C# drawing rectangle in a specific color channel

Question

I am trying to draw 2 rectangles 1 in green channel 2nd in red channel of a bitmap.

This is the code:

Bitmap bitmap_guess = new Bitmap(C_PALETTE_X_PIXEL_MAX, C_PALETTE_Y_PIXEL_MAX, PixelFormat.Format24bppRgb);
Graphics graphics = Graphics.FromImage(bitmap_guess);
Rectangle box_rect = new Rectangle(0, 0, C_BOX_PIXELS_WIDTH, C_BOX_PIXELS_HEIGHT);

matrix = new Matrix();
matrix.RotateAt(thc, new System.Drawing.PointF(xc, yc), MatrixOrder.Prepend);
graphics.Transform = matrix;
graphics.FillRectangle(new SolidBrush(Color.Green), xc, yc, box_rect.Width, box_rect.Height);

matrix = new Matrix();
matrix.RotateAt(th, new System.Drawing.PointF(x, y), MatrixOrder.Prepend);
graphics.Transform = matrix;
graphics.FillRectangle(new SolidBrush(Color.Red), x, y, box_rect.Width, box_rect.Height);

The problem is that when I draw the 2nd rectangle in red color it overwrites the 1st rectangles overlapping pixels to 0's. I'd want the rectangles to to change color when they overlap instead of plainly overwriting the previous pixel values. IE - to draw in a single channel instead of all 3.

I'm specifically looking for a Graphics operation for this.

Kind regards, Laov

Answer 1

This might help you Merging two images in C#/.NET

You then could draw each rectangle into its own image and merge the results...