[SOLVED] Why setting TBitmap's PixelFormat property resets the Canvas
Brush/Pen?
lazarus / 39-Graphics

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Gustavo 'Gus' Carreno · 2021-09-19 06:22

Hey Y'all,

Still asking for the lazCAPTCHA thingamaboob I made

While trying to set the background to white I got a solution on this
thread but the solution kinda spawn another issue.

I'm now baffled by why setting the PixelFormat property of a TBitmap
resets the Canvas Pen and/or Brush!!

Let me explain it with code.

If I do this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i var 
Bitmap: TBitmap;begin  Bitmap:= TBitmap.Create;  Bitmap.Width:=
cWidth;  Bitmap.Height:= cHeight;  Bitmap.Canvas.Brush.Color:=
clWhite;  Bitmap.PixelFormat:= pf24bit; 
Bitmap.Canvas.FillRect(0,0,cWidth,cHeight);end;Then the FillRect is
ineffective at painting a white rectangle. I'm presuming that setting
the PixelFormat resets the Brush, but I'm not sure and that's why i
need a clarification.

BUT if I do this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i var 
Bitmap: TBitmap;begin  Bitmap:= TBitmap.Create;  Bitmap.PixelFormat:=
pf24bit;  Bitmap.Width:= cWidth;  Bitmap.Height:= cHeight; 
Bitmap.Canvas.Brush.Color:= clWhite; 
Bitmap.Canvas.FillRect(0,0,cWidth,cHeight);end;Then the FillRect does
indeed draw a white rectangle.

Is this a bug that needs to be reported to the issue tracking on
GitLab?

Many thanks in advance!!

Cheers,
Gus

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speter · 2021-09-19 23:41

When the pixelformat is changed the current colours may no longer be
valid (for example if it is set to 2bit then only black/white would be
valid), so it makes sense for them to be reset.

cheers
S.

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Gustavo 'Gus' Carreno · 2021-09-19 23:46

Hey speter,

Quote from: speter on September 20, 2021, 01:41:28 amWhen the
pixelformat is changed the current colours may no longer be valid (for
example if it is set to 2bit then only black/white would be valid), so
it makes sense for them to be reset.

I was afraid that was the answer and it makes all the sense.

I'm just too of a newbie in terms of images that I wasn't sure.

Thank you SOOOO much for clarifying

Cheers,
Gus

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flowCRANE · 2021-09-20 11:39

To keep the code fast and logical, you should always set the
PixelFormat before setting the bitmap size and drawing. Look how the
setter of this property look like:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TCustomBitmap.SetPixelFormat(AValue: TPixelFormat);begin  if AValue =
FPixelFormat then Exit;  {$IFDEF VerboseLCLTodos}{$note todo copy
image into new format }{$ENDIF}  FreeImage;  FPixelFormat :=
AValue;end;
When the format is changed, the image is freed, so previous settings
are no longer valid.

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Gustavo 'Gus' Carreno · 2021-09-20 13:05

Hey Furious Programming,

Quote from: furious programming on September 20, 2021, 01:39:50 pmTo
keep the code fast and logical, you should always set the PixelFormat
before setting the bitmap size and drawing.

First of all, thank you SO very much for that source deep dive, I
appreciate the effort!!

And now it's clear why it fails.

I'm now sure that the change I made to have the PixelFormat set right
after Bitmap:= TBitmap.Create; is the best practice.

Again, many thanks for the help, quite appreciated indeed!!

Cheers,
Gus