How to access TFrame's canvas?

Question

Using: Delphi XE2, VCL 32-bit application, Windows 8

I'm trying to paint the background of my frame onto a panel (I'm using TJvPanel, because it exposes the OnPaint event) which is a child control of the frame.

After reading this post and adding a canvas as a field, I am still not successful.

After calling ShowAddReceiptPanel, it should draw the frame's (TfrmMyFrame) window contents with all the controls already on it (which include a grid and a pagecontrol) on the foreground panel, grayscaled, after being processed by the ProEffectImage method, but instead it shows an opaque white background. Am I missing something?

Here's my code:

type
  TfrmMyFrame = class(TFrame)
    pnlHdr: TPanel;
    pnlAddNewBG: TJvPanel;
    procedure pnlAddNewBGPaint(Sender: TObject);
  private
    { Private declarations }
    FBGImg: TProEffectImage;
    Fcnvs: TCanvas;

    procedure PaintWindow(DC: HDC); override;
    procedure ShowAddReceiptPanel;
    procedure HideAddReceiptPanel;
    procedure ResizePanel_pnlAddNewBG;

  public
    { Public declarations }
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  end;

constructor TfrmMyFrame.Create(AOwner: TComponent);
begin
  inherited;

  FBGImg := TProEffectImage.Create(nil);
  Fcnvs := TCanvas.Create;

end;

destructor TfrmMyFrame.Destroy;
begin
  if Assigned(FBGImg) then
    FBGImg.Free;

  if Assigned(Fcnvs) then
    Fcnvs.Free;

  inherited;
end;

procedure TfrmMyFrame.ShowAddReceiptPanel;
begin
  ResizePanel_pnlAddNewBG;
  pnlAddNewBG.Visible := True;
end;

procedure TfrmMyFrame.PaintWindow(DC: HDC);
begin
  inherited;

  Fcnvs.Handle := DC;
end;

procedure TfrmMyFrame.pnlAddNewBGPaint(Sender: TObject);
var
  l, t, w, h: Integer;
  srct, drct: TRect;
begin

  // Copy Frame canvas to BGImg bitmap
  l := 0;
  t := pnlHdr.Height;
  w := ClientWidth;
  h := ClientHeight - t;

  srct := TRect.Create(l, t, w, h);
  FBGImg.Width := w;
  FBGImg.Height := h;
  drct := TRect.Create(l, t, w, h);
  FBGImg.Canvas.CopyMode := cmSrcCopy;
  FBGImg.Canvas.CopyRect(drct, Fcnvs, srct);
//  FBGImg.Picture.SaveToFile('c:\tmp\a.bmp');

  FBGImg.Effect_AntiAlias;
  FBGImg.Effect_GrayScale;

  // Draw BGImg onto Option panel
  TJvPanel(Sender).Canvas.CopyMode := cmSrcCopy;
  TJvPanel(Sender).Canvas.Draw(0, 0, FBGImg.Picture.Graphic);
end;

procedure TfrmMyFrame.ResizePanel_pnlAddNewBG;
var
  x1, y1, x2, y2: Integer;
  bmp: TBitmap;
begin
  x1 := 0;
  y1 := pnlHdr.Height;
  x2 := ClientWidth;
  y2 := ClientHeight - y1;

  pnlAddNewBG.SetBounds(x1, y1, x2, y2);
end;

The if Assigned tests in your destructor can and should be removed. Just call Free which includes and Assigned check. — David Heffernan, Apr 05 '13 at 11:00
Thanks! Free includes an Assigned check - when did they introduce that feature? — Steve F, Apr 05 '13 at 11:03
Er, in Delphi 1. I refer you to this: http://stackoverflow.com/questions/8548843/why-should-i-not-use-if-assigned-before-using-or-freeing-things/8550628#8550628 — David Heffernan, Apr 05 '13 at 11:05
https://stackoverflow.com/questions/65050463/why-canvas-is-hidden-in-all-vcl-controls — Gabriel, Apr 30 '21 at 12:52

David Heffernan · Accepted Answer · 2013-04-05T11:12:38.170

2

The DC that you assign to your canvas handle is only valid during the PaintWindow call. You use it outside that function when it is not valid and hence the behaviour that you observe.

