Issue about calling Canvas.Polygon

[c0d3r 17]

Hi,

 

For the last couple of days I tried figure out the issue about Canvas.Polygon wasn't working properly.   Here is my routine:

 

procedure DrawTriangle(ACanvas: TCanvas; AColor: TColor; ARect: TRect);
begin
  ACanvas.Brush.Style := bsSolid;
  ACanvas.Brush.Color := ColorToRGB(clRed); //ColorToRGB(AColor);
  ACanvas.Pen.Color := ColorToRGB(clRed); //ColorToRGB(AColor);
  //ACanvas.Pen.Mode := pmCopy;
  ACanvas.Polygon([Point(ARect.Right - 1, ARect.Bottom - 10), Point(ARect.Right - 1, ARect.Bottom - 1),
                   Point(ARect.Right - 10, ARect.Bottom - 1)]);
end;

 

if I passed clRed as one of the Color parameter on the red background, it drawed a white triangle,  as you see from the below screenshot,. It was only painted right at the most right column.  Even I hard coded to set color to clRed, it still drawed a white triangle.

 

 

 

 

I just can't figure out what I was doing wrong.

 

Edited January 29, 2021 by c0d3r

[c0d3r 17]

if I use clBlack instead:

 

[FPiette 384]

2 hours ago, c0d3r said:

ACanvas.Brush.Color := ColorToRGB(clRed); //ColorToRGB(AColor);

You should not pass thru ColorToRGB.

[Anders Melander 1784]

I use the following to draw corners (top-right in this example):

const
  CornerSize = 8;
var
  Triangle: array[0..2] of TPoint;
begin
  ACanvas.SaveDC;
  try

    ACanvas.Brush.Color := clRed;
    ACanvas.Brush.Style := bsSolid;

    ACanvas.Pen.Handle := 0; // Work around for messed up pen handle sporadically causing line not to be drawn
    ACanvas.Pen.Color := GetHighLightColor(ACanvas.Brush.Color);
    ACanvas.Pen.Style := psSolid;

    Triangle[0].X := AViewInfo.BoundsRect.Right-1;
    Triangle[0].Y := AViewInfo.BoundsRect.Top;
    Triangle[1].X := Triangle[0].X - CornerSize;
    Triangle[1].Y := Triangle[0].Y;
    Triangle[2].X := Triangle[0].X;
    Triangle[2].Y := Triangle[0].Y + CornerSize;
    
    ACanvas.Polygon(Triangle);

  finally
    ACanvas.RestoreDC;
  end;

 

[MarkShark 27]

@Anders Melander Are your SaveDC and RestoreDC calls from a class helper for TCanvas?  I don't do a lot of canvas drawing and only recently need to use the API functions for this and I don't fully understand when calling Canvas.Refresh is needed... and I'm wondering if your calls handle that.   Thanks!

[Anders Melander 1784]

Just now, MarkShark said:

Are your SaveDC and RestoreDC calls from a class helper for TCanvas?

They are Windows API functions: https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-savedc

 

[MarkShark 27]

4 minutes ago, Anders Melander said:

They are Windows API functions: https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-savedc

 

Thanks! I notice you're calling it like ACanvas.SaveDC so I thought maybe you were encapsulating the API functions (my TCanvas doesn't seem to have those as methods.)  I do use the api calls directly, but obviously your code is cleaner.

[c0d3r 17]

6 hours ago, FPiette said:

You should not pass thru ColorToRGB.

Tried without, it doesn't work either.

[Anders Melander 1784]

1 hour ago, MarkShark said:

Thanks! I notice you're calling it like ACanvas.SaveDC so I thought maybe you were encapsulating the API functions (my TCanvas doesn't seem to have those as methods.)  I do use the api calls directly, but obviously your code is cleaner.

My mistake. The ACanvas in my code is a DevExpress TcxCanvas - I didn't think about that.

