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Drawing into Bitmaps
Date: Mon, 27 Jan 1992 22:36 PST
From: email@example.com (John K. Myers)
I need the following functionality: a macro that encloses a body of drawing
routines, with the argument of an existing bitmap, such that the figures
drawn by the drawing routines in absolute coordinates are drawn into the bitmap;
figures drawn overlapping or outside of the edges of the bitmap are clipped;
and the alu is preserved (allowing selective erasing, flipping, etc.).
You already got the right answer for this, but I thought that I
would point out that tv:with-output-to-bitmap actually returns
five values, the bitmap and the four "bounding box" edge
coordinates. Thus, you can find out where in the "absolute"
coordinate system the top-left edge of the bitmap should be
placed by capturing the second and third return values, and then
use those offsets as the TO-X and TO-y args in your call to
My current routine is the following:
(defmacro draw-into-bitmap (my-bitmap &rest body)
"Wrapper for drawing into a bitmap, instead of onto the screen.
The body *must* use magic variable STREAM as the output.
UNFORTUNATELY AUTO-ADJUSTS THE BITMAT TO THE UPPER LEFT.
Remember in bitmaps, X points to the right, Y points down.
`(multiple-value-bind (my-width my-height ignore-span-width) (decode-raster-array ,my-bitmap)
(let ((buffer (tv:with-output-to-bitmap (stream) ,@body)))
(multiple-value-bind (new-width new-height ignore-span-width) (decode-raster-array buffer)
(setq my-width (min my-width new-width))
(setq my-height (min my-height new-height))
(bitblt tv:alu-ior my-width my-height buffer 0 0 ,my-bitmap 0 0) ;1 bits set; 0 bits left alone