Graphics not working on Mac/PowerPC
lazarus / 39-Graphics

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WalterK · 2011-04-15 15:03

I'm using version 0.9.30 on my PowerPC Mac running OS 10.5.8...

I''m trying to draw some lines in a TPanel (GraphPanel).  The below
code (boiled down) has no visible output on my Mac, but does draw a
line on my Windows laptop (Vista).

   maxY := GraphPanel.Height;
   maxX := GraphPanel.Width;
GraphPanel.Canvas.Pen.Color:=clBlack;
GraphPanel.Canvas.Pen.Width:=4;
   GraphPanel.Canvas.Line(0,0,maxx,maxy);
   GraphPanel.Invalidate;

Can someone tell me what I'm doing wrong?

TIA.

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eny · 2011-04-15 15:10

Maybe it's because a TPanel isn't supposed to be used as a drawing
surface.
For example it doesn't have an (accessible) OnPaint method.

Maybe it works if you manually set the OnPaint method of TPanel and do
all painting in there.

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Arbee · 2011-04-15 15:19

I do know there is indeed a difference in how (or when) Windows and
Mac update the screen.  I haven't been able to exactly pinpoint what
that difference is.

Your code does not say where it is actually invoked (in some onxxxxxx
event I gather), so it is hard to tell what heppens.

Incidentally ... a TPaintbox won't work either?

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Arbee · 2011-04-15 15:49

Or ... a TImage.

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WalterK · 2011-04-15 19:43

OK.  It's working now...

I used a TPanel because it has a Canvas and I didn't know any better. 
I was drawing the lines upon a button click.  Apparently this is
major-ignorant.

I changed to use a TPaintBox and put the line drawing code inside an
OnPaint routine and it works. (I'll suspect this will work under
Windows also.)

Thanks.

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circular · 2011-04-15 20:45

If you call Invalidate after you draw, it erases everything.