How to copy Bitmap pixels to other Bitmap preservi

Question

Could some rewrite the following function to use any optimized mechanism? I'm pretty sure that this is not the way to proceed, copying pixel by pixel.

I have read about AlphaBlend, or BitBlt, but I'm not used to native code.

public static Bitmap GetAlphaBitmap(Bitmap srcBitmap)
{
    Bitmap result = new Bitmap(srcBitmap.Width, srcBitmap.Height, PixelFormat.Format32bppArgb);

    Rectangle bmpBounds = new Rectangle(0, 0, srcBitmap.Width, srcBitmap.Height);

    BitmapData srcData = srcBitmap.LockBits(bmpBounds, ImageLockMode.ReadOnly, srcBitmap.PixelFormat);

    try
    {
        for (int y = 0; y <= srcData.Height - 1; y++)
        {
            for (int x = 0; x <= srcData.Width - 1; x++)
            {
                Color pixelColor = Color.FromArgb(
                    Marshal.ReadInt32(srcData.Scan0, (srcData.Stride * y) + (4 * x)));

                result.SetPixel(x, y, pixelColor);
            }
        }
    }
    finally
    {
        srcBitmap.UnlockBits(srcData);
    }

    return result;
}

IMPORTANT NOTE: The source image has a wrong pixel format (Format32bppRgb), so I need to adjust the alpha channel. This is the only mechanism that works for me.

The reason why the src image has a wrong pixel format is explained here.

I tried the following options without luck:

• Creating a new image and draw the src image using the Graphics.DrawImage from src. Did not preserve the alpha.
• Creating a new image using the Scan0 form src. Works fine, but has a problem when the GC dispose the src image (explained in this other post);

This solution is the only that really works, but I know that is not optimal. I need to know how to do it using the WinAPI or other optimal mechanism.

Thank you very much!

Answer 1

Assuming the source image does infact have 32 bits per pixel, this should be a fast enough implementation using unsafe code and pointers. The same can be achieved using marshalling, though at a performance loss of around 10%-20% if I remember correctly.

Using native methods will most likely be faster but this should already be orders of magnitude faster than SetPixel.

public unsafe static Bitmap Clone32BPPBitmap(Bitmap srcBitmap)
{
    Bitmap result = new Bitmap(srcBitmap.Width, srcBitmap.Height, PixelFormat.Format32bppArgb);

    Rectangle bmpBounds = new Rectangle(0, 0, srcBitmap.Width, srcBitmap.Height);
    BitmapData srcData = srcBitmap.LockBits(bmpBounds, ImageLockMode.ReadOnly, srcBitmap.PixelFormat);
    BitmapData resData = result.LockBits(bmpBounds, ImageLockMode.WriteOnly, result.PixelFormat);

     int* srcScan0 = (int*)srcData.Scan0;
     int* resScan0 = (int*)resData.Scan0;
     int numPixels = srcData.Stride / 4 * srcData.Height;
     try
     {
         for (int p = 0; p < numPixels; p++)
         {
             resScan0[p] = srcScan0[p];
         }
     }
     finally
     {
         srcBitmap.UnlockBits(srcData);
         result.UnlockBits(resData);
     }

    return result;
}

Here is the safe version of this method using marshalling:

public static Bitmap Copy32BPPBitmapSafe(Bitmap srcBitmap)
{
    Bitmap result = new Bitmap(srcBitmap.Width, srcBitmap.Height, PixelFormat.Format32bppArgb);

    Rectangle bmpBounds = new Rectangle(0, 0, srcBitmap.Width, srcBitmap.Height);
    BitmapData srcData = srcBitmap.LockBits(bmpBounds, ImageLockMode.ReadOnly, srcBitmap.PixelFormat);
    BitmapData resData = result.LockBits(bmpBounds, ImageLockMode.WriteOnly, result.PixelFormat);

    Int64 srcScan0 = srcData.Scan0.ToInt64();
    Int64 resScan0 = resData.Scan0.ToInt64();
    int srcStride = srcData.Stride;
    int resStride = resData.Stride;
    int rowLength = Math.Abs(srcData.Stride);
    try
    {
        byte[] buffer = new byte[rowLength];
        for (int y = 0; y < srcData.Height; y++)
        {
            Marshal.Copy(new IntPtr(srcScan0 + y * srcStride), buffer, 0, rowLength);
            Marshal.Copy(buffer, 0, new IntPtr(resScan0 + y * resStride), rowLength);
        }
    }
    finally
    {
        srcBitmap.UnlockBits(srcData);
        result.UnlockBits(resData);
    }

    return result;
}

Edit: Your source image has a negative stride, which means the scanlines are stored upside-down in memory (only on the y axis, rows still go from left to right). This effectively means that .Scan0 returns the first pixel of the last row of the bitmap.

As such I modified the code to copy one row at a time.

notice: I've only modified the safe code. The unsafe code still assumes positive strides for both images!

Answer 2

Try the Bitmap Clone method.

Answer 3

A utility class in my Codeblocks library http://codeblocks.codeplex.com allows you to transform a source image to any other image using LINQ.

See this sample here: http://codeblocks.codeplex.com/wikipage?title=Linq%20Image%20Processing%20sample&referringTitle=Home

While the sample transforms the same image format between source and destination, you could change things around, as well.

Note that I have clocked this code and it is much faster than even unsafe code for large images because it uses cached full-row read ahead.