delphi change canvas pixel color - delphi

Question

i need to convert all pixels of a canvas
found this function after a quick search in google
but dont work correct , but it seems must work good!!
function RGBBitmapTo1Bit(OriginalBitmap : TBitmap) : TBitmap;
var
x, y : integer;
begin
result := TBitmap.Create;
result.width := OriginalBitmap.width;
result.height := OriginalBitmap.height;
for x := 1 to OriginalBitmap.width do
for y := 1 to OriginalBitmap.height do
begin
result.Canvas.Pixels[x, y] := clBlack;
end;
end;
this function dont make any change on the file
for example i used like this
procedure TForm1.Button2Click(Sender: TObject);
var
imgf : TBitmap;
begin
if od1.Execute then
begin
imgf := TBitmap.Create;
imgf.LoadFromFile(od1.FileName);
RGBBitmapTo1Bit(imgf);
imgf.SaveToFile(ExtractFilePath(od1.FileName)+'test.bmp');
end;
but the output and input files are the same!!!
how can i assign a color to a pixel rightly!?

Answer 1

Your code has three problems with it:
Pixels are 0-indexed in both dimensions, so you need to change your loops accordingly.
for x := 0 to OriginalBitmap.width-1 do
for y := 0 to OriginalBitmap.height-1 do
your function DOES NOT modify the original TBitmap, it allocates and modifies a new TBitmap and then returns that to the caller, but the caller is ignoring that new bitmap, expecting the original TBitmap to have been modified instead. You are saving the original TBitmap to file, which is why you don't see any of the pixels changed.
You are leaking memory for both TBitmap objects;
Try this instead:
function RGBBitmapTo1Bit(OriginalBitmap : TBitmap) : TBitmap;
var
x, y : integer;
begin
Result := TBitmap.Create;
Result.Width := OriginalBitmap.Width;
Result.Height := OriginalBitmap.Height;
for x := 0 to OriginalBitmap.Width-1 do
for y := 0 to OriginalBitmap.Height-1 do
begin
Result.Canvas.Pixels[x, y] := clBlack;
end;
end;
procedure TForm1.Button2Click(Sender: TObject);
var
imgf : TBitmap;
imgf2 : TBitmap;
begin
if od1.Execute then
begin
imgf := TBitmap.Create;
try
imgf.LoadFromFile(od1.FileName);
imgf2 := RGBBitmapTo1Bit(imgf);
try
imgf2.SaveToFile(ExtractFilePath(od1.FileName)+'test.bmp');
finally
imgf2.Free;
end;
finally
imgf.Free;
end;
end;
end;
Or this:
procedure RGBBitmapTo1Bit(OriginalBitmap : TBitmap);
var
x, y : integer;
begin
for x := 0 to OriginalBitmap.Width-1 do
for y := 0 to OriginalBitmap.Height-1 do
begin
OriginalBitmap.Canvas.Pixels[x, y] := clBlack;
end;
end;
procedure TForm1.Button2Click(Sender: TObject);
var
imgf : TBitmap;
begin
if od1.Execute then
begin
imgf := TBitmap.Create;
try
imgf.LoadFromFile(od1.FileName);
RGBBitmapTo1Bit(imgf);
imgf.SaveToFile(ExtractFilePath(od1.FileName)+'test.bmp');
finally
imgf.Free;
end;
end;
end;
That being said, RGBBitmapTo1Bit() is slow in both versions. A faster version would be more like this:
function RGBBitmapTo1Bit(OriginalBitmap : TBitmap) : TBitmap;
begin
Result := TBitmap.Create;
Result.Width := OriginalBitmap.Width;
Result.Height := OriginalBitmap.Height;
Result.Canvas.Brush.Color := clBlack;
Result.Canvas.FillRect(Rect(0, 0, Result.Width, Result.Height));
end;
function RGBBitmapTo1Bit(OriginalBitmap : TBitmap) : TBitmap;
begin
Result := TBitmap.Create;
Result.Canvas.Brush.Color := clBlack;
Result.Width := OriginalBitmap.Width;
Result.Height := OriginalBitmap.Height;
end;
Or:
procedure RGBBitmapTo1Bit(OriginalBitmap : TBitmap);
begin
OriginalBitmap.Canvas.Brush.Color := clBlack;
OriginalBitmap.Canvas.FillRect(Rect(0, 0, OriginalBitmap.Width, OriginalBitmap.Height));
end;
It also does not do what its name suggests - convert a bitmap to 1bit. To do that, you have to set the TBitmap.PixelFormat property instead:
function RGBBitmapTo1Bit(OriginalBitmap : TBitmap) : TBitmap;
begin
Result := TBitmap.Create;
Result.PixelFormat := pf1Bit;
Result.Canvas.Brush.Color := clBlack;
Result.Width := OriginalBitmap.Width;
Result.Height := OriginalBitmap.Height;
end;
Or:
procedure RGBBitmapTo1Bit(OriginalBitmap : TBitmap);
begin
OriginalBitmap.PixelFormat := pf1Bit;
OriginalBitmap.Canvas.Brush.Color := clBlack;
OriginalBitmap.Canvas.FillRect(Rect(0, 0, OriginalBitmap.Width, OriginalBitmap.Height));
end;