope of the invention should thus be determined from the following clams and equivalents thereto.