Maintain Alpha when compressing PNG

[dazuaz]

Hey, great library!, is there an option to maintain alpha channel when converting PNG to webp, also, when I try to compress PNG, I only get a very small silluete of the image

[colbyn]

Oh great question, and cool seeing someone taking an interest in my work! 🙂

PNG Issue

Regarding,

when I try to compress PNG, I only get a very small silluete of the image

can you upload an example of the source image, the resulting image, and the CLI command used?

Also the PNG optimization logic isn't at the same quality as the implementations for lossy codecs such as JPEG and WebP.

Supporting Transparency

Regarding,

is there an option to maintain alpha channel when converting PNG to webp

This is a no.

At the time, my concern was compressing media using any means possible, and alpha channels generally introduce significant overhead in this regard (well depending on the given codec). Also my focus was lossy codecs and only newer formats support transparency.

Though I can see why this may be necessary.

But anyway, the short answer is no.

The long answer, for those interested: the Imager frontend decodes a multitude of image formats into an intermediate (lowest common denominator) pixel format, i.e. YUV420P pixel buffers, which all subsequent components accept as input. VMAF for instance, only supports YUV420P and a few others. So supporting transparency will involve some changes to this intermediate format, which can be done... I'd imagine it wouldn't be too hard.

PS

• Honestly, I'd love to move away from using YUV420P.

• Regarding PNG support. The current implementation is based on non-GPL code... Part of me wanted to see Imager as the FFmpeg of automated media compression. So therefore, perhaps, I'd like to add optional support for pngquant... But this may complicate things from a licensing perspective (e.g. I generally try to avoid runtime dependencies since it can be frustrating for the end-user).