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Convert 24Bit to 32Bit with alpha in C#

Posted on 2009-05-08
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Last Modified: 2012-05-06
The Code I wrote below should work. But it will not convert a 24 bit to 32 bit image. It should but apparently Clone doesn't work that way.

Anyone have any alternate solutions?
public static Bitmap SetAlpha(Bitmap Target, int alpha)
        {
 
            /*if (Target.PixelFormat == PixelFormat.Format24bppRgb)
            {
 
                Bitmap newBitmap = new Bitmap(Target.Width, Target.Height, PixelFormat.Format32bppArgb);
 
                BitmapData returnData1 = Target.LockBits(new Rectangle(0, 0, Target.Width, Target.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, Target.PixelFormat);
                BitmapData returnData2 = newBitmap.LockBits(new Rectangle(0, 0, newBitmap.Width, newBitmap.Height), System.Drawing.Imaging.ImageLockMode.WriteOnly, newBitmap.PixelFormat);
 
 
 
            }*/
 
            //Clone the Target to return Bitmap, this will change to pixel format correctly
            Bitmap returnBitmap = Target.Clone(new Rectangle(0, 0, Target.Width, Target.Height), PixelFormat.Format32bppArgb);
 
 
 
            //New fast Code using Lock bits to get and set alpha channel. Don't use Get and Set Pixel it's 10x slower
            BitmapData returnData = returnBitmap.LockBits(new Rectangle(0, 0, returnBitmap.Width, returnBitmap.Height), System.Drawing.Imaging.ImageLockMode.ReadWrite, returnBitmap.PixelFormat);
 
            int PixelSize = 4;
 
            unsafe
            {
 
                for (int y = 0; y < returnData.Height; y++)
                {
                    //Gets the pixelRow for Target Bitmap
                    byte* row = (byte*)returnData.Scan0 + (y * returnData.Stride);
 
                    for (int x = 0; x < returnData.Width; x++)
                    {
                        //Set alpha pixel byte to the same as the mask
 
                        byte oldAlpha = row[x * PixelSize + 3];
                        byte B = row[x * PixelSize + 2];
                        byte G = row[x * PixelSize + 1];
                        byte R = row[x * PixelSize + 0];
                        byte setAlpha = Convert.ToByte(alpha);
 
 
                        //Check for Null pixel
                        if ((R == 0) && (G == 0) && (B == 0) && (oldAlpha < 10)) { R = 254; B = 254; G = 254; setAlpha = oldAlpha; }
                        
 
                        //if (returnBitmap.Height == 290)
                        //{ B = 254; G = 254; R = 254; }
                        //if (alpha > oldAlpha)
                        row[x * PixelSize + 0] = R;
                        row[x * PixelSize + 1] = G;
                        row[x * PixelSize + 2] = B;
                        row[x * PixelSize + 3] = setAlpha;
                    }
 
                }
            }
 
 
            //Restore Bitmap Data Directly back to the bitmap.
            returnBitmap.UnlockBits(returnData);
            
 
            //Dispose of input bitmaps
            Target.Dispose();
 
            return returnBitmap;
        }

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Question by:RDdice
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Accepted Solution

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anyoneis earned 500 total points
ID: 24350255
I don't know why the clone doesn't work as you are using it. Try this tweak:

            //Clone the Target to return Bitmap, this will change to pixel format correctly
            Bitmap returnBitmap = new Bitmap(Target);
            returnBitmap = returnBitmap.Clone(new Rectangle(0, 0, Target.Width, Target.Height), PixelFormat.Format32bppArgb);

dwr
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LVL 11

Expert Comment

by:anyoneis
ID: 24350272
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LVL 7

Author Comment

by:RDdice
ID: 24350286
Ya doesn't work. Passing a 24 bit bitmap to it, it doesn't always return a 32 bit for some reason and i'm not sure why. Could be something to do with the bitmaps I'm using.

Bitmap.MakeTransparent Does return a 32 bit in every instance, but of course makes 1 color transparent.
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LVL 11

Expert Comment

by:anyoneis
ID: 24350422
I can't get it to fail, but I am not trying a bunch of different images. Sorry that didn't help (but it was fun playing with it :-)

Are you saying that a given image does not always convert, or that some images never convert with this code?
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LVL 7

Author Closing Comment

by:RDdice
ID: 31579630
Clone works, but not 100% of the time, seems like an internal bug defendant on the target image.
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