How to draw a gradient background

My previous post I wrote about how to reduce flicker on a gradient background and did not mention how to create a gradient background. Here is how I create a gradient background.

Interface

procedure GradVertical(Canvas: TCanvas; Rect: TRect; FromColor, ToColor: TColor);

Implemention


procedure TForm1.GradVertical(Canvas: TCanvas; Rect: TRect; FromColor, ToColor: TColor);
var
    Y                     : Integer;
    dr, dg, db            : Extended;
    C1, C2                : TColor;
    r1, r2, g1, g2, b1, b2: Byte;
    R, G, B               : Byte;
    cnt                   : Integer;
begin
    C1 := FromColor;
    r1 := GetRValue(C1);
    g1 := GetGValue(C1);
    b1 := GetBValue(C1);

    C2 := ToColor;
    r2 := GetRValue(C2);
    g2 := GetGValue(C2);
    b2 := GetBValue(C2);

    dr := (r2 - r1) / Rect.Bottom - Rect.Top;
    dg := (g2 - g1) / Rect.Bottom - Rect.Top;
    db := (b2 - b1) / Rect.Bottom - Rect.Top;

    cnt   := 0;
    for Y := Rect.Top to Rect.Bottom - 1 do
    begin
        R := r1 + Ceil(dr * cnt);
        G := g1 + Ceil(dg * cnt);
        B := b1 + Ceil(db * cnt);

        Canvas.Pen.Color := RGB(R, G, B);
        Canvas.MoveTo(Rect.Left, Y);
        Canvas.LineTo(Rect.Right, Y);
        Inc(cnt);
    end;
end;


procedure TForm1.ButtonClick(Sender: TObject);
var
    Rect: TRect;
begin
    Rect := GetClientRect;
    GradVertical(Self.Canvas, Rect, clBlack, clRed);
end;