Getting pixel data from `ID2D1Bitmap1.Map()` are not correct

[d2phap]

Hi @smourier

I create this method to get the pixel color:

private IComObject<ID2D1Bitmap1>? _imageD2D;
private IComObject<ID2D1DeviceContext6>? _device;

public byte[] GetColor(int x, int y)
{
    var bmpProps = new D2D1_BITMAP_PROPERTIES1()
    {
        bitmapOptions = D2D1_BITMAP_OPTIONS.D2D1_BITMAP_OPTIONS_CANNOT_DRAW | D2D1_BITMAP_OPTIONS.D2D1_BITMAP_OPTIONS_CPU_READ,
        pixelFormat = new D2D1_PIXEL_FORMAT()
        {
            alphaMode = D2D1_ALPHA_MODE.D2D1_ALPHA_MODE_PREMULTIPLIED,
            format = DXGI_FORMAT.DXGI_FORMAT_B8G8R8A8_UNORM,
        },
        dpiX = 96.0f,
        dpiY = 96.0f,
    };

    var size = _imageD2D.GetSize().ToD2D_SIZE_U();
    using var bitmap1 = _device.CreateBitmap<ID2D1Bitmap1>(size, bmpProps);
    bitmap1.CopyFromBitmap(_imageD2D);

    var map = bitmap1.Map(D2D1_MAP_OPTIONS.D2D1_MAP_OPTIONS_READ);
    var startIndex = ((y * size.width) + x) * 4;
    var dataSize = (int)(size.width * size.height * 4);

    var bytes = new byte[dataSize];
    Marshal.Copy(map.bits, bytes, 0, dataSize);
    bitmap1.Unmap();

    var b = bytes[startIndex];
    var g = bytes[startIndex + 1];
    var r = bytes[startIndex + 2];
    var a = bytes[startIndex + 3];

    var color = new byte[] { r, g, b, a };

    return color;
}

Problems

When I debug it, with this image (4 x 4 pixels), there are 2 problems:

1. The bytes only contains the first row of the pixel data, the rest are just 0.
2. The pixel data with alpha (x=2, y=0) are not correct:
   • Expectation: [b=47, g=196, r=227, a=92]
   • Actual value: [b=17, g=71, r=82, a=92]

Debug data

Could you please help? Thank you!

[smourier]

Hi,

DirectN is only a bunch of .NET declarations that directly map to underlying techs, it has very little intelligence on its own, so it seems it's more a D2D issue (if it's an issue, are you sure it's not expected?)

If you have a full reproducing project, and I find some time, I can investigate.

[d2phap]

Thanks @smourier for the reply. I realized I am using WicNet instead of DirectN! So I created a ticket here: https://github.com/smourier/WicNet/issues/4

• The problem about alpha pixel color values is fixed by dividing to alpha/255f because the alpha mode is D2D1_ALPHA_MODE_PREMULTIPLIED.
• The 1st problem still persists, you can find the full project in https://github.com/smourier/WicNet/issues/4