/// <summary>
/// Multiplies alpha by an amount.
/// </summary>
/// <param name="img">
/// The affected image.
/// </param>
/// <param name="alpha">A value from 0 to 1 to multiply with.</param>
public static unsafe void ColorImage(Bitmap img, float alpha)
{
BitmapData bmpData = img.LockBits(
new Rectangle(0, 0,
img.Width,
img.Height),
ImageLockMode.ReadOnly,
img.PixelFormat);
byte *row = (byte *)bmpData.Scan0;
for (int y = 0; y < img.Height; y++)
{
for (int x = 0; x < img.Width; x++)
{
int ptr = y * bmpData.Stride + x * 4;
ColorBgra unmultipliedPixel = ColorBgra.FromBgra(row[ptr], row[ptr + 1], row[ptr + 2], row[ptr + 3]).ConvertFromPremultipliedAlpha();
unmultipliedPixel.A = (byte)(unmultipliedPixel.A * alpha);
ColorBgra premultiplied = unmultipliedPixel.ConvertToPremultipliedAlpha();
row[ptr + 3] = premultiplied.A;
row[ptr + 2] = premultiplied.R;
row[ptr + 1] = premultiplied.G;
row[ptr] = premultiplied.B;
}
}
img.UnlockBits(bmpData);
}
/// <summary>
/// Overwrites RGB channels and multiplies alpha.
/// </summary>
/// <param name="img">
/// The affected image.
/// </param>
/// <param name="color">The color to overwrite the image with.</param>
/// <param name="alpha">A value from 0 to 1 to multiply with.</param>
public static unsafe void ColorImage(Bitmap img, Color?col, float alpha)
{
BitmapData bmpData = img.LockBits(
new Rectangle(0, 0,
img.Width,
img.Height),
ImageLockMode.ReadOnly,
img.PixelFormat);
byte * row = (byte *)bmpData.Scan0;
Color color = col ?? default;
ColorBgra pixel;
for (int y = 0; y < img.Height; y++)
{
for (int x = 0; x < img.Width; x++)
{
int ptr = y * bmpData.Stride + x * 4;
if (col != null)
{
pixel = ColorBgra.FromBgra(color.B, color.G, color.R, (byte)(row[ptr + 3] * alpha)).ConvertToPremultipliedAlpha();
}
else
{
ColorBgra unmultipliedPixel = ColorBgra.FromBgra(row[ptr], row[ptr + 1], row[ptr + 2], row[ptr + 3]).ConvertFromPremultipliedAlpha();
unmultipliedPixel.A = (byte)(unmultipliedPixel.A * alpha);
pixel = unmultipliedPixel.ConvertToPremultipliedAlpha();
}
row[ptr + 3] = pixel.A;
row[ptr + 2] = pixel.R;
row[ptr + 1] = pixel.G;
row[ptr] = pixel.B;
}
}
img.UnlockBits(bmpData);
}