How to paint a translucent (not transparent) rectangle

Problem/Question/Abstract:

I want to have a background image and then draw a translucent, red rectangle over it. The effect would then be like looking through a piece of red glass. How to achieve that?

Answer:

There are a number of different techniques, which vary in the overall effect. A simple algorithm, that doesn't model specular reflection or refraction, is demonstrated by this code:


procedure DrawTransparentRectangle(Canvas: TCanvas; Rect: TRect;
  Color: TColor; Transparency: Integer);
var
  X: Integer;
  Y: Integer;
  C: TColor;
  R, G, B: Integer;
  RR, RG, RB: Integer;
begin
  RR := GetRValue(Color);
  RG := GetGValue(Color);
  RB := GetBValue(Color);
  for Y := Rect.Top to Rect.Bottom - 1 do
    for X := Rect.Left to Rect.Right - 1 do
    begin
      C := Canvas.Pixels[X, Y];
      R := Round(0.01 * (Transparency * GetRValue(C) + (100 - Transparency) * RR));
      G := Round(0.01 * (Transparency * GetGValue(C) + (100 - Transparency) * RG));
      B := Round(0.01 * (Transparency * GetBValue(C) + (100 - Transparency) * RB));
      Canvas.Pixels[X, Y] := RGB(R, G, B);
    end;
end;


This routine is meant to illustrate the principle; in reality, you'd use something other than the (very slow) Pixels property to access the individual pixels of the canvas. For example, if you were dealing with bitmaps, you could use the Scanline property.

The Transparency parameter ranges from 0 (completely opaque) to 100 (completely transparent). With this simple algorithm, transparency values greater than 50 work best. Note that this algorithm is non-physical. The results are not what you'd get with a real piece of colored glass.