I am writing an application that needs to convert RGB to grayscale images. The conversion works but converting an image of 3648 * 2736 pixel takes round about 7 secs. I know that set and getpixel take some time. But I think that it shouldn't take so long if you are using Lockbits even though the image is not small. (please correct me if that is wrong). Maybe I just did a fatal mistake within my code.
public static long ConvertToGrayScaleV2(Bitmap imageColor, bool useHDTVConversion)
{
Stopwatch stpw = new Stopwatch();
stpw.Start();
System.Drawing.Imaging.BitmapData imageColorData = imageColor.LockBits(new Rectangle(new Point(0, 0), imageColor.Size),
System.Drawing.Imaging.ImageLockMode.ReadWrite, imageColor.PixelFormat);
IntPtr PtrColor = imageColorData.Scan0;
int strideColor = imageColorData.Stride;
byte[] byteImageColor = new byte[Math.Abs(strideColor) * imageColor.Height];
System.Runtime.InteropServices.Marshal.Copy(PtrColor, byteImageColor, 0, Math.Abs(strideColor) * imageColor.Height);
int bytesPerPixel = getBytesPerPixel(imageColor);
byte value;
if (bytesPerPixel == -1)
throw new Exception("Can't get bytes per pixel because it is not defined for this image format.");
for (int x = 0, position; x < imageColor.Width * imageColor.Height; x++)
{
position = x * bytesPerPixel;
if (useHDTVConversion)
{
value = (byte)(byteImageColor[position] * 0.0722 + byteImageColor[position + 1] * 0.7152 + byteImageColor[position + 2] * 0.2126);
}
else
{
value = (byte)(byteImageColor[position] * 0.114 + byteImageColor[position + 1] * 0.587 + byteImageColor[position + 2] * 0.299);
}
byteImageColor[position] = value;
byteImageColor[position+1] = value;
byteImageColor[position+2] = value;
}
System.Runtime.InteropServices.Marshal.Copy(byteImageColor, 0, PtrColor, Math.Abs(strideColor) * imageColor.Height);
imageColor.UnlockBits(imageColorData);
stpw.Stop();
return stpw.ElapsedMilliseconds;
}
public static int getBytesPerPixel(Image img)
{
switch (img.PixelFormat)
{
case System.Drawing.Imaging.PixelFormat.Format16bppArgb1555: return 2;
case System.Drawing.Imaging.PixelFormat.Format16bppGrayScale: return 2;
case System.Drawing.Imaging.PixelFormat.Format16bppRgb555: return 2;
case System.Drawing.Imaging.PixelFormat.Format16bppRgb565: return 2;
case System.Drawing.Imaging.PixelFormat.Format1bppIndexed: return 1;
case System.Drawing.Imaging.PixelFormat.Format24bppRgb: return 3;
case System.Drawing.Imaging.PixelFormat.Format32bppArgb: return 4;
case System.Drawing.Imaging.PixelFormat.Format32bppPArgb: return 4;
case System.Drawing.Imaging.PixelFormat.Format32bppRgb: return 4;
case System.Drawing.Imaging.PixelFormat.Format48bppRgb: return 6;
case System.Drawing.Imaging.PixelFormat.Format4bppIndexed: return 1;
case System.Drawing.Imaging.PixelFormat.Format64bppArgb: return 8;
case System.Drawing.Imaging.PixelFormat.Format64bppPArgb: return 8;
case System.Drawing.Imaging.PixelFormat.Format8bppIndexed: return 1;
default: return -1;
}
}
If you're converting to greyscale, try using a ColorMatrix transformation instead. from: https://web.archive.org/web/20141230145627/http://bobpowell.net/grayscale.aspx
Image img = Image.FromFile(dlg.FileName);
Bitmap bm = new Bitmap(img.Width,img.Height);
Graphics g = Graphics.FromImage(bm);
ColorMatrix cm = new ColorMatrix(new float[][]{ new float[]{0.5f,0.5f,0.5f,0,0},
new float[]{0.5f,0.5f,0.5f,0,0},
new float[]{0.5f,0.5f,0.5f,0,0},
new float[]{0,0,0,1,0,0},
new float[]{0,0,0,0,1,0},
new float[]{0,0,0,0,0,1}});
/*
//Gilles Khouzams colour corrected grayscale shear
ColorMatrix cm = new ColorMatrix(new float[][]{ new float[]{0.3f,0.3f,0.3f,0,0},
new float[]{0.59f,0.59f,0.59f,0,0},
new float[]{0.11f,0.11f,0.11f,0,0},
new float[]{0,0,0,1,0,0},
new float[]{0,0,0,0,1,0},
new float[]{0,0,0,0,0,1}});
*/
ImageAttributes ia = new ImageAttributes();
ia.SetColorMatrix(cm);
g.DrawImage(img,new Rectangle(0,0,img.Width,img.Height),0,0,img.Width,img.Height,GraphicsUnit.Pixel,ia);
g.Dispose();