Question

Lately I got low level graphics programming job to do (implementing raster graphics algorithms).
That’s why I started to look for tools (classes) which would be helpful to draw primitives (lines, rectangles, ellipses etc.) on the low level of abstraction (even using unsafe code).

I consider if I should implement my algorithms in WPF, WindowsForms, WinAPI or other environment. For now I’m going to give a try to WPF. I found some examples how to draw pixels on bitmap but unfortunately I have problem with understanding this code (source code from MSDN):

    // The DrawPixel method updates the WriteableBitmap by using 
    // unsafe code to write a pixel into the back buffer. 
    static void DrawPixel(MouseEventArgs e)
    {
        int column = (int)e.GetPosition(i).X;
        int row = (int)e.GetPosition(i).Y;

        // Reserve the back buffer for updates.
        writeableBitmap.Lock();

        unsafe
        {
            // Get a pointer to the back buffer. 
            int pBackBuffer = (int)writeableBitmap.BackBuffer;

            // Find the address of the pixel to draw.
            pBackBuffer += row * writeableBitmap.BackBufferStride;
            pBackBuffer += column * 4;//??

            // Compute the pixel's color. 
            int color_data = 255 << 16; // R
            color_data |= 128 << 8;   // G
            color_data |= 255 << 0;   // B 

            // Assign the color data to the pixel.
            *((int*)pBackBuffer) = color_data;//??
        }

        // Specify the area of the bitmap that changed.
        writeableBitmap.AddDirtyRect(new Int32Rect(column, row, 1, 1));

        // Release the back buffer and make it available for display.
        writeableBitmap.Unlock();
    }

1. What is this pBackBuffer += column * 4; line for? Why 4?
2. What does it *((int*)pBackBuffer) = color_data; mean? I know pointers from C/C++ but in C# there is IntPtr, int* and this line int pBackBuffer = (int)writeableBitmap.BackBuffer; suggests that we can treat equally int and IntPtr which is also not clear for me.
3. Which programming environment should I use? WinAPI, WPF or other?

I would be very thankful if someone could explain me this unsafe code.

Answer 1

1: What is this pBackBuffer += column * 4; line for? Why 4?

Assuming that pixel is ARGB, it is 4 bytes per pixel. Since column is X coordinate, it must be multiplied by 4.

2: What does it ((int)pBackBuffer) = color_data; mean?

(int*)pBackBuffer – treat value of int pBackBuffer as pointer to an int

and

*((int*)pBackBuffer) = color_data – store color_data to that pointer

IntPtr and int are not quite the same. IntPtr is 32-bit when running on 32-bit OS and 64-bit when running on 64-bit OS. int is always 32-bit.