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In some situations, color is associated with each pattern position (“Color JB2″) instead of the creation of a separate foreground color layer.Įxposing the OCR Layer for Searching and IndexingĪ variety of tools can be used to expose the XML hidden text layer of any DjVu file. IW44 encoding is very similar to that of JPEG 2000, but it also supports a half-decoding that enables browser plug-ins to be very efficient while ” very little memory, it enables the masking feature that guarantees that no IW44 bits will be wasted encoding pixels that will be covered by text/foreground pixels, and it is usually about three times faster. IW44 is a progressive format which supports in-place updates when DjVu files are viewed over a slow connection, background layers, text colors and images are decoded progressively and the view is gradually refined in the plug-in window as more data arrives. The “color” of colored text is typically represented as a separate, low-resolution layer called the “foreground color layer” and compressed ” the wavelet-based compression technique IW44, which is the same method as the one used for the background layer.
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In addition, the segmenter’s ability to accurately separate text from graphics leads to dramatically improved optical character recognition (OCR) results when scanning complex color layouts. Since the greatest obstacles to compression in a document are created by the sharp edges of the text component, segmenting the text into a separate layer enables DjVu to apply aggressive compression techniques to the relatively soft-edged graphics that remain, resulting in DjVu’s high overall compression ratios while preserving a visually lossless appearance. These include the compression methods known as JB2 and IW44. Instead, they separated or “segmented” the document image into elements that could then be compressed independently ” the best possible approach – and resolution – for each element.
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It relies on advanced content analysis techniques to segment a scanned color page into layers and achieve high compression ratios, low memory utilization, extremely fast rendering on screen and indexing of the material.ĭjVu offers a means not only of preserving documents, but also making them truly accessible and usable across and throughout the digital world regardless of available bandwidth or the origin, complexity or size of the originals.ĭjVu’s architects asked, “Why use 200 dpi or more for image and graphical elements when a much lower resolution would be sufficient, and why use a full-color compression technique for text, which is typically black and white or low-color?” DjVu uses a mixed raster content (MRC) imaging model, an approach consistent with that endorsed by the International Telecommunications Union (ITU).