Private Type RGBAL
    a As Long
End Type

Private Type RGBAB
    r As Byte
    g As Byte
    b As Byte
    a As Byte
End Type

Function BlendColor(ByVal Src0 As Long, ByVal Src1 As Long, ByVal Alpha As Byte) As Long
    Dim v0 As Single
    Dim v1 As Single
    Dim p0 As RGBAB
    Dim p1 As RGBAB
    Dim t0 As RGBAL
    Dim t1 As RGBAL
    
    t0.a = Src0
    t1.a = Src1
    
    LSet p0 = t0
    LSet p1 = t1
    
    v0 = Alpha / 255
    v1 = 1 - v0
    
    p0.r = p0.r * v0 + p1.r * v1
    p0.g = p0.g * v0 + p1.g * v1
    p0.b = p0.b * v0 + p1.b * v1

    LSet t0 = p0
    BlendColor = t0.a
End Function

function AlphaBlendProcess(const ColorA, ColorB: integer; const Alpha: single = 1 {1= 100%, 0.5= 50%}): integer;
var NAlpha : single;
    CA : array[0..3] of byte Absolute ColorA;
    CB : array[0..3] of byte Absolute ColorB;
    CR : array[0..3] of byte Absolute result;
begin
  NAlpha := 1 - alpha;
  CR[0]  := byte( round( (CA[0]*Alpha) + (CB[0]*NAlpha) ) );
  CR[1]  := byte( round( (CA[1]*Alpha) + (CB[1]*NAlpha) ) );
  CR[2]  := byte( round( (CA[2]*Alpha) + (CB[2]*NAlpha) ) );
  CR[3]  := 0;
end;
{ exemple :
  C := AlphaBlendProcess($54749C, $FFFFCC, 0.5); { C = $00AABAB4 }
}


{ version avec precalcul (+ performances / gain =~ 50%) }
type
  TAlphaTable = array[byte, byte] of byte;

procedure PreCalcAlphaTable(var AlphaTable: TAlphaTable; const Alpha: single= 1 {1= 100%, 0.5= 50%});
var A,B: integer;
    NAlpha: single;
begin
  NAlpha := 1 - Alpha;
  for A := 0 to 255 do
    for B := 0 to 255 do
      AlphaTable[A,B] := byte( round( (A*Alpha) + (B*NAlpha) ) );
end;

function AlphaBlendProcess(const ColorA, ColorB: integer; const AlphaTable: TAlphaTable): integer;
var
  CA : array[0..3] of byte Absolute ColorA;
  CB : array[0..3] of byte Absolute ColorB;
  CR : array[0..3] of byte Absolute result;
begin
  CR[0]  := AlphaTable[CA[0], CB[0]];
  CR[1]  := AlphaTable[CA[1], CB[1]];
  CR[2]  := AlphaTable[CA[2], CB[2]];
  CR[3]  := 0;
end;
{ exemple :
  PreCalcAlphaTable(AlphaTable, 0.5);
  for Y := 0 to MaxY do
    for X := 0 to MaxX do
      ColorPixelR[X,Y] := AlphaBlendProcess(ColorPixelA[X,Y], ColorPixelB[X,Y], AlphaTable); 
}