From: Amos Shapir amos@nsof.co.il
Subject: Do bitblt function codes work?
Date: Sun, 31 Dec 1995 05:29:41 -0500 [thread overview]
Message-ID: <19951231102941.l39hs-9hUYGS3LrTWutrjMd2WM3Fc3orHDo1xxbKHP8@z> (raw)
In your message of Thu, 21 Dec 1995 08:53:56 GMT you wrote:
> > From: nsof.co.IL!amos (Amos Shapir)
> > On a PC with vga and "out of the box" drivers, the following program
> > generates only 4 distinct images, not 16. It seems somewhere along
> > the line to/from the bitblt server, the lower two bits of the function
> > codes get hardwired to (binary)10.
> This is correct behavior. Since you're drawing the line with code
> segment(d, Pt(mid.x, r.min.y), Pt(mid.x, r.max.y), ~0, t);
> using drawing function 0<=t<=15, you might expect 16 different
> results. But since the source function is the constant ~0, not a general
> value, symmetries fold up and the number of results is reduced.
> The possible answers are 0->0, 0->1, 1->0, and 1->1. It is not
> the full matrix of possibilities because the source is a single value (1).
> For related information, see the comment in /sys/src/libgnot/gsegment.c.
I have read this comment, and gather that this behavior is a feature;
but I still can't see the rationale behind it. How can the "value"
parameter be anything but a constant?
I tried all values, and I still can't find any combination that ANDs
the pixel values of two bitmaps; e.g. in the given example, draw just
the part of the segment which falls within the disc.
Happy new year, anyway
Amos Shapir Net: amos@nsof.co.il
Paper: nSOF Parallel Software, Ltd.
Givat-Hashlosha 48800, Israel
Tel: +972 3 9388551 Fax: +972 3 9388552 GEO: 34 55 15 E / 32 05 52 N
next reply other threads:[~1995-12-31 10:29 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
1995-12-31 10:29 Amos [this message]
-- strict thread matches above, loose matches on Subject: below --
1996-01-02 10:38 Amos
1995-12-31 14:58 dhog
1995-12-21 18:53 Rob
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