how to get better shrunk images

[Michael Katz]

I'm drawing on a raster surface. I want to take the image on a surface and shrink it by a factor. I have code that looks like this:

SkImageInfo info = SkImageInfo::MakeN32( width, height, kUnpremul_SkAlphaType );
_surface = SkSurface::MakeRaster( info );

_canvas = _surface->getCanvas();

(draw some stuff on _canvas)

sk_sp<SkImage> image = _surface->makeImageSnapshot();

SkImageInfo info2 = SkImageInfo::MakeN32( width / 2, height / 2, kUnpremul_SkAlphaType );

_surface2 = SkSurface::MakeRaster( info2 );

_canvas2 = _surface2->getCanvas();

_canvas2->scale( 0.5, 0.5 );

_canvas2->drawImage( image, 0, 0 );

What I get out looks like the left column below. The right column shows what I expect. Is there a way to get that better quality out of Skia? Am I doing the whole process wrong? I played around with SkSamplingOptions to use a SkCubicResampler, but I think that's just for upsizing, not downsizing?

[craste...@gmail.com]

Have you looked into SkSamplingOptions ?

[Michael Ludwig]

By default drawImage just uses nearest neighbor sampling, which will not handle this sort of progressive downsizing. You should use SkSamplingOptions{SkFilterMode::kLinear} in your drawImage call and it should be very similar to what's produced by paint.NET

[Michael Katz]

Thanks. Perhaps that could be called out more clearly in the documentation as the way to get the best quality shrinking: https://api.skia.org/classSkCanvas.html#ab37570ba2c305284a79e88ad57a86313. From what I can see, that documentation doesn't even mention the FilterMode option with the drawImage() function. So I was trying every variant I could with the SamplingOptions and nothing was helping.

[Michael Katz]

Thanks. Perhaps that could be called out better in the documentation as the way to do the highest quality shrinking. Here: https://api.skia.org/classSkCanvas.html. Or here https://api.skia.org/SkSamplingOptions_8h.html#a12f307632434a5814d5c6ad50f1c1551.

On Tuesday, June 27, 2023 at 7:08:23 AM UTC-7 michae...@google.com wrote: