Enhancement: Investigate control over output PNG IDAT chunk size, might reduce file size of smaller palette-based images

[guoyunhe]

Question about an existing feature

What are you trying to achieve?

I have some PNG images. I want to use sharp to compress them and reduce file sizes.

I tried a few options. However, the output size (13.3KB) is always bigger than input size (11.4KB).

When you searched for similar issues, what did you find that might be related?

Please provide a minimal, standalone code sample, without other dependencies, that demonstrates this question

https://github.com/retrobrainz/image-compress-test

Please provide sample image(s) that help explain this question

See the repo above

[lovell]

It looks like input.png is a palette-based PNG with a meaning-of-lifesque 42 colours:

$ pngchunks input.png | grep PLTE
Chunk: Data Length 126 (max 2147483647), Type 1163152464 [PLTE]

(3 channels * 42 colours = 126 bytes)

You can reduce the number of colours in the palette with colours, for example:

  .png({
    palette: true
    effort: 10,
+   colours: 64,
  })

[guoyunhe]

I added colors: 64 and the result:

1. output.png (12.8KB) is still bigger than input.png (11.4KB)
2. Image quality is downgraded. Some colors are visually different.

https://github.com/retrobrainz/image-compress-test

[lovell]

It looks like the input image was optimised using a tool such as pngcrush. Even with compressionLevel set to its highest value I see the following difference:

$ pngchunks input.png | grep IDAT
Chunk: Data Length 11482 (max 2147483647), Type 1413563465 [IDAT]

$ pngchunks output.png | grep IDAT
Chunk: Data Length 8192 (max 2147483647), Type 1413563465 [IDAT]
Chunk: Data Length 4911 (max 2147483647), Type 1413563465 [IDAT]

We previously investigated setting the max IDAT/buffer size in libvips but found the savings were not very useful, however this specific image might be the case that proves otherwise - see libvips/libvips#1357

[guoyunhe]

This image (screenshot) was generated by https://github.com/libretro/retroarch (Hope it is helpful)

[lovell]

Thanks, it appears to write a single IDAT chunk (similar to pngcrush).

https://github.com/libretro/RetroArch/blob/bd15d7c02d2614ae878cd2947ba0664269cf2ee0/libretro-common/formats/png/rpng_encode.c#L339-L341

I'll mark this as a future possible enhancement for now.