I think that you should be able to solve your problem by calling the PaintTo method. Create a bitmap of the right size and pass its canvas to PaintTo.

edited Apr 05 '13 at 11:12

answered Apr 05 '13 at 10:55

David Heffernan

601,492
42
1,072
1,490

Do you only want to paint the background? Or the frame's children too. – David Heffernan Apr 05 '13 at 11:02
Need to paint the frame's children. The front panel will show a black and white image of what lies beneath it - that is the whole purpose. – Steve F Apr 05 '13 at 11:05
My second paragraph should cover it. – David Heffernan Apr 05 '13 at 11:06
Please can you give an example of the code, as I am totally new to sending messages. Also where do I put that call in the context of my existing code? – Steve F Apr 05 '13 at 11:08
Is there a variation of PaintTo that paints only a rectangle (TRect) instead of the whole canvas? I need to omit a rectangular region at the top of the frame (that contains a header panel). – Steve F Apr 05 '13 at 11:30
Nevermind, I can adjust the drawing location by offsetting the Y by the needed amount: TJvPanel(Sender).Canvas.Draw(0, - pnlHdr.Height, FBGImg.Picture.Graphic); – Steve F Apr 05 '13 at 11:33
I do not think so. You need to get the whole image and then strip off the unwanted portion into a second image. – David Heffernan Apr 05 '13 at 11:43
Actually your solution is far better! – David Heffernan Apr 05 '13 at 11:46

score 1 · Answer 2 · answered Apr 05 '13 at 11:16

1

A TFrame does not have a canvas. You could create/add one, as TCustomControl does, but you do not have to. A canvas is just a handy wrapper around a Windows device context. The PaintWindow routine is called whenever the frame has to be (partially) redrawn. The parameter exhibits the DC, or you could obtain one with GetDC.

Then pseudo-code would be as follows:

procedure TfrmMyFrame.PaintWindow(DC: HDC);
begin
  - Resize BG image and hide it (otherwise image itself will be copied too)
  - Paint the frame's contents to the image with:
    Self.PaintTo(FBGImg.Canvas.Handle, 0, 0)
  - Process the special effects on FBGImg
  - Paint the image onto DC with:
    BitBlt(DC, 0, 0, ClientWidth, ClientHeight, FBGImage.Canvas.Handle, 0, 0, SRCCOPY);
end;

answered Apr 05 '13 at 11:16

NGLN

43,011
8
105
200

GetWindowDC does not always return the same DC. – David Heffernan Apr 05 '13 at 11:17
Oops, `GetWindowDC` returns the non-client part indeed. – NGLN Apr 05 '13 at 11:20
More than that the DC could be user supplied. As in "paint on my bitmap". – David Heffernan Apr 05 '13 at 11:28

score 0 · Answer 3 · answered Feb 06 '15 at 15:29

0

An easy way to get access to a Canvas on a TFrame is to add a TPaintBox on top of it with Align := alClient and using its Canvas property.

I expect this method to work with any version of Delphi, also in the future, and therefore use it instead of the PaintWindow method, which seems to be tricky.

answered Feb 06 '15 at 15:29

Marthein

153
1
5

That does not, however, *get access to a Canvas on a TFrame* - it gets access to *a TPaintBox on the TFrame*, which is not the same thing at all. – Ken White Feb 06 '15 at 15:54
`TPaintBox.Canvas` is a Canvas, in my opinion just as much as a `Canvas` created in `PaintWindow`. I first used the `PaintWindow` method, but ran into problems with fonts automatically changing. Therefore I switched to using the `TPaintBox`, which is simple and does not do anything with the Windows API around Delphi code. – Marthein Mar 30 '15 at 13:45

How to access TFrame's canvas?

3 Answers3