Technical Q&A QD14

Spooling a pixMap into a Window

Q: When a picture that contains a pixMap is spooled into a window, how and when is the depth of the pixMap in the picture converted to the depth of the screens the window is on?

A: When a picture is spooled in, if QuickDraw encounters any bitmap opcode, it allocates a pixMap of the same depth as the data associated with the bitmap opcode, expands the data into the temporary pixMap, and then calls StdBits. StdBits is what triggers the depth and color conversions as demanded by the color environment (depth, color table, B & W settings) of the devices the target port may span (as when a window crosses two or more screens).

If there's not enough memory in the application heap or in the temporary memory pool, QuickDraw bands the image down to one scan line and calls StdBits for each of these bands. Note that if you're providing your own bitsProc, QuickDraw will call it instead of StdBits.

This process is the same when the picture is in memory, with the obvious exception that all the picture data is present; the color mapping occurs when StdBits does its stuff.