Zola: Low quality of resized images

[BojanKogoj]

Currently, all resized images are JPEG (according to docs), which doesn't really work well with computer-generated images, such as screenshots. There is also no transparency which makes it even worse.

For example, the following image

becomes (at 100% quality)

Any way to create PNGs in the future?

[Keats]

Looks like it should keep jpeg/png if possible yes.

[MatteoGgl]

I'd be willing to try working on this, but beware: I'm a Rust beginner with a day job as a PHP developer. :smiley:

I was thinking of a possible implementation in two steps: the resize_image function should keep the format of the images it's fed by default, so if I try to process a PNG it'll keep the format. As a second step, we could add an optional argument to resize_image that specifies the desired extension, like ext=jpg.

Thoughts?

[Keats]

Go for it!
What you said does make sense to me, although I don't know the support of images outside of jpeg in Rust :/

cc @vojtechkral who wrote the original code for feedback as well.

[MatteoGgl]

I'll look at the image crate and we'll see if something can be done then!

[vojtechkral]

image AFAIK does support encoding of PNGs, so that sould not be a problem. As for preserving the format, I'm not sure you'd really want to do that. For example, making a bmp thumb for a bmp image doesn't make much sense to me, you most likely want png. Perhaps the best way would be to create jpgs for lossy formats and png for lossless formats.

I think the best way would be to have a format option with possible choices "auto" (which would do the above), "png" and "jpeg", defaulting to "auto". Should not be difficult to implement...

[BojanKogoj]

I agree with @vojtechkral
Having options jpeg, png and auto (with default to auto) would be really nice.

[vojtechkral]

Btw. what's with the project rename? :) I must've missed that. Should we also replace pictures of Gutenberg with those of Émile Zola in the documentation? :)

[Keats]

Explained here: https://www.vincentprouillet.com/blog/releasing-zola-0-5-0/#a-bit-of-history-and-changing-name-to-zola

No need to change the pictures

[MatteoGgl]

@vojtechkral I like this implementation, that's in the same general direction of what I was going for, just more polished :grin:

[bdjnk]

The essential problem here is not, so far as I understand it, the encoding, but rather the resize filter. Gaussian, which is what is currently used, is producing unexpected blur. For reducing image size, which I believe is the common use case being targeted, Lanczos3 will almost always produce better results (certainly sharper). Switching should be a one line fix.

[bdjnk]

I just ran some tests, and changing the filter doesn't seem to have made a whit of difference, but I could have done it wrong.

[vojtechkral]

I created a pull request. The same thumbnail using PNG looks like this:

Transparency seems to be preserved as far as I can tell.