[SOLVED] Trouble getting a TBitmap to display on a TPanel.
lazarus / 39-Graphics

----------------------------------------------------------------------
indydev · 2023-07-25 16:23

When I put the following code into the FormCreate event, the Bitmap
does not display.

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
TForm1.FormCreate(Sender: TObject);var   AgentPic: TBitmap;   PicRect:
TRect;begin    AgentPic := TBitmap.Create;  try   
AgentPic.LoadFromFile('Agent.bmp');    Panel4.Canvas.Brush.Color :=
Panel4.Color;    Panel4.Canvas.FillRect(Panel4.ClientRect);    PicRect
:= Rect(24, 8, 54, 38);     Panel4.Canvas.StretchDraw(PicRect,
AgentPic);  finally    AgentPic.Free;  end;end;   
If I put the same code in the FormResize event, and then 'resize' the
form, the bitmap displays. The same code in onShow, or onRepaint does
not display (onResize removed). If I leave the code in
FormResize(Sender);  and call it from FormCreate(Sender); it does not
display.

I am confused as to why the bitmap does not display in FormCreate, or
FormShow, or FormPaint (assigned to their respective 'on' events);

If someone knows what is actually triggering the display in the
FormResize(Sender); procedure so I can get this bitmap displayed
without having to resize the form, I would appreciate your help.

Thanks.

----------------------------------------------------------------------
Handoko · 2023-07-25 17:06

OnCreate event is for your program to initialize data. If you do any
drawing/painting here, nothing will be drawn on the screen, and it may
cause exception. The reason is, The form, canvas, etc aren't actually
being created when the execution runs into the OnCreate event. As the
name suggests, it is for you to initialize your data.

OnRepaint? I checked my TForm, I couldn't find OnRepaint. No comment.

OnResize, as the name suggests it will be fired when you resize the
form. Put you code here if you need to do the calculation for
rearranging the layout if the size of the form is changed.

OnPaint. Any drawing/painting should be put here. As the name suggests
OnPaint, any non-painting related code shouldn't put here. For your
case, loading an image from disk, shouldn't be here. You may not
notice the problem, but if the file is huge and you're running the
code on a slow computer, the program will be noticeable delay. The
better solution is load the picture only once and store in the memory,
so there will be no reloading next time you need it.

OnShow. Don't put any drawing/painting code here. I rarely use it, no
comment.

----------------------------------------------------------------------
Josh · 2023-07-25 18:05

Hi

any component drawing should be done in  components onpaint event, as
you cant guarantee if component is ready for painting/updating any
other way. Also the LCL will decide when the component needs repaint;
and then your code gets run also. You can call onpaint youself, but do
so only when you need to, it is possible you can cause an infinate
loop if done incorrectly.
project demo attached, not the neatest but should work.

----------------------------------------------------------------------
indydev · 2023-07-25 18:19

Ok, figured it out. My slip up here. I was calling the code to display
from the TForm and not the TPanel itself. Thank you both for
responding. Handoko's explanation made me be more methodical about my
Bitmap, but Josh's code in the Panel's own paint method made me
realize I was trying to call from the Form's paint method (which I
thought would work, but obviously doesn't), when I should call from
the Panel's paint method.

great catch!