Preserve color palette for PNG images

[aitva]

Original post on Discourse.

On my website, the PNG images resized with Hugo are bigger than the original.

For example, with an image called apercu-slack.png:

• the original is 1280x920px and weight 26ko
• the resized version is 1024x736px and weight 60ko

The original image:

The resized version:

[bep]

https://github.com/foobaz/lossypng maybe

[aitva]

I have made a repo to test the quality parameter of Golang's image/png library. I will try to add imaging and lossypng.

https://github.com/aitva/golang-png-encoding

[bep]

@aitva much appreciated. Note that imaging is only a shallow wrapper around err = png.Encode(w, img).

[aitva]

Hi, I've spent some time toying with the image library and here is what I found:

• just encoding and decoding the PNG image through go increases its size
• the PNG image I am using comes with a color palette, which greatly reduce its size
• passing the image through imaging removes its color palette, greatly increasing its size
• using the lossypng library does not help, the image quality is affected and the image size increase

The good news is that there is an easy fix! We can reuse the original color palette on the reduced image.
Thus, we go back to something very close to the original size and the image is not altered:

• original image: 1280x920px and 26297 bytes
• original image through go: 1280x920px and 31368 bytes
• resized image: 1024x736px and 47339 bytes
• resized and indexed image: 1024x736px and 23617 bytes

There are more details at https://github.com/aitva/golang-png-encoding

[bep]

Thanks for this, really appreciated.

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