faq33.htm

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   <TITLE>Create memory DCs and BitBlt to the screen with BCB.</TITLE>

   <META NAME="Author" CONTENT="Harold Howe">

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<H3>

Create memory DCs and BitBlt to the screen with BCB.

</H3>

<P>

 The VCL <TT>TBitmap</TT> class contains a <TT>Canvas</TT> that

encapsulates a memory DC for the bitmap. You can create a memory DC by creating

a <TT>TBitmap</TT> object and drawing on its canvas.

</P>

<pre>

<b>void</b> <b>__fastcall</b> TForm1<b>:</b><b>:</b>FormPaint<b>(</b>TObject <b>*</b>Sender<b>)</b>

<b>{</b>

    Graphics<b>:</b><b>:</b>TBitmap <b>*</b>bmp <b>=</b> <b>new</b> Graphics<b>:</b><b>:</b>TBitmap<b>;</b>

    bmp<b>-></b>Width  <b>=</b> ClientWidth<b>;</b>

    bmp<b>-></b>Height <b>=</b> ClientHeight<b>;</b>

    bmp<b>-></b>Canvas<b>-></b>Rectangle<b>(</b><font color="blue">0</font><b>,</b><font color="blue">0</font><b>,</b>ClientWidth<b>/</b><font color="blue">2</font><b>,</b> ClientHeight<b>/</b><font color="blue">2</font><b>)</b><b>;</b>

    Canvas<b>-></b>Draw<b>(</b><font color="blue">0</font><b>,</b><font color="blue">0</font><b>,</b>bmp<b>)</b><b>;</b>

    <b>delete</b> bmp<b>;</b>

<b>}</b>

</pre>

<P>

<B>Notes:</B> The <TT>Canvas</TT> property of <TT>TBitmap</TT> is <TT>NULL</TT>

until you use the <TT>Canvas</TT> for the first time. In the code above, <TT>Canvas</TT> is <TT>NULL</TT>

until the <TT>Rectangle</TT> call. It doesn't matter how you access the <TT>Canvas</TT>.

The <TT>Canvas</TT> is created in the first call to the <TT>GetCanvas</TT> function

(the read function for the <TT>Canvas</TT> property).

</P>

<P>

The <TT>Draw</TT> method of <TT>TCanvas</TT> results in an API <TT>StretchBlt</TT>

function call (<TT>StretchBlt</tt> will be called if source graphic is a bitmap,

<TT>DrawIcon</TT> will be called if the graphic is an icon). <TT>StretchBlt</TT> blasts the

contents of the memory bitmap into the display canvas. The <TT>CopyRect</TT> method of

<TT>TCanvas</TT> also calls the <TT>StretchBlt</TT> function. However, <TT>CopyRect</TT> copies

from a source <TT>Canvas</TT> into another <TT>Canvas</TT>, and the source <TT>Canvas</TT>

does not have to be the same size as the destination.

</P>

<P>

Setting the <TT>Width</TT> and <TT>Height</TT> of the <TT>TBitmap</TT> object is

important when you are not loading a bitmap image into the object. If you do load

a bitmap into the object from a resource or BMP file, the <TT>Width</TT> and <TT>Height</TT>

properties will be set to match the loaded image. In this case, omit the assignments to

<TT>Height</TT> and <TT>Width</TT>. For example:

</P>

<pre>

Graphics<b>:</b><b>:</b>TBitmap <b>*</b>bmp <b>=</b> <b>new</b> Graphics<b>:</b><b>:</b>TBitmap<b>;</b>

bmp<b>-></b>LoadFromResourceName<b>(</b> <b>(</b><b>int</b><b>)</b> HInstance<b>,</b> <font color="blue">"IDD_BITMAP1"</font><b>)</b><b>;</b>

bmp<b>-></b>Canvas<b>-></b>Rectangle<b>(</b><font color="blue">10</font><b>,</b><font color="blue">10</font><b>,</b>ClientWidth<b>/</b><font color="blue">2</font><b>,</b> ClientHeight<b>/</b><font color="blue">2</font><b>)</b><b>;</b>

Canvas<b>-></b>Draw<b>(</b><font color="blue">0</font><b>,</b><font color="blue">0</font><b>,</b>bmp<b>)</b><b>;</b>

<b>delete</b> bmp<b>;</b>

</pre>





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