Image canvas drawing ends up on form instead

问题

I'm trying to draw some custom shapes on the canvas of a TImage, but the output ends up on the TImage parent form's canvas instead. It appears I have to convert my points from local to absolute to make this work, but that causes problems too. See example:

• Does anyone know why Image.Canvas.DrawArc (etc) draws relative to the parent form instead of relative to the Image?
• If I go through the trouble of .LocalToAbsolute... why does the arc look so different?

The project is simple: Firemonkey HD form with TPanel in the middle and TImage inside the TPanel (aligned to client). The button just executes the code.

Here's the code:

    unit Unit1;

interface

uses
  System.SysUtils, System.Types, System.UITypes, System.Classes, System.Variants, 
  FMX.Types, FMX.Graphics, FMX.Controls, FMX.Forms, FMX.Dialogs, FMX.StdCtrls,
  FMX.Objects;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Panel1: TPanel;
    Image1: TImage;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.fmx}


procedure TForm1.Button1Click(Sender: TObject);
begin
  Image1.Canvas.BeginScene;

  {Trying to draw arc and line on Image's canvas - Doesn't work; it draws to Form's canvas instead}
  Image1.Canvas.DrawArc(PointF(0,0), PointF(10, 10), 0.0, 45.0, 1.0);
  Image1.Canvas.DrawLine(PointF(0.0, 0.0), PointF(100, 100), 1.0);

  {Trying to draw arc and line on Image's canvas - This works; by why should such steps be necessary and why is the arc so different?}
  Image1.Canvas.DrawArc(Image1.LocalToAbsolute(PointF(0,0)), Image1.LocalToAbsolute(PointF(10, 10)), 0.0, 45.0, 1.0);
  Image1.Canvas.DrawLine(Image1.LocalToAbsolute(PointF(0.0, 0.0)), Image1.LocalToAbsolute(PointF(100, 100)), 1.0);

  Image1.Canvas.EndScene;
end;

end.

回答1:

As I mentioned before, this is for XE5. Here is you code:

procedure TForm3.Button1Click(Sender: TObject);
begin
  Image1.Bitmap.Canvas.BeginScene;

  {Trying to draw arc and line on Image's canvas - Doesn't work; it draws to Form's canvas instead}
  Image1.Bitmap.Canvas.DrawArc(PointF(0,0), PointF(10, 10), 0.0, 145.0, 1.0);
  Image1.Bitmap.Canvas.DrawLine(PointF(0.0, 0.0), PointF(200, 200), 1.0);

  {Trying to draw arc and line on Image's canvas - This works; by why should such steps be necessary and why is the arc so different?}
  Image1.Bitmap.Canvas.DrawArc(Image1.LocalToAbsolute(PointF(0,0)), Image1.LocalToAbsolute(PointF(10, 10)), 0.0, 45.0, 1.0);
  Image1.Bitmap.Canvas.DrawLine(Image1.LocalToAbsolute(PointF(0.0, 0.0)), Image1.LocalToAbsolute(PointF(100, 100)), 1.0);

  Image1.Bitmap.Canvas.EndScene;

end;

procedure TForm3.FormCreate(Sender: TObject);
begin
  Image1.Bitmap:=TBitmap.Create(trunc(image1.Width),trunc(image1.Height));
end;

You didn't paste bitmap's creation procedure.

回答2:

Well i've had some problems with this too and you should use a Bitmap as @LHristov said. The only thing is that there is a bug that doesn't set the bitmap size correctly and the fix for this is as follows:

procedure TForm3.FormCreate(Sender: TObject);
begin
  bitmap := TBitmap.Create;
  bitmap.SetSize(round(Image1.Width), round(Image1.Height));
  Image1.MultiResBitmap.Bitmaps[1].Assign(bit);
  Image1.Bitmap := Image1.MultiResBitmap.Bitmaps[1];
  Image1.Bitmap.Clear(TAlphaColorRec.White);
end;

procedure TForm3.Button1Click(Sender: TObject);
begin
  Image1.Bitmap.Canvas.BeginScene;

  Image1.Bitmap.Canvas.DrawArc(PointF(0,0), PointF(10, 10), 0.0, 145.0, 1.0);
  Image1.Bitmap.Canvas.DrawLine(PointF(0.0, 0.0), PointF(200, 200), 1.0);

  Image1.Bitmap.Canvas.DrawArc(Image1.LocalToAbsolute(PointF(0,0)), Image1.LocalToAbsolute(PointF(10, 10)), 0.0, 45.0, 1.0);
  Image1.Bitmap.Canvas.DrawLine(Image1.LocalToAbsolute(PointF(0.0, 0.0)), Image1.LocalToAbsolute(PointF(100, 100)), 1.0);

  Image1.Bitmap.Canvas.EndScene;

end;

Hope this solves it for you!

来源：https://stackoverflow.com/questions/24769644/image-canvas-drawing-ends-up-on-form-instead