How to create and use a colour palette

Problem/Question/Abstract:

How to create and use a color palette

Answer:

Solve 1:

Below are functions that help to create a palette (an identity palette, BTW) from an array of RGBQuads (such as you would find in the palette section of a .BMP file). I stole this from the WinG documentation, and converted it to Delphi. First call ClearSystemPalette, then you can get an identity palette by calling CreateIdentityPalette. If you plan to try palette animation, work in a 256-color mode, and change all the PC_NOCOLLAPSE entries below to PC_RESERVED. Besides creating the palette, the other pieces to the puzzle are:



1. Override the form's GetPalette method, so that it returns the new palette.


2. Select and realize the new palette just before you paint.

OldPal := SelectPalette(Canvas.Handle, NewPalette, False);
RealizePalette(Canvas.Handle);
{ Do your painting here }
SelectPalette(Canvas.Handle, OldPal, False);


3. Remember to release the palette when you are done using DeleteObject


4. If you are used using the RGB function to get color values, use the PaletteRGB function in its place.

function CreateIdentityPalette(const aRGB; nColors: Integer): HPALETTE;
type
  QA = array[0..255] of TRGBQUAD;
var
  Palette: PLOGPALETTE;
  PalSize: Word;
  ScreenDC: HDC;
  I: Integer;
  nStaticColors: Integer;
  nUsableColors: Integer;
begin
  PalSize := SizeOf(TLOGPALETTE) + SizeOf(TPALETTEENTRY) * 256;
  GetMem(Palette, PalSize);
  try
    with Palette^ do
    begin
      palVersion := $0300;
      palNumEntries := 256;
      ScreenDC := GetDC(0);
      try
        { For SYSPAL_NOSTATIC, just copy the color table into a PALETTEENTRY
          array and replace the first and last entries with black and white }
        if (GetSystemPaletteUse(ScreenDC) = SYSPAL_NOSTATIC) then
        begin
          { Fill in the palette with the given values, marking each with PalFlag }

{$R-}
          for i := 0 to (nColors - 1) do
            with palPalEntry[i], QA(aRGB)[I] do
            begin
              peRed := rgbRed;
              peGreen := rgbGreen;
              peBlue := rgbBlue;
              peFlags := PC_NOCOLLAPSE;
            end;
          { Mark any unused entries with PalFlag }
          for i := nColors to 255 do
            palPalEntry[i].peFlags := PC_NOCOLLAPSE;
          { Make sure the last entry is white -- This may replace an entry in the array!}
          I := 255;
          with palPalEntry[i] do
          begin
            peRed := 255;
            peGreen := 255;
            peBlue := 255;
            peFlags := 0;
          end;
          { And the first is black -- This may replace an entry in the array!}
          with palPalEntry[0] do
          begin
            peRed := 0;
            peGreen := 0;
            peBlue := 0;
            peFlags := 0;
          end;
{$R+}
        end
        else
        begin
          { For SYSPAL_STATIC, get the twenty static colors into the
            array, then fill in the empty spaces with the given color table }

          { Get the static colors from the system palette }
          nStaticColors := GetDeviceCaps(ScreenDC, NUMRESERVED);
          GetSystemPaletteEntries(ScreenDC, 0, 256, palPalEntry);
{$R-}
          { Set the peFlags of the lower static colors to zero }
          nStaticColors := nStaticColors shr 1;
          for i := 0 to (nStaticColors - 1) do
            palPalEntry[i].peFlags := 0;
          { Fill in the entries from the given color table}
          nUsableColors := nColors - nStaticColors;
          for I := nStaticColors to (nUsableColors - 1) do
            with palPalEntry[i], QA(aRGB)[i] do
            begin
              peRed := rgbRed;
              peGreen := rgbGreen;
              peBlue := rgbBlue;
              peFlags := PC_NOCOLLAPSE;
            end;
          { Mark any empty entries as PC_NOCOLLAPSE }
          for i := nUsableColors to (255 - nStaticColors) do
            palPalEntry[i].peFlags := PC_NOCOLLAPSE;
          { Set the peFlags of the upper static colors to zero }
          for i := (256 - nStaticColors) to 255 do
            palPalEntry[i].peFlags := 0;
        end;
      finally
        ReleaseDC(0, ScreenDC);
      end;
    end;
    { Return the palette }
    Result := CreatePalette(Palette^);
  finally
    FreeMem(Palette, PalSize);
  end;
end;

