converting a PNGImage to grayscale using delphi - delphi

Question

hi there
here it is my code:
procedure TForm4.Button1Click(Sender: TObject);
var
png: TPNGImage;
data: PRGBQarray;
p: ^tagRGBQuad;
i, o: integer;
begin
png := TPNGImage.Create;
try
png.LoadFromFile('C:\Untitled.png');
for o := 1 to 100 do
begin
data:=png.Scanline[o];
for I := 1 to 400 do
begin
p := #data^[i];
p.rgbGreen := p.rgbBlue;
p.rgbRed := p.rgbGreen;
end;
end;
img.picture.Assign(png);
finally
png.Free;
end;
end;
it doesn't work and it makes the pic messy, I'm sure it's because of the rgbReserved.
what should i do?

Answer 1

This is how to greyify a bitmap. (And, yes, if you want to greyify a PNG, you first need to get the bitmap data out of it. I think the VCL will do this for you.)
type
PRGB32Array = ^TRGB32Array;
TRGB32Array = packed array[0..MaxInt div SizeOf(TRGBQuad)-1] of TRGBQuad;
procedure MakeGrey(Bitmap: TBitmap);
var
w, h: integer;
y: Integer;
sl: PRGB32Array;
x: Integer;
grey: byte;
begin
Bitmap.PixelFormat := pf32bit;
w := Bitmap.Width;
h := Bitmap.Height;
for y := 0 to h - 1 do
begin
sl := Bitmap.ScanLine[y];
for x := 0 to w - 1 do
with sl[x] do
begin
grey := (rgbBlue + rgbGreen + rgbRed) div 3;
rgbBlue := grey;
rgbGreen := grey;
rgbRed := grey;
end;
end;
end;
Sample usage:
procedure TForm4.Button1Click(Sender: TObject);
var
bm: TBitmap;
begin
bm := TBitmap.Create;
try
bm.LoadFromFile('C:\Users\Andreas Rejbrand\Pictures\Porträtt, litet, kvadratiskt.bmp');
MakeGrey(bm);
Canvas.Draw(0, 0, bm);
finally
bm.Free;
end;
end;

Answer 2

Andreas's answer will give you a good, fast approximation, but you'll lose some quality, because red, green and blue don't mix with equal intensities in the human eye. If you want to "get it right", instead of
grey := (rgbBlue + rgbGreen + rgbRed) div 3;
try this:
grey := round(rgbRed * .3) + round(rgbGreen * .59) + round(rgbBlue * .11);
You'll get a bit of a performance hit over the simple average, though it probably won't be noticeable unless you're on a very large image.

Answer 3

I know the question has already been answered but here is my 2c worth...
The following code comes from the PNGComponents package (PngFunctions.pas) produced by Thany.
//
//The Following code comes from the PNGComponents package from Thany...
//
procedure MakeImageGrayscale(Image: TPNGObject; Amount: Byte = 255);
procedure GrayscaleRGB(var R, G, B: Byte);
var
X: Byte;
begin
X := Round(R * 0.30 + G * 0.59 + B * 0.11);
R := Round(R / 256 * (256 - Amount - 1)) + Round(X / 256 * (Amount + 1));
G := Round(G / 256 * (256 - Amount - 1)) + Round(X / 256 * (Amount + 1));
B := Round(B / 256 * (256 - Amount - 1)) + Round(X / 256 * (Amount + 1));
end;
var
X, Y, PalCount: Integer;
Line: Pointer;
PaletteHandle: HPalette;
Palette: array[Byte] of TPaletteEntry;
begin
//Don't do anything if the image is already a grayscaled one
if not (Image.Header.ColorType in [COLOR_GRAYSCALE, COLOR_GRAYSCALEALPHA])
then begin
if Image.Header.ColorType = COLOR_PALETTE
then begin
//Grayscale every palette entry
PaletteHandle := Image.Palette;
PalCount := GetPaletteEntries(PaletteHandle, 0, 256, Palette);
for X := 0 to PalCount - 1
do GrayscaleRGB(Palette[X].peRed, Palette[X].peGreen, Palette[X].peBlue);
SetPaletteEntries(PaletteHandle, 0, PalCount, Palette);
Image.Palette := PaletteHandle;
end
else begin
//Grayscale every pixel
for Y := 0 to Image.Height - 1
do begin
Line := Image.Scanline[Y];
for X := 0 to Image.Width - 1
do GrayscaleRGB(PRGBLine(Line)^[X].rgbtRed, PRGBLine(Line)^[X].rgbtGreen, PRGBLine(Line)^[X].rgbtBlue);
end;
end;
end;
end;
There is a set of routines, that was originally published by the author of the PNGImage components, that can be found on Code Central that shows how to do other things like Alpha blending two images, rotation, overlay, etc. CodeCentral Link

Answer 4

This really should have been a comment to #Mason's routine to turn RGB into GreyScale, but since I don't know how to make a comment show code, I'm making it an answer instead.
This is how I do the conversion:
FUNCTION RGB2GRAY(R,G,B : BYTE) : BYTE; Register; ASSEMBLER;
ASM
IMUL EAX,19595
IMUL EDX,38470
IMUL ECX,7471
ADD EAX,EDX
ADD EAX,ECX
SHR EAX,16
END;
FUNCTION GreyScale(C : TColor) : TColor; Register; ASSEMBLER;
ASM
MOVZX EDX,AH
MOV ECX,EAX
SHR ECX,16
MOVZX EAX,AL
CALL RGB2GRAY
MOVZX EAX,AL
MOV AH,AL
SHL EAX,8
MOV AL,AH
END;
I don't know if it is NTSC formula or whatever, but they seem to work in my programs :-).