Question

I have a method which needs to be as fast as it possibly can, it uses unsafe memory pointers and its my first foray into this type of coding so I know it can probably be faster.

    /// <summary>
    /// Copies bitmapdata from one bitmap to another at a specified point on the output bitmapdata
    /// </summary>
    /// <param name="sourcebtmpdata">The sourcebitmap must be smaller that the destbitmap</param>
    /// <param name="destbtmpdata"></param>
    /// <param name="point">The point on the destination bitmap to draw at</param>
    private static unsafe void CopyBitmapToDest(BitmapData sourcebtmpdata, BitmapData destbtmpdata, Point point)
    {
        // calculate total number of rows to draw.
        var totalRow = Math.Min(
            destbtmpdata.Height - point.Y,
            sourcebtmpdata.Height);


        //loop through each row on the source bitmap and get mem pointers
        //to the source bitmap and dest bitmap
        for (int i = 0; i < totalRow; i++)
        {
            int destRow = point.Y + i;

            //get the pointer to the start of the current pixel "row" on the output image
            byte* destRowPtr = (byte*)destbtmpdata.Scan0 + (destRow * destbtmpdata.Stride);
            //get the pointer to the start of the FIRST pixel row on the source image
            byte* srcRowPtr = (byte*)sourcebtmpdata.Scan0 + (i * sourcebtmpdata.Stride);

            int pointX = point.X;
            //the rowSize is pre-computed before the loop to improve performance
            int rowSize = Math.Min(destbtmpdata.Width - pointX, sourcebtmpdata.Width);
            //for each row each set each pixel
            for (int j = 0; j < rowSize; j++)
            {
                int firstBlueByte = ((pointX + j)*3);

                int srcByte = j *3;
                destRowPtr[(firstBlueByte)] = srcRowPtr[srcByte];
                destRowPtr[(firstBlueByte) + 1] = srcRowPtr[srcByte + 1];
                destRowPtr[(firstBlueByte) + 2] = srcRowPtr[srcByte + 2];
            }


        }
    }

So is there anything that can be done to make this faster? Ignore the todo for now, ill fix that later once I have some baseline performance measurements.

UPDATE: Sorry, should have mentioned that the reason i’m using this instead of Graphics.DrawImage is because im implementing multi-threading and because of that I cant use DrawImage.

UPDATE 2: I’m still not satisfied with the performance and i’m sure there’s a few more ms that can be had.

Answer 1

There was something fundamentally wrong with the code that I cant believe I didn’t notice until now.

byte* destRowPtr = (byte*)destbtmpdata.Scan0 + (destRow * destbtmpdata.Stride);

This gets a pointer to the destination row but it does not get the column that it is copying to, that in the old code is done inside the rowSize loop. It now looks like:

byte* destRowPtr = (byte*)destbtmpdata.Scan0 + (destRow * destbtmpdata.Stride) + pointX * 3;

So now we have the correct pointer for the destination data. Now we can get rid of that for loop. Using suggestions from Vilx- and Rob the code now looks like:

        private static unsafe void CopyBitmapToDestSuperFast(BitmapData sourcebtmpdata, BitmapData destbtmpdata, Point point)
    {
        //calculate total number of rows to copy.
        //using ternary operator instead of Math.Min, few ms faster
        int totalRows = (destbtmpdata.Height - point.Y < sourcebtmpdata.Height) ? destbtmpdata.Height - point.Y : sourcebtmpdata.Height;
        //calculate the width of the image to draw, this cuts off the image
        //if it goes past the width of the destination image
        int rowWidth = (destbtmpdata.Width - point.X < sourcebtmpdata.Width) ? destbtmpdata.Width - point.X : sourcebtmpdata.Width;

        //loop through each row on the source bitmap and get mem pointers
        //to the source bitmap and dest bitmap
        for (int i = 0; i < totalRows; i++)
        {
            int destRow = point.Y + i;

            //get the pointer to the start of the current pixel "row" and column on the output image
            byte* destRowPtr = (byte*)destbtmpdata.Scan0 + (destRow * destbtmpdata.Stride) + point.X * 3;

            //get the pointer to the start of the FIRST pixel row on the source image
            byte* srcRowPtr = (byte*)sourcebtmpdata.Scan0 + (i * sourcebtmpdata.Stride);

            //RtlMoveMemory function
            CopyMemory(new IntPtr(destRowPtr), new IntPtr(srcRowPtr), (uint)rowWidth * 3);

        }
    }

Copying a 500×500 image to a 5000×5000 image in a grid 50 times took: 00:00:07.9948993 secs. Now with the changes above it takes 00:00:01.8714263 secs. Much better.