Image resize/scale - bitmap vs. image (findings)

[AndersMad]

@mattleibow asks "What is the recommended way to resize bitmaps" here https://groups.google.com/d/msg/skia-discuss/r-2svp3dK_8/2MRh6HKyEQAJ

The response is to use Image and not Bitmap as Bitmap is on a route to obsolete. Thats a big FYI i spose. But when i tried using image vs. bitmap:

Bitmap: Using e.g. bmp.Resize(bmpDest, SKBitmapResizeMethod.Lanczos3) the result is very nice and beats System.Drawing in quality and size (and twice as fast).

Image: BUT when resizing an image using surface/canvas/paint and even with paint.IsAntialias = false and paint.FilterQuality = SKFilterQuality.High set - the result is pretty bad in blurriness - its smudgy: canvas.DrawImage(img, r1, r2, paint);

This might likely be a root Skia problem (or me overlooking something?) - but when considering that Chrome uses Skia for its canvas elements - the forum responses here is to make sure that anti-alias is turned off - so maybe there is a missing property for this? Or maybe its just the Skia lack of being able to set the resize method using image draw.

Anyone else bumped into this?

[mattleibow]

@AndersMad I have not tried this, but I think this is the way to control it:

SKPaint paint = ...;
paint.FilterQuality = SKFilterQuality.High;

Gets or sets the filter quality of the current paint.
This affects the quality (and performance) of drawing scaled images.
https://developer.xamarin.com/api/property/SkiaSharp.SKPaint.FilterQuality/

Let me know what happens - and you can try the other values and see if that is faster/better.

[AndersMad]

Sorry, forgot that in my comment! Yes I had and no - it does not help: If set to low quality it is just terrible and high is blurred..

It's probably in the root Skia so i will post other place - but now people are aware: If doing scaling don't use SKImage - "keep on" using SKBitmap.

For others coming from System.Drawing: Using SKBitmapResizeMethod.Lanczos3 seems to be almost the same as InterpolationMode.HighQualityBicubic etc. - except the end result (Skia that is) gives a much smaller file size with twice the speed - even though tiny details actually looks better (tested with jpeg q@75).. So thanks for this excellent .NET wrapper! :)

[mattleibow]

Closing this as it is not an issue. Thanks for posting your findings, it is always appreciated.

[dsyno]

There is definitely something wrong with SKImage. As @AndersMad discovered, SKImage resizes very blurry. Whereas SKBitmap resizes fine.

There are several posts online stating that SKBitmap is being done away with/becoming obsolete, and to use SKImage instead. However, this issue is a showstopper. @mattleibow

[mattleibow]

@dsyno Could you open a new issue with images showing the difference? And maybe the code you used to get there?

I will investigate and see what Google has to say if there is a problem.

[dsyno]

Thanks @mattleibow. New issue created #520

[jdelrue]

This still seems to be a problem, with images far worse than pictures.