Draw rectangle on a Window DC

[Dennis Lee Bieber]

Anand fed this fish to the penguins on Tuesday 05 November 2002 01:03 
am:

> Thanks Dennis. That technique worked.
> But since I use a XOR operator the outline of the rectangle
> does not have a unique color, but depends on the pixel color at (x,y).
>

        When the user releases the "drag" you can redraw the final rectangle 
in the desired solid color.

        The other means of maintaining a fixed color for rubber banding 
requires either lots of bit-planes with the resultant very large color 
lookup table OR the use of a display list with the allowance of display 
noise until finished.

        In the first case, say you need 128 colors for the display, and only 
one "overlay" (the rubber band). You allocate a color table for 256 
colors (1 overlay bit, 7 color bits). The first 128 entries in the 
lookup table are the desired colors -- the NEXT 128 entries are all set 
to the "rubberband" color. Drawing the line will be slow on most 
true-color/non-bitplane hardware as you draw the line by: 
set_pixel(get_pixel(x,y) OR 0x80, x, y) and "undraw" the line by 
set_pixel(get_pixel(x,y) AND 0x7F, x, y).

        Bit-plane hardware (the Commodore Amiga was one such system) 
simplifies the operation because they have settings for which 
bitplane(s) a draw operation will affect -- no need for the OR/AND bit 
manipulation. set_mask(0x80) {says only one bit will be modified in 
subsequent operations}; draw_line(0xFF, x1, y1, x2, y2) {draws}; 
draw_line(0x00, x1, y1, x2, y2) {erases line}.


        The second case requires a display list manager (GKS was one such). In 
this situation, you create a new "segment" with a unique ID. You draw 
with whatever color, and to erase when the rubberband moves you draw 
the background color. This will result in making a mess of the 
previously drawn data BUT... when the user releases the "drag" you 
delete the "segment", set the active segment to the base drawing 
segment, draw the final rectangle, and invoke a refresh of the display 
list (the manager than processes all the drawing commands that have 
been stored to recreate the display).

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