PNG quality parameter has no effect

[xiaoxuwei12345]

The quality parameter doesn't affect PNG output. When converting the same HEIC file with different quality values (0.3, 0.5, 0.7), the resulting PNG files have identical file sizes.

Test result:

• quality: 0.3 → size: 14472794 bytes
• quality: 0.7 → size: 14472794 bytes (same)

This appears to be because canvas.toBlob() ignores the quality parameter for PNG format (as per MDN docs). The documentation shows PNG supports quality parameter, but it doesn't actually work.

Could you please clarify this in the documentation or provide an alternative way to control PNG file size?

Thank you very much!

[hoppergee]

Sorry, I missed this. I'll take a look in 2 days.

[hoppergee]

A Number between 0 and 1 indicating the image quality to be used when creating images using file formats that support lossy compression (such as image/jpeg or image/webp). A user agent will use its default quality value if this option is not specified, or if the number is outside the allowed range.
https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob#quality

I think this is the reason. The quality only works for lossy image, however PNG is lossless.

Could you please clarify this in the documentation or provide an alternative way to control PNG file size?

Maybe convert it to JPEG is a way to go. Or maybe we can add an extra parameter scale, which you can use it to adjust png by reduct dimensions. How do you think? @xiaoxuwei12345