Problems with draw()

[kadaAS3]

hello, I'm very new to AS3.0 and I have problems with alpha. The next code is from my animation class and its main purpose is draw frame to other bitmapdata object.

// Copy the frame from original BitmapData.
			destBitmapTempData.copyPixels(bitmapData, rect, zeroPoint);
			
			// Set the newly copied BitmapData to Bitmap.
			modifiedBitmap.bitmapData = destBitmapTempData;
			
			// Change alpha
			modifiedBitmap.alpha = 0.5;
			
			// Get Matrix from the modifiedBitmap.
			matrix = modifiedBitmap.transform.matrix;
			
			// Change it if necessary
			if (flipHor)
			{
				matrix.scale( -1, 1);
				matrix.translate(width, 0);
			}
			if (flipVer)
			{
				matrix.scale( 1, -1);
				matrix.translate(0, height);
			}
			
			// Clear previous frame.
			destBitmapData.fillRect(zeroRect, 0x00000000);
			
			// Draw the new frame.
			destBitmapData.draw(modifiedBitmap, matrix);
	
			// Copy the new frame to canvas.
			canvasBD.copyPixels(destBitmapData, zeroRect, position, null, null, true);

The thing is that when I use copyPixels, the frame loses its alpha value. For example if I do new bitmap object from destBitmapData and addChild it, then the alpha remains, but not when I use copyPixels

The canvasBD is created:

canvasBD = new BitmapData(screenRect.width, screenRect.height, true, 0x00FFFFFF);

Any ideas how do I get alpha remain to that frame?