Why was the Lanczos filter removed?

[Stefan O.]

If I downscale an image using lanczos filter in Affinity Photo it has great quality compared to the linear filter.

There was once a Lanczos filter option in SKIA:

• https://learn.microsoft.com/en-us/dotnet/api/skiasharp.skbitmapresizemethod?view=skiasharp-2.88
• https://github.com/mono/SkiaSharp/issues/520

Why was it removed? Will it ever come back?

I find downscaled images using SKIA not looking very good.

The mitchell & catmull_rom filters are no replacement.
They look very blocky, because they don't seem to support midmaps.
(Maybe that's an skiko issue, not sure about that.)

I had those blocks on linear to until I figured out that I need to specify a MipmapMode.

Using skiko, a Kotlin SKIA wrapper, my code looks like this:

val bitmap = Bitmap()
bitmap.allocN32Pixels(thumbnailSize.width, thumbnailSize.height)
this.scalePixels(bitmap.peekPixels()!!, FilterMipmap(FilterMode.LINEAR, MipmapMode.LINEAR), false)
val downscaledImage = Image.makeFromBitmap(bitmap)

How are the plans for the Lanczos filter?

[Greg Daniel]

I can't really answer the Lanczos question, but as to your question about mipmaps they should give you good quality on downsampling. You can look at this example fiddle here, https://fiddle.skia.org/c/75079f18906f12241fe9a4760bd11254. So my guess is if you are not getting the expected quality than there may be a bug in your wrapper of Skia, i.e. skiko. Though if you are able to reproduce the issue using raw Skia (or ideally in fiddle.skia.org) we'd be very interested in investigating it more.

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[Stefan O.]

Yes, for LINEAR and NEAREST they work.

But I can’t specify them on cubic sampling (Mitchell and Catmull Rom), so the output looks blocky.

I can do the multiple resizes by 50% myself, but that’s slower due do the overhead.

[Stefan O.]

Okay, I opened up an issue.

I did not find another issue asking for Lanczos, so I might be the first one who cares. 😅

https://issues.skia.org/issues/331406965

[John Stiles]

I believe the references to Lanczos you are finding originate from here:

https://source.chromium.org/chromium/chromium/src/+/main:skia/ext/image_operations.h;l=74-76;drc=098756533733ea50b2dcb1c40d9a9e18d49febbe

This is Chrome image code that is layered on top of Skia, not part of Skia proper.

It doesn't surprise me that SkiaSharp absorbed that code as well since it's certainly useful.

You might be able to convince Skiko to adopt it as well, but I don't think we have any plans to migrate it from Chrome into Skia at this time.

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/fe650a5e-9888-45da-a599-73f814bcc0c5n%40googlegroups.com.

[Stefan O.]

Thank you! That actually explains a lot to me! :)