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Special Forums UNIX and Linux Applications Compression (gzip) for image analysis Post 302591427 by Corona688 on Thursday 19th of January 2012 12:25:23 PM
Old 01-19-2012
gzip isn't suitable. gzip wouldn't know which bits mean what pixels. gzip might also find patterns you don't want, identical things in the wrong places of the image. It's also lossless, so an intensity difference of 1 matters just as much as an intensity difference of 255 as far as it's concerned. It really doesn't know what the picture looks like.

This paper can't have meant lossless compression, or any kind of general compression -- it must have meant some kind of lossy image compression like jpg which doesn't care if the pixels are bit-for-bit identical.

I don't think 'cat' is an appropriate way to combine the images, either.

I think what they're looking for is the resulting size of the image when saved with identical levels of lossy compression. S(I1) and S(I2) should be very similar. What S(I3) is would depend. How does the paper say to combine these images?
 

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IMAGEPNG(3)								 1							       IMAGEPNG(3)

imagepng - Output a PNG image to either the browser or a file

SYNOPSIS
bool imagepng (resource $image, [string $filename], [int $quality], [int $filters]) DESCRIPTION
Outputs or saves a PNG image from the given $image. PARAMETERS
o $ image -An image resource, returned by one of the image creation functions, such as imagecreatetruecolor(3). o $filename -The path to save the file to. If not set or NULL, the raw image stream will be outputted directly. Note NULL is invalid if the $quality and $filters arguments are not used. o $quality - Compression level: from 0 (no compression) to 9. o $filters - Allows reducing the PNG file size. It is a bitmask field which may be set to any combination of the PNG_FILTER_XXX constants. PNG_NO_FILTER or PNG_ALL_FILTERS may also be used to respectively disable or activate all filters. RETURN VALUES
Returns TRUE on success or FALSE on failure. CHANGELOG
+--------+--------------------------------+ |Version | | | | | | | Description | | | | +--------+--------------------------------+ | 5.1.3 | | | | | | | Added the $filters parameter. | | | | | 5.1.2 | | | | | | | Added the $quality parameter. | | | | +--------+--------------------------------+ EXAMPLES
<?php $im = imagecreatefrompng("test.png"); header('Content-Type: image/png'); imagepng($im); imagedestroy($im); ?> SEE ALSO
imagegif(3), imagewbmp(3), imagejpeg(3), imagetypes(3), imagesavealpha(3). PHP Documentation Group IMAGEPNG(3)
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