如何获得的位图transparancy，而不需要先画？ [英] How to get bitmap transparancy without the need to paint first?

问题描述

一个新创建的位图似乎有默认情况下（白色）的背景。至少，在像素属性证实的查询。但是，为什么是，背景颜色不作为透明色当透明设置为true？

A newly created bitmap seems to have a (white) background by default. At least, a query on the Pixels property confirms. But why is that background color not used as the transparent color when Transparent is set true?

考虑这个简单的测试code：

Consider this simple test code:

procedure TForm1.Button1Click(Sender: TObject);
var
  Bmp: TBitmap;
begin
  Bmp := TBitmap.Create;
  try
    Bmp.Width := 100;
    Bmp.Height := 100;
    Bmp.Transparent := True;
    Canvas.Draw(0, 0, Bmp);                              // A white block is drawn
    Bmp.Canvas.Brush.Color := Bmp.Canvas.Pixels[0, 99];  // = 'clWhite'
    Bmp.Canvas.FillRect(Rect(0, 0, 100, 100));
    Canvas.Draw(0, 100, Bmp);                            // "Nothing" is drawn
  finally
    Bmp.Free;
  end;
end;

由于某些原因，整个位面都必须涂，才可以显示为透明，这听起来有点古怪。

For some reason, the entire bitmap surface has to be painted before it can appear transparent, which sounds kind of odd.

以下变化试图消除调用 FillRect ，都具有相同的结果（无transparancy）：

The following variations are tried to eliminate the call to FillRect, all with the same outcome (no transparancy):

• 只有设置 Brush.Color ，


• Brush.Handle：= CreateSolidBrush（clWhite），


• 的PixelFormat：= pf32Bit ，


• IgnorePalette：= TRUE ，


• TransparantColor：= clWhite ，


• TransparantMode：= tmFixed ，


• Canvas.Pixels [0，99]：= clWhite 这让只有像素透明，


• 修改：= TRUE

• Only setting Brush.Color,
• Brush.Handle := CreateSolidBrush(clWhite),
• PixelFormat := pf32Bit,
• IgnorePalette := True,
• TransparantColor := clWhite,
• TransparantMode := tmFixed,
• Canvas.Pixels[0, 99] := clWhite which makes only thát pixel transparent,
• Modified := True.

所以，愿望是只绘制新创建的位图的一部分，并获得剩余的表面透明。

So, the wish is to paint only a portion of a newly created bitmap and get the remaining surface transparent.

使用：德尔福7，赢7/64

Using: Delphi 7, Win 7/64.

推荐答案

只要设置 TransparentColor和 Canvas.Brush.Color设置位图尺寸了。

Just set TransparentColor and Canvas.Brush.Color before setting dimensions of the bitmap.

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