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Article #16247: Painting a rainbow color spectrum onto a form

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FAQ1247D.txt   Painting a rainbow color spectrum onto a form
Category   :Miscellaneous
Platform    :All
Product    :All 32 bit  

Question:
How do I paint the color spectrum of a rainbow, and if the 
spectrum is clicked on, how do I calculate what color was 
clicked on?

Answer:
The following example demonstrates painting a color spectrum, 
and calculating the color of a given point on the spectrum. 
Two procedures are presented: PaintRainbow() and 
ColorAtRainbowPoint(). The PaintRainbow() procedure paints a 
spectrum from red to magenta if the WrapToRed parameter is 
false, or paint red to red if the WrapToRed parameter is true. 
The rainbow can progress either in a horizontal or
vertical progression. The ColorAtRainbowPoint() function 
returns a TColorRef containing the color at a given point in 
the rainbow.

Example:

procedure PaintRainbow(Dc : hDc; {Canvas to paint to}
                       x : integer; {Start position X}
                       y : integer;  {Start position Y}
                       Width : integer; {Width of the rainbow}
                       Height : integer {Height of the rainbow};
                       bVertical : bool; {Paint verticallty}
                       WrapToRed : bool); {Wrap spectrum back to red}
var
  i : integer;
  ColorChunk : integer;
  OldBrush : hBrush;
  OldPen : hPen;
  r : integer;
  g : integer;
  b : integer;
  Chunks : integer;
  ChunksMinus1 : integer;
  pt : TPoint;
begin
  OffsetViewportOrgEx(Dc,
                      x,
                      y,
                      pt);

  if WrapToRed = false then
    Chunks := 5 else
    Chunks := 6;
  ChunksMinus1 := Chunks - 1;

  if bVertical = false then
    ColorChunk := Width div Chunks else
    ColorChunk := Height div Chunks;

 {Red To Yellow}
  r := 255;
  b := 0;
  for i := 0 to ColorChunk do begin
    g:= (255 div ColorChunk) * i;
    OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r, g, b)));
    if bVertical = false then
      PatBlt(Dc, i, 0, 1, Height, PatCopy) else
      PatBlt(Dc, 0, i, Width, 1, PatCopy);
    DeleteObject(SelectObject(Dc, OldBrush));
  end;

 {Yellow To Green}
  g:=255;
  b:=0;
  for i := ColorChunk  to (ColorChunk * 2) do begin
    r := 255 - (255 div ColorChunk) * (i - ColorChunk);
    OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r, g, b)));
    if bVertical = false then
      PatBlt(Dc, i, 0, 1, Height, PatCopy) else
      PatBlt(Dc, 0, i, Width, 1, PatCopy);
    DeleteObject(SelectObject(Dc, OldBrush));
  end;

 {Green To Cyan}
  r:=0;
  g:=255;
  for i:= (ColorChunk * 2) to (ColorChunk * 3) do begin
    b := (255 div ColorChunk)*(i - ColorChunk * 2);
    OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r, g, b)));
    if bVertical = false then
      PatBlt(Dc, i, 0, 1, Height, PatCopy) else
      PatBlt(Dc, 0, i, Width, 1, PatCopy);
    DeleteObject(SelectObject(Dc,OldBrush));
  end;

 {Cyan To Blue}
  r := 0;
  b := 255;
  for i:= (ColorChunk * 3) to (ColorChunk * 4) do begin
    g := 255 - ((255 div ColorChunk) * (i - ColorChunk * 3));
    OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r, g, b)));
    if bVertical = false then
      PatBlt(Dc, i, 0, 1, Height, PatCopy) else
      PatBlt(Dc, 0, i, Width, 1, PatCopy);
    DeleteObject(SelectObject(Dc, OldBrush));
  end;

