private void CalculateLuminance(SKBitmap src)
{
if (src == null)
{
throw new ArgumentNullException("src");
}
var imageInfo = src.Info;
var width = imageInfo.Width;
var height = imageInfo.Height;
var colorType = imageInfo.ColorType;
if (colorType == SKColorType.Index8)
{
// use the color table for indexed images
for (int row = 0; row < height; row++)
{
var index = width * row;
for (int col = 0; col < width; col++, index++)
{
var pixel = src.GetIndex8Color(col, row);
SetLuminance(index, pixel.Red, pixel.Green, pixel.Blue, pixel.Alpha);
}
}
}
else if (colorType == SKColorType.Rgba8888 ||
colorType == SKColorType.RgbaF16 ||
colorType == SKColorType.Bgra8888)
{
// Read pixels from unmanaged memory to set luminance
// https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/graphics/skiasharp/bitmaps/pixel-bits
IntPtr pixelsAddr = src.GetPixels();
unsafe
{
uint *ptr = (uint *)pixelsAddr.ToPointer();
for (int row = 0; row < height; row++)
{
var index = width * row;
for (int col = 0; col < width; col++, index++)
{
uint colorVal = *ptr++;
var alpha = (byte)((colorVal >> 24) & 0xff);
var red = (byte)((colorVal >> 16) & 0xff);
var green = (byte)((colorVal >> 8) & 0xff);
var blue = (byte)((colorVal) & 0xff);
SetLuminance(index, red, green, blue, alpha);
}
}
}
}
else
{
// unknown type or other color types, use the 'old' way allocating more managed memory
// can potentially migrate this to the above case, but leaving here for safety
var pixels = src.Pixels;
var length = pixels.Length;
for (var index = 0; index < length; index++)
{
var pixel = pixels[index];
SetLuminance(index, pixel.Red, pixel.Green, pixel.Blue, pixel.Alpha);
}
}
}
