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Here's a simple program that illustrates how to draw on a canvas, save the
drawing to a bitmap (or file), clear the canvas and redisplay a previously
saved image. Oh, and scribble too if you want.
We'll use a TImage component to provide the canvas. Even though
forms have canvas properties, drawing on them generally causes
more headaches than it's worth. First, it is impossible to redraw the
image when the form is redisplayed. Also the canvas covers the entire form
so it's easy to draw on top of other controls ( buttons. labels, etc.), usually
with undesirable results. Both of these problems disappear when we use
I placed several buttons on the form:
- Create ellipses adds 10 ellipses of random size and color to the
canvas, You can see here how clipping is beneficial.
TColor fields are made of 3 bytes representing the red, green and blue
components (RGB format). So I colored each ellipse using Random(256*256*256);
Don't be concerned about efficiency loss from not pre-multiplying these
numbers - Delphi compiler will do it for you one time so the object code
will contain only the product. We also could have written Random($FFFFFF);
to specify the same result as 3 hexadecimal bytes.
- Clear simply draws a white rectangle over the entire screen.
- Save canvas to list creates a TBitmap, copies the canvas to
it, and saves the bitmap as an object in the listbox items
stringlist. Copyrect is the standard procedure to
- Save canvas to file only takes a couple of lines of code to save the
current canvas to a file. Savedialog provides the user
interface to get the filename for us.
Retrieving the images from the listbox when the user selects and item is
pretty easy - just copy the bitmap from the selected item back to the canvas,
again using Copyrect.
Finally I added a scribble feature to allow user drawing on the canvas.
This uses a OnMouseDown exit to set the initial drawing parameters (Pen
width and color) and to set a Drawing boolean variable so that OnMouseMove
will know whether a line is to be drawn. And of course,
OnMouseUp exits just to reset the Drawing flag to
false. I learned here that cursor refresh while drawing sometimes erases a portion of the line being drawn. The solution trick
is to set the Cursor to CrNone before the line is drawn and CrDefault
Plenty of room to expand on this if you want -- user control of pen
colors and widths, erase or undo mode, customized cursors, for
Click here to download source