 

I think that if you use SaveDC/RestoreDC with a TCanvas then you'll need to lock the canvas to guard against the DC being changed. Something like this:

ACanvas.Lock;
try
  SaveDC(ACanvas.Handle);
  try
    ...
  finally
    RestoreDC(ACanvas.Handle, -1);
  end;
finally
  ACanvas.Unlock;
end;

 

Edited January 29, 2021 by Anders Melander

[c0d3r 17]

@Anders Melander tried yours but still the same issue:

procedure DrawTriangle(ACanvas: TCanvas; AColor: TColor; ARect: TRect);
var
  SaveIndex: Integer;
begin
  ACanvas.Lock;
  try
    SaveIndex := SaveDC(ACanvas.Handle);
    try
      ACanvas.Brush.Style := bsSolid;
      ACanvas.Brush.Color := clBlack;
      ACanvas.Pen.Color := clBlack;
      ACanvas.Polygon([Point(ARect.Right - 1, ARect.Bottom - 10), Point(ARect.Right - 1, ARect.Bottom - 1),
                       Point(ARect.Right - 10, ARect.Bottom - 1)]);
    finally
      RestoreDC(ACanvas.Handle, SaveIndex);
    end;
  finally
   ACanvas.Unlock;
  end;
end;

 

Edited January 29, 2021 by c0d3r

[Anders Melander 1784]

I know the SaveDC/RestoreDC doesn't solve your problem. In my code it's just there to leave the canvas in the same state as it was in when I got it. The key point from my code was the clearing of the pen handle since in my case I had problems with the pen color.

 

Try reversing the order of the pen and brush assignments. I've had similar problems and as far as I remember that was what I did to solve it.

[c0d3r 17]

13 minutes ago, Anders Melander said:

I know the SaveDC/RestoreDC doesn't solve your problem. In my code it's just there to leave the canvas in the same state as it was in when I got it. The key point from my code was the clearing of the pen handle since in my case I had problems with the pen color.

 

Try reversing the order of the pen and brush assignments. I've had similar problems and as far as I remember that was what I did to solve it.

Tried reversing the order of pen and brush assignments,  doesn't work either.

[c0d3r 17]

Solved the issue, but I don't understand the reason why, I had like:

 

SaveIndex := SaveDC(Canvas.Handle);

try

   IntersectClipRect(Canvas.Handle, .....);

   PaintRows(Canvas, ...);

finally

    RestoreDC(Canvas.Handle, SaveIndex);

end;

 

The codes in procedure PaintRows:

 

SaveIndex2 := SaveDC(ACanvas.Handle);

try

   IntersectClipRect(ACanvas.Handle, .....);

   .....

   PaintCells(ACanvas, ...);

   ......

finally

    RestoreDC(ACanvas.Handle, SaveIndex2);

end;

 

By removing the SaveDC/RestoreDC from procedure PaintRows, it solves,  but Why?   can't nest call SaveDC/RestoreDC??

Edited January 29, 2021 by c0d3r

[Anders Melander 1784]

7 minutes ago, c0d3r said:

can't nest call SaveDC/RestoreDC??

Yes, they're designed to be nested.

I have no idea about why it doesn't work though. Maybe TCanvas gets confused when the HDC is modified outside its control.

[balabuev 102]

The issue happens due to the fact that you use two not fully compatible ways to deal with the device context (DC):

 

1) Delphi TCanvas wrapper, which have a lot of lazy initializing stuff. For example, when you change the color of the Brush, the brush object is marked as invalid internally, and its handle will be recreated later - only when needed.

2) WinAPI directly, like SaveDC/RestoreDC, which skips mentioned Delphi internal invalidation.

 

TCanvas.Refresh method is specifically exist to resort cases like this. Try to call it just before ACanvas.Polygon.

 

I can even describe, why red color is a kind of special color, which do not work (as opposed to any other color, like black).

 

And I agree - ColorToRGB is not needed here.

Edited January 31, 2021 by balabuev

[balabuev 102]

On 1/29/2021 at 6:07 PM, c0d3r said:

Solved the issue, but I don't understand the reason why, I had like:

 

SaveIndex := SaveDC(Canvas.Handle);

try

   IntersectClipRect(Canvas.Handle, .....);

   PaintRows(Canvas, ...);

finally

    RestoreDC(Canvas.Handle, SaveIndex);

end;

SaveDC/RestoreDC in such cases are usually used to restore original clipping region, since you change it in the code.

 

 

Edited January 31, 2021 by balabuev