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From: jwwalker@usceast.UUCP (Jim Walker)
Newsgroups: comp.sys.mac.programmer
Subject: bitmap printing problem
Keywords: printing bitmap CopyBits ImageWriter
Message-ID: <3240@usceast.UUCP>
Date: 5 May 90 21:00:43 GMT
Distribution: na
Organization: University of South Carolina, Columbia
Lines: 39


I'm trying to use draft-mode bitmap printing (as described in IM V,
p. 415) to print a full-page bitmap at 144dpi on an ImageWriter.  I
prepare the bitmap in an offscreen bitmap, and use CopyBits to copy
it to the printing GrafPort.  However, this was failing for some
bitmaps;  the printer would just print a digit at the left hand side
of the paper and stop.

I thought that the problem might relate to the warning in IM I that
"A good rule of thumb is not to copy more than 3K bytes."  So I tried
copying the bitmap as a number of horizontal strips, as follows:

	#define CHUNK_BITS  100000L
	#define SLOP    4
	SetPort( (GrafPtr) g_print_port_p );
	dest_rect = (*g_print_port_p).gPort.portRect;
	chunks = (int) ( ((long)dest_rect.right *
		(long)dest_rect.bottom + CHUNK_BITS) / CHUNK_BITS );
	chunk_depth = dest_rect.bottom / chunks;
	dest_rect.top = 0;
	dest_rect.bottom = chunk_depth;
	for (which_chunk = 0; which_chunk < chunks; which_chunk++)
	{
		CopyBits( &g_offscreen_GrafPort.portBits,
			&(*g_print_port_p).gPort.portBits,
			&dest_rect,
			&dest_rect,
			srcCopy, nil );
		dest_rect.top = dest_rect.bottom + SLOP;
		dest_rect.bottom += chunk_depth;
	}

That seems to have cured the problem I mentioned, but because of SLOP,
the printed image has some thin horizontal white lines through it.
But if I decrease SLOP at all, then the Imagewriter backs up at each
chunk boundary, causing noisy and inefficient printing.
-- 

   Jim Walker  jwwalker@usceast.cs.scarolina.edu  76367.2271@compuserve.com