 {Blue To Magenta}
  g := 0;
  b := 255;
  for i:= (ColorChunk * 4) to (ColorChunk * 5) do begin
    r := (255 div ColorChunk) * (i - ColorChunk * 4);
    OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r, g, b)));
    if bVertical = false then
      PatBlt(Dc, i, 0, 1, Height, PatCopy) else
      PatBlt(Dc, 0, i, Width, 1, PatCopy);
    DeleteObject(SelectObject(Dc, OldBrush))
  end;

  if WrapToRed <> false then begin
   {Magenta To Red}
    r := 255;
    g := 0;
    for i := (ColorChunk * 5) to ((ColorChunk * 6) - 1) do begin
      b := 255 -((255 div ColorChunk) * (i - ColorChunk * 5));
      OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r,g,b)));
      if bVertical = false then
        PatBlt(Dc, i, 0, 1, Height, PatCopy) else
        PatBlt(Dc, 0, i, Width, 1, PatCopy);
      DeleteObject(SelectObject(Dc,OldBrush));
    end;
  end;

 {Fill Remainder}
  if (Width - (ColorChunk * Chunks) - 1 ) > 0 then begin
    if WrapToRed <> false then begin
      r := 255;
      g := 0;
      b := 0;
    end else begin
      r := 255;
      g := 0;
      b := 255;
    end;
    OldBrush := SelectObject(Dc, CreateSolidBrush(Rgb(r, g, b)));
    if bVertical = false then
      PatBlt(Dc,
             ColorChunk * Chunks,
             0,
             Width - (ColorChunk * Chunks),
             Height,
             PatCopy) else
      PatBlt(Dc,
             0,
             ColorChunk * Chunks,
             Width,
             Height - (ColorChunk * Chunks),
             PatCopy);
    DeleteObject(SelectObject(Dc,OldBrush));
  end;
  OffsetViewportOrgEx(Dc,
                      Pt.x,
                      Pt.y,
                      pt);
end;

function ColorAtRainbowPoint(ColorPlace : integer;
                             RainbowWidth : integer;
                             WrapToRed : bool) : TColorRef;
var
  ColorChunk : integer;
  ColorChunkIndex : integer;
  ColorChunkStart : integer;
begin
  if ColorPlace = 0 then begin
    result := RGB(255, 0, 0);
    exit;
 end;
{WhatChunk}
  if WrapToRed <> false then
    ColorChunk := RainbowWidth div 6 else
    ColorChunk := RainbowWidth div 5;
    ColorChunkStart := ColorPlace div ColorChunk;
    ColorChunkIndex := ColorPlace mod ColorChunk;
  case ColorChunkStart of
   0 : result := RGB(255,
                     (255 div ColorChunk) * ColorChunkIndex,
                     0);
   1 : result := RGB(255 - (255 div ColorChunk) * ColorChunkIndex,
                     255,
                     0);
   2 : result := RGB(0, 255, (255 div ColorChunk) * ColorChunkIndex);
   3 : result := RGB(0,
                     255 - (255 div ColorChunk) * ColorChunkIndex,
                     255);
   4 : result := RGB((255 div ColorChunk) * ColorChunkIndex,
                     0,
                     255);
   5 : result := RGB(255,
                     0,
                     255 - (255 div ColorChunk) * ColorChunkIndex);
  else
    if WrapToRed <> false then
      result := RGB(255, 0, 0) else
      result := RGB(255, 0, 255);
  end;{Case}
end;


procedure TForm1.FormPaint(Sender: TObject);
begin
  PaintRainbow(Form1.Canvas.Handle,
               0,
               0,
               Form1.ClientWidth,
               Form1.ClientHeight,
               false,
               true);

end;

procedure TForm1.FormResize(Sender: TObject);
begin
  InvalidateRect(Form1.Handle, nil, false);
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var
  Color : TColorRef;
begin
  Color := ColorAtRainbowPoint(y,
                               Form1.ClientWidth,
                               true);
  ShowMessage(IntToStr(GetRValue(Color)) + #32 +
              IntToStr(GetGValue(Color)) + #32 +
              IntToStr(GetBValue(Color)));
end;


7/16/98 4:31:28 PM
 

Last Modified: 01-SEP-99