Always optimize favicon after resizing

[benoit74]

Currently, when favicon used as illustration is not proper size, we resize it. In fact this does not reduce the file size. We should call optimize_png (available in scraperlib) after the resize operation so that image is both resized and optimized

[benoit74]

We should definitely use new utility introduced in openzim/python-scraperlib#260

[aaryansinhaa]

Hey @benoit74 I want to take this issue.
Implementation seems to be straight forward.

[benoit74]

Please do

[aaryansinhaa]

@benoit74 why should we only optimize when resizing? if an icon is of correct dimensions but large in size, I believe it would be better to optimize at all paths. I saw the implementation of optimize_png, and it seems to be pixel preserving.

[benoit74]

re-optimizing an already optimized image could produce artifacts from my experience ; I'm assuming images which are not modified don't need re-optimizing because they should already be optimized. Do you have a different experience?

[aaryansinhaa]

but I believe that convert_svg2png and convert_image do not optimize the image, they only convert the image to png.

[benoit74]

Oh, yes, when converting we should indeed optimize. I was speaking about the case where favicon is already a PNG. In such a case I don't think we should optimize again