procedure ClearSystemPalette;
var
  Palette: PLOGPALETTE;
  PalSize: Word;
  ScreenDC: HDC;
  I: Word;
const
  ScreenPal: HPALETTE = 0;
begin
  PalSize := SizeOf(TLOGPALETTE) + SizeOf(TPALETTEENTRY) * 255; {256th = [0] }
  GetMem(Palette, PalSize);
  try
    FillChar(Palette^, PalSize, 0);
    Palette^.palVersion := $0300;
    Palette^.palNumEntries := 256;
{$R-}
    for I := 0 to 255 do
      with Palette^.palPalEntry[I] do
        peFlags := PC_NOCOLLAPSE;
{$R+}
    { Create, select, realize, deselect, and delete the palette }
    ScreenDC := GetDC(0);
    try
      ScreenPal := CreatePalette(Palette^);
      if ScreenPal <> 0 then
      begin
        ScreenPal := SelectPalette(ScreenDC, ScreenPal, FALSE);
        RealizePalette(ScreenDC);
        ScreenPal := SelectPalette(ScreenDC, ScreenPal, FALSE);
        DeleteObject(ScreenPal);
      end;
    finally
      ReleaseDC(0, ScreenDC);
    end;
  finally
    FreeMem(Palette, PalSize);
  end;
end;


Solve 2:

unit VideoFcns;

interface
uses Windows;

procedure GrayColorTable(const clrtable: PRGBQUAD; const threshold: integer = -1);
procedure BinaryColorTable(const clrtable: PRGBQUAD; const threshold: integer);

implementation

procedure GrayColorTable(const clrtable: PRGBQUAD; const threshold: integer);
var
  j: integer;
  cp: PRGBQUAD;
begin
  if threshold <> -1 then
  begin
    BinaryColorTable(clrtable, threshold);
    exit;
  end;
  cp := clrtable;
  for j := 0 to 255 do
  begin
    {here you can set rgb components the way you like}
    cp^.rgbBlue := j;
    cp^.rgbGreen := j;
    cp^.rgbRed := j;
    cp^.rgbReserved := 0;
    inc(cp);
  end;
end;

procedure BinaryColorTable(const clrtable: PRGBQUAD; const threshold: integer);
var
  j: integer;
  g: integer;
  cp: PRGBQUAD;
begin
  cp := clrtable;
  for j := 0 to 255 do
  begin
    if j < threshold then
      g := 0
    else
      g := 255;
    cp^.rgbBlue := g;
    cp^.rgbGreen := g;
    cp^.rgbRed := g;
    cp^.rgbReserved := 0;
    inc(cp);
  end;
end;


Here is an example how palette is used:


procedure TBmpByteImage.FillBMPInfo(BMPInfo: pointer; const Wi, He: integer);
var
  p: ^TBitmapInfo;
begin
  p := BMPInfo;
  p^.bmiHeader.biSize := sizeof(p.bmiHeader);
  if Wi <> 0 then
    p^.bmiHeader.biWidth := Wi
  else
    p^.bmiHeader.biWidth := w;
  if He <> 0 then
    p^.bmiHeader.biHeight := He
  else
    p^.bmiHeader.biHeight := h;
  p^.bmiHeader.biPlanes := 1;
  p^.bmiHeader.biBitCount := 8;
  p^.bmiHeader.biCompression := BI_RGB;
  p^.bmiHeader.biClrUsed := 0;
  p^.bmiHeader.biClrImportant := 0;
end;

function TBmpByteImage.CreateDIB(const threshold: integer): HBITMAP;
var
  dc: HDC;
  bmpInfo: ^TBitmapInfo;
  BMPData: pointer;
  hBmp: HBITMAP;
  x, y: integer;
  cp1, cp2: pbyte;
begin
  GetMem(bmpInfo, sizeof(bmpInfo.bmiHeader) + sizeof(RGBQUAD) * 256);
  FillBMPInfo(BMPInfo);
  {I am using a grey palette}
  GrayColorTable(@bmpInfo^.bmiColors[0], threshold);
  dc := CreateDC('DISPLAY', nil, nil, nil);
  hBmp := CreateDIBSection(dc, bmpInfo^, DIB_RGB_COLORS, BMPData, 0, 0);
  DeleteDC(dc);
  FreeMem(bmpInfo, sizeof(bmpInfo.bmiHeader) + sizeof(RGBQUAD) * 256);
  cp2 := BMPData;
  for y := h - 1 downto 0 do
  begin
    cp1 := @g^[y]^[0];
    for x := 0 to w - 1 do
    begin
      cp2^ := cp1^;
      inc(cp1);
      inc(cp2);
    end;
  end;
  CreateDIB := hBmp;
end;

{and  finally draw bitmap }

procedure TBmpByteImage.Draw(const where: TImage; const threshold: integer);
var
  hBmp: HBITMAP;
  Bitmap1: TBitmap;
begin
  hBmp := CreateDIB(threshold);
  if hBmp = 0 then
    exit;
  Bitmap1 := TBitmap.Create;
  with Bitmap1 do
  begin
    Handle := hBmp;
    Width := w;
    Height := h;
  end;
  where.picture.Bitmap := Bitmap1;
  Bitmap1.Free;
  GlobalFree(hBmp);
end;