png images bit depth is modified in resized images

[wpfed]

Hi,

When I upload a 8 bit png image graphic, WordPress resizes as usual but modifies the bit depth to 24 or 32 bit, depending if their is a alpha channel. Without Alpha channel it is 24bit, with it's 32bit.

The problem with that is that the file sizes end up being multiple times bigger than the original image.

I opened a ticket with ImageMagick ​https://github.com/ImageMagick/ImageMagick/discussions/3825

Some of the replies included these solutions:

Setting bit-depth to 8 in the way done in the script may not be adequate for PNG. I think the only way to ensure that is to preface the output name with PNG8:filename.png. But I am not sure if it will preserve you transparency if the input has transparency. But the best thing to resolve that would be simply to try it.

Scaling the image, depending upon the scaling method, most likely will add more colors than the 256 limit for 8-bit color. So the image has to be forced to quantize colors to the 256 limit in order to save as 8-bits. Otherwise, it is saved as 24 or 32-bits. So specifying PNG8:filename.png will save as 8-bits.

PNG00: Inherit PNG color and bit depth from input image(​https://legacy.imagemagick.org/Usage/formats/#png_formats)

Also here are related tickets I was able to find:

https://core.trac.wordpress.org/ticket/36477
https://core.trac.wordpress.org/ticket/30402
https://core.trac.wordpress.org/ticket/48736

Thanks!

[wpfed]

Update from the ImageMagik pros:

I found that if you, say, blur the file and save with PNG8:filename.png, the 8-bit alpha format is preserved with only a small increase in file size. ImageMagick cannot achieve the same level of optimization as specialized programs such as you used. So the file size went from 105K to 112K. So have WordPress change the code so that when 8-bits is desired, add the PNG8: prefix to the filename.