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Old 05-25-2016, 03:38 PM   #7
Tex2002ans
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Join Date: Jul 2012
Device: Kobo Forma, Nook
Quote:
Originally Posted by jcsalomon View Post
I’ve also gotten conflicting suggestions on PNG vs. GIF, and searching this forum doesn’t yield anything that looks authoritative. PNG it is for now, unless I get a good reason to change.
PNG is the superior format, and it can do everything GIF can do better (besides animation... which doesn't apply in EPUBs).
  • Color
    • GIF: Up to 256 colors.
    • PNG: Millions of colors.
      • There is also Indexed PNGs (up to 256 colors), which encompasses the entirety of GIF.
  • Transparency
    • GIF: Either on/off (and hackish).
    • PNG: Has an entire Alpha channel (8-bit transparency).
  • Compression
    • GIF: None or LZW.
    • PNG: A multitude of different types of compression algorithms can be used.
      • As an example, in 2013 Google released the Zopfli compression algorithm, which compresses ~3-8% more than the previous best algorithms.
      • In almost all cases, the equivalent PNG will be smaller than the GIF. (GIF only wins in very tiny icon-sized images because of less overhead).

Quote:
Originally Posted by jcsalomon View Post
Seems I have a partial answer to image transparency, at least. The release notes for Kindlegen version 2.5 build 0626-3a91e28 includes

[...]

So not only will the transparency not work, it will also trigger a PNG→JPEG conversion. (ETA: Experimentation shows that kindlegen actually converts to a GIF image, without transparency.)
Hmmm, that is good to hear they at least convert it to GIF. Although GIF transparency =/= PNG transparency.

When I wrote my "Tutorial: Formulas to PNG", KindleGen still converted Transparent PNG -> JPG, which was just PREPOSTEROUS:

https://www.mobileread.com/forums/sho...d.php?t=223254

I haven't really tested it since then, and have been converting all transparent formulas -> white background beforehand. (It also compresses much better, and Indexed PNGs with transparency are... it depends [see the thread for the Kindle black box bug I ran into depending on the PNG compression tool]).

Quote:
Originally Posted by jcsalomon View Post
Image color: The images are scanned gray-scale line-art. [...] I understand that gray-scale PNG is the best format for this, but should I leave it as black & gray on white, or convert to transparent?
For those types of images, you could save them as Grayscale PNGs (lossless)... or I recommend converting them to Indexed PNG (lossy). You can lower the amount of grays to <=256 (see my Tutorial above).

Bug to watch out for: Depending on which PNG compression tools you use, you should probably avoid 2-bit (4 color) and 4-bit (16 color) PNGs as some devices have display bugs. See GrannyGrump's topic here:

https://www.mobileread.com/forums/sho...d.php?t=222916

I personally just settled on 17 color Indexed PNGs. But anywhere from 17-256 Indexed colors should be safe (since this forces it to be an 8-bit PNG).

Last edited by Tex2002ans; 05-25-2016 at 07:39 PM.
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