c# - GDI+ Compose bitmap with transparency -

i'd rewrite method using graphics.drawimage(). no matter pixelformat of bitmaptodrawon , bitmapdrawn values (i tried combinations) should typically pixelformat.format32bpprgb , pixelformat.format32bppargb (or pixelformat.format32bpppargb), cannot obtain result.

this compose operation seems pretty obvious me, looks missing something.

private static void composebitmapwithtransparency(bitmap bitmaptodrawon, bitmap bitmapdrawn) {    debug.assert(bitmaptodrawon != null);    debug.assert(bitmaptodrawon.pixelformat == pixelformat.format32bpprgb);     debug.assert(bitmapdrawn != null);    debug.assert(bitmapdrawn.pixelformat == pixelformat.format32bppargb);     var width = bitmaptodrawon.width;    debug.assert(bitmapdrawn.width == width);     var height = bitmaptodrawon.height;    debug.assert(bitmapdrawn.height == height);     var rect = new rectangle(0, 0, width, height);    var bitmaptodrawondata = bitmaptodrawon.lockbits(          rect,          imagelockmode.readwrite,          bitmaptodrawon.pixelformat);    var bitmapdrawndata = bitmapdrawn.lockbits(          rect,          imagelockmode.readonly,          bitmapdrawn.pixelformat);     unsafe {       var pbitmaptodrawon = (structpixel*)bitmaptodrawondata.scan0;       var pbitmapdrawn = (structpixel*)bitmapdrawndata.scan0;        (int y = 0; y < height; y++) {          (int x = 0; x < width; x++) {             byte r = pbitmaptodrawon->r;             byte g = pbitmaptodrawon->g;             byte b = pbitmaptodrawon->b;              byte rx = pbitmapdrawn->r;             byte gx = pbitmapdrawn->g;             byte bx = pbitmapdrawn->b;              // transparency of bitmapdrawn taken account.             byte alphax = pbitmapdrawn->alpha;             byte alphaxcomplement = (byte)(255 - alphax);              pbitmaptodrawon->r = (byte)((r * alphaxcomplement + rx * alphax) >> 8);             pbitmaptodrawon->g = (byte)((g * alphaxcomplement + gx * alphax) >> 8);             pbitmaptodrawon->b = (byte)((b * alphaxcomplement + bx * alphax) >> 8);              pbitmaptodrawon++;             pbitmapdrawn++;          }       }    }     bitmapdrawn.unlockbits(bitmapdrawndata);    bitmaptodrawon.unlockbits(bitmaptodrawondata); } 

the code structpixel:

   [structlayout(layoutkind.explicit)]    public struct structpixel {       [fieldoffset(0)]       public byte b;       [fieldoffset(1)]       public byte g;       [fieldoffset(2)]       public byte r;       [fieldoffset(3)]       public byte alpha;    }