Can we compress PNG files ?

Let us begin by addressing the title of this blog - PNG is supposed to be a lossless image format which can only have lossless compression, correct ?

If you came in here with this understanding you are correct, since PNG employs a lossless technique we do not have tools like chroma subsampling at our disposal to compress the pristine/source image. However PNG can be compressed in a “lossy” manner if you downsample colors per channel.

Color Quantization - This is the technique that allows us to reduce the file size of a PNG image file. Assuming the source image was created at 24 bit color depth(8 bits/pixel) we could bring this down to 65K colors or even all the way down to 256 colors. For most types of image file you would hardly notice any differences, one key reason being our vision is much more sensitive to luminance/light/brightness than color ( Read more about color in this blog - https://coderevere.com/single-blog.php?blog=colorspacemodel ).

This is the primary technique that tools like TinyPNG use to reduce the file size of a PNG file. By reducing color information using techniques like quantization technique the file sizes drop significantly in most cases. These tools not only help reduce file sizes but do it while ensuring they preserve transparency.

I used Image Magick to reduce the file size of the logo of this website, leveraging the color reduction technique mentioned above.