faq35.htm

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   <TITLE>Paint on the contents of a TImage control.</TITLE>

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

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<H3>

Paint on the contents of a TImage control.

</H3>

<P>

<TT>TImage</TT> provides a <TT>Canvas</TT> property that allows you to draw on the contents

of the image. Changes made to the image become a permanent part of the image. If you

copy the resulting image to the clipboard or save the image to a file, your changes

will appear right with the image. Here is a code snippet that draws an arrow onto

an image.

<pre>

  <b>const</b> TPoint Arrow1<b>[</b><b>]</b> <b>=</b> <b>{</b> <b>{</b><font color="blue">80</font><b>,</b><font color="blue">105</font><b>}</b> <b>,</b> <b>{</b><font color="blue">106</font><b>,</b><font color="blue">66</font><b>}</b><b>,</b> <b>{</b><font color="blue">80</font><b>,</b><font color="blue">87</font><b>}</b><b>,</b><b>{</b><font color="blue">94</font><b>,</b><font color="blue">98</font><b>}</b><b>}</b><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>Pen<b>-></b>Color <b>=</b> clBlue<b>;</b>

  Image1<b>-></b>Canvas<b>-></b>Pen<b>-></b>Width <b>=</b> <font color="blue">3</font><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>MoveTo<b>(</b>Arrow1<b>[</b><font color="blue">0</font><b>]</b><b>.</b>x<b>,</b>Arrow1<b>[</b><font color="blue">0</font><b>]</b><b>.</b>y<b>)</b><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>LineTo<b>(</b>Arrow1<b>[</b><font color="blue">1</font><b>]</b><b>.</b>x<b>,</b>Arrow1<b>[</b><font color="blue">1</font><b>]</b><b>.</b>y<b>)</b><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>MoveTo<b>(</b>Arrow1<b>[</b><font color="blue">0</font><b>]</b><b>.</b>x<b>,</b>Arrow1<b>[</b><font color="blue">0</font><b>]</b><b>.</b>y<b>)</b><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>LineTo<b>(</b>Arrow1<b>[</b><font color="blue">2</font><b>]</b><b>.</b>x<b>,</b>Arrow1<b>[</b><font color="blue">2</font><b>]</b><b>.</b>y<b>)</b><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>MoveTo<b>(</b>Arrow1<b>[</b><font color="blue">0</font><b>]</b><b>.</b>x<b>,</b>Arrow1<b>[</b><font color="blue">0</font><b>]</b><b>.</b>y<b>)</b><b>;</b>

  Image1<b>-></b>Canvas<b>-></b>LineTo<b>(</b>Arrow1<b>[</b><font color="blue">3</font><b>]</b><b>.</b>x<b>,</b>Arrow1<b>[</b><font color="blue">3</font><b>]</b><b>.</b>y<b>)</b><b>;</b>

</pre>

<P>

<B>Note:</B> You can only draw on the <TT>Canvas</TT> property of <TT>TImage</TT> if the

<TT>Picture</TT> is empty or contains a bitmap as its graphic. You cannot draw on the

<TT>Canvas</TT> of <TT>TImage</TT> if the image contains an icon or a metafile as its

graphic. The <TT>GetCanvas</TT> read method of <TT>TImage</TT> demonstrates why.

</P>

<pre>

<font color="navy">// function converted to C++</font>

TCanvas <b>*</b> TImage<b>:</b><b>:</b>GetCanvas<b>(</b><b>)</b>

<b>{</b>

    Graphics<b>:</b><b>:</b>TBitmap <b>*</b>Bitmap<b>;</b>



    <b>if</b> <b>(</b>Picture<b>-></b>Graphic <b>==</b> NULL<b>)</b>

    <b>{</b>

        Bitmap <b>=</b> <b>new</b> Graphics<b>:</b><b>:</b>TBitmap

        Bitmap<b>-></b>Width  <b>=</b> Width<b>;</b>

        Bitmap<b>-></b>Height <b>=</b> Height<b>;</b>



        <font color="navy">// assign new bitmap to the Graphic property. This calls the</font>

        <font color="navy">// SetGraphic write method of TPicture. SetGraphic calls Assign,</font>

        <font color="navy">// equivalent to calling Picture->Graphic->Assign(Bitmap);</font>

        Picture<b>-></b>Graphic <b>=</b> Bitmap<b>;</b>



        <font color="navy">// Delete the temp bitmap pointer. Because Assign was called,</font>

        <font color="navy">// the contents of the HBITMAP is retained in Picture->Graphic</font>

        <b>delete</b> Bitmap<b>;</b>

    <b>}</b>



    <b>if</b> <b>(</b><font color="navy">/* Picture->Graphic is a TBitmap object */</font> <b>)</b>

        <b>return</b> Picture<b>-></b>Bitmap<b>-></b>Canvas<b>;</b>

    <b>else</b>

        <b>throw</b> EInvalidOperation<b>(</b> <font color="navy">/* some args here maybe */</font><b>)</b><b>;</b>

<b>}</b>

</pre>

<P>

Notice that if the <TT>Picture</TT> property is empty, a new bitmap object is created for

you to draw in. If a bitmap is loaded, then <TT>TImage::GetCanvas</TT> simply returns the

<TT>Canvas</TT> of the <TT>Bitmap</TT>. The code generates an exception if the

<TT>Picture</TT> is an icon or a metafile.

</P>

<P>

<B>Note:</B> As a result of how <TT>TImage::GetCanvas</TT> works, there is no

difference beteen <TT>Image-&gt;Canvas</TT> and <TT>Image-&gt;Picture-&gt;Bitmap-&gt;Canvas</TT>.

</P>

<P>

<B>Note:</B> The width and the height of an <TT>Image</TT> control's drawing canvas are

identical to the original bitmap size. This may differ from the <TT>Width</TT> and

<TT>Height</TT> of the image if the bitmap has been stretched. For example, place an

<TT>Image</TT> control onto a new form. Size the form so it is 500 x 400 pixels. Set the

<TT>Align</TT> property of the <TT>Image</TT> control to <TT>alClient</TT>,

and load in the CHEMICAL.BMP splash screen that ships with BCB. Finally, change

the <TT>Stretch</TT> property to true to make the picture expand to fit the entire form.

At this point, <TT>Image1-&gt;Width</TT> will equal <TT>Form1-&gt;ClientWidth</TT>, but

<TT>Image1-&gt;Picture-&gt;Width</TT> will be the width of CHEMICAL.BMP (240).

</P>

<P>

<B>Note:</B> When you draw on an image, try to use only the standard Windows

colors or a color that is already conained in the palette of the Image.

</P>





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