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j.p.s · 10-05-2014, 08:45 PM

The whole point of JPEG is compression. Its computational cost and lossiness are worthwhile because its compression ratio for photographic images can be enormous with only barely detectable quality degradation. But the characteristics of line drawings are different from photographs, so JPEGs of line drawings suffer in comparison to PNG, both in compression ratio and image quality. Even for photographs, JPEGs should not be used until the final image, because quality degradation accumulates with each step.

Gray scale images are usually represented as rows of 8 bit pixels with values of 255 being white, 0 black, and the rest the intermediate shades of gray. An image 600 pixels wide and 800 pixels high would require 480000 bytes, which is known as the uncompressed size. The heroes-1.jpg at the Joe Abercrombie site is 2048 by 1448 pixels, or 2965504 bytes uncompressed. The size of the JPEG image is 2764773 bytes, a mere 6.7% compression. I had thought that Joques was exaggerating for effect when he wrote: "No compression at all, it seems." in https://www.mobileread.com/forums/sho...=242848&page=2, but for JPEG 6.7% compression is pretty much no compression at all. A straight conversion from JPEG to PNG results in a file 1711169 bytes long, for a compression of 42.3% and 38.1% smaller than the JPEG. Without having to encode the JPEG artifacts, the PNG would be significantly smaller.

These are still 256 gray level images, but e-ink can only display 16 gray levels, which are also plenty for showing line drawings on LCDs. 4 bit (16 color) PNGs are even less storage space than 8 bit PNGs. This particular image also has a lot of scanning noise, which boosts the image size in bytes for both JPEG and PNG. This is easily seen by stretching the contrast of the bright parts of the image (by mapping bright gray / dull white to full black). This can be easilt done with Tools->Color Tools->Curves in GIMP and I assume Photoshop. One of the best ways I know to make JPEG artifacts glaringly obvious is to invoke Enhance->Equalize in the display application from ImageMagick. Saving a line drawing as a 4 bit PNG cleans up a good bit of the scanning noise in addition to shrinking the file size. A 16 level PNG of heroes-1 is 811205 bytes long, 72.6% compression.

According to Joques, the full epub book is 3.6MB and the 5 maps are 714x502 and 300KB each, or 41.6% of the book. I made 714x505 4bit PNG versions of the Maps from the large versions on the web site. They were all 125KB, a reduction of over 50%, reducing them to less than 20% of the book.

This is just one case and results can vary a lot in both directions, but for line drawing, PNG should almost always be a big win over JPEG. I wasn't able to extract images from the azw3 version of the free sample and the azw version was obviously modified by the kindle publishing process, but I was able to determine that it also came from a noisy scan. Cleanly scanned to a lossless file type and touched up will compress better than noisy scans straight to JPEG, even if the final file type is JPEG. Starting with a line drawing generated with a computer application will do much better still.

10-05-2014, 08:45 PM	#10
j.p.s Grand Sorcerer Posts: 5,804 Karma: 103362673 Join Date: Apr 2011 Device: pb360	JPEG vs PNG Case Study The whole point of JPEG is compression. Its computational cost and lossiness are worthwhile because its compression ratio for photographic images can be enormous with only barely detectable quality degradation. But the characteristics of line drawings are different from photographs, so JPEGs of line drawings suffer in comparison to PNG, both in compression ratio and image quality. Even for photographs, JPEGs should not be used until the final image, because quality degradation accumulates with each step. Gray scale images are usually represented as rows of 8 bit pixels with values of 255 being white, 0 black, and the rest the intermediate shades of gray. An image 600 pixels wide and 800 pixels high would require 480000 bytes, which is known as the uncompressed size. The heroes-1.jpg at the Joe Abercrombie site is 2048 by 1448 pixels, or 2965504 bytes uncompressed. The size of the JPEG image is 2764773 bytes, a mere 6.7% compression. I had thought that Joques was exaggerating for effect when he wrote: "No compression at all, it seems." in https://www.mobileread.com/forums/sho...=242848&page=2, but for JPEG 6.7% compression is pretty much no compression at all. A straight conversion from JPEG to PNG results in a file 1711169 bytes long, for a compression of 42.3% and 38.1% smaller than the JPEG. Without having to encode the JPEG artifacts, the PNG would be significantly smaller. These are still 256 gray level images, but e-ink can only display 16 gray levels, which are also plenty for showing line drawings on LCDs. 4 bit (16 color) PNGs are even less storage space than 8 bit PNGs. This particular image also has a lot of scanning noise, which boosts the image size in bytes for both JPEG and PNG. This is easily seen by stretching the contrast of the bright parts of the image (by mapping bright gray / dull white to full black). This can be easilt done with Tools->Color Tools->Curves in GIMP and I assume Photoshop. One of the best ways I know to make JPEG artifacts glaringly obvious is to invoke Enhance->Equalize in the display application from ImageMagick. Saving a line drawing as a 4 bit PNG cleans up a good bit of the scanning noise in addition to shrinking the file size. A 16 level PNG of heroes-1 is 811205 bytes long, 72.6% compression. According to Joques, the full epub book is 3.6MB and the 5 maps are 714x502 and 300KB each, or 41.6% of the book. I made 714x505 4bit PNG versions of the Maps from the large versions on the web site. They were all 125KB, a reduction of over 50%, reducing them to less than 20% of the book. This is just one case and results can vary a lot in both directions, but for line drawing, PNG should almost always be a big win over JPEG. I wasn't able to extract images from the azw3 version of the free sample and the azw version was obviously modified by the kindle publishing process, but I was able to determine that it also came from a noisy scan. Cleanly scanned to a lossless file type and touched up will compress better than noisy scans straight to JPEG, even if the final file type is JPEG. Starting with a line drawing generated with a computer application will do much better still.