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Mask drawing is the technique for placing a non-rectangular image on top of
another image. Here is an example and the Delphi code to
accomplish the trick.
The complication is that Windows only knows how to draw
rectangles. So what to we do with the dark gray area around the
balloon? We get rid of it a 4 step process likes this:
| Step 1: Create a mask image of the balloon that has all 1 pixels
(white) values where the dark gray is, and 0 (black) pixels where the
balloon image existed. |
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| Step 2: Make a copy of the
balloon with all of dark gray pixels changed to Black. (The implementation
of this step takes two draw operations and is shown as Steps 2A and 2B in
the demo program output). |
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Step 3: Put the mask on the cloud
image using a logical AND operation. The key points here:
 | 0 AND anything is 0 (Black and anything is Black) |
| 1 AND anything remains unchanged (White and anything remains
unchanged) |
So ANDing our Black balloon image with the cloud image creates a
Black balloon shaped hole in the clouds without changing the surrounding image.
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Step 4: OR the image created in Step 2 using the same
coordinates that we used to draw the mask in Step 3. For logical OR
operations
| 0 OR anything remains unchanged (Black OR
anything remains unchanged) |
| 1 OR anything is 1 (White or anything is
White) |
It's the first OR property that we use in this step. In
the image created in Step 2, everything outside the balloon shape proper is
Black so when ORed with clouds, the clouds remain unchanged.
For the balloon shape itself, Step 3 put a Black balloon shaped hole in the
clouds which when ORed with the balloon pixels, fills the Black hole with
the balloon image.
Cool!
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If you have access to Delphi source code, you can browse the
TransparentStretchBlt function in the Graphics unit to find
Delphi's implementation of this. TBitmap's Draw
method calls it when Transparent property is True. The BrushCopy
method of TCanvas uses it if BrushStyle is bsClear;
TImageList uses the Windows API function ImageList_DrawEx to
accomplish the same thing, (I assume in the same manner.
Download Delphi source for
MaskDraw demo, just to prove that it really works.
| Created: September 11, 2002 |
Modified: May 18, 2009
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