[QUESTION] SKBitmap.InstallPixels and general pixel memory ownership guidelines?

[ziriax]

I'm having trouble to understand the ownership of pixel memory.

For example, the following code always saves a correct image A, and mostly saves a corrupt image B, or crashes randomly (at least in 1.68.3):

using System.IO;
using SkiaSharp;

namespace SkiaCropBug
{
	class Program
	{
		static SKBitmap Crop(int i)
		{
			using var codec = SKCodec.Create("kitten.jpg");
			using var bm1 = SKBitmap.Decode(codec);
			using var pm1 = bm1.PeekPixels();
			using var pm2 = pm1.ExtractSubset(SKRectI.Create(70, 20, 50, 45));

			var bm2 = new SKBitmap(pm2.Info, SKBitmapAllocFlags.None);
			bm2.InstallPixels(pm2);

			using var stream = File.Create($"frames/crop_{i:02}_A.png");
			bm2.Encode(stream, SKEncodedImageFormat.Png, 9);

			return bm2;
		}

		static void Main(string[] args)
		{
			Directory.CreateDirectory("frames");

			for (int i = 0; i < 100; ++i)
			{
				using var bm2 = Crop(i);
				using var stream = File.Create($"frames/crop_{i:02}_B.png");
				bm2.Encode(stream, SKEncodedImageFormat.Png, 9);
			}
		}
	}
}

Moving the bm1 out of the loop fixes the problem, but now I would have to pass two bitmaps around, while surely it should be possible for bm1 and bm2 to share the same underlying SkPixelRef?

As a temporary workaround, I'm going to copy the pixels, but I would like to know the general rule here.

From the code above, it feels natural that ownership is managed for you, since I'm not passing any IntPtr around. However, looking the native code, SKPixmap doesn't store a SKPixelRef, only a pointer...

So the managed bit is just an illusion, even though I'm coding C# here, I still have to think carefully about C/C++ ownership?

As far as I understand, the rules are:

• SKPixmap never frees memory, it is purely a (non reference counted!) view on the raw pixels, using a pointer
• SKBitmap and SKImage internally use an SKPixelRef (reference counted), but this reference counting is lost as soon as a pixmap is involved...
• So basically, one should never create a bitmap or image from a pixmap, unless this creation copies the pixels over.

Is this correct?

[ziriax]

Mmm, it seems in this case, I could have used the SKBitmap.ExtractSubSet method, that does share the underlying native SkPixelRef...