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From: ken@turtlevax.UUCP (Ken Turkowski)
Newsgroups: net.graphics
Subject: Re: rasterops
Message-ID: <914@turtlevax.UUCP>
Date: Mon, 30-Sep-85 14:00:16 EDT
Article-I.D.: turtleva.914
Posted: Mon Sep 30 14:00:16 1985
Date-Received: Wed, 2-Oct-85 05:30:07 EDT
References: <588@stc-b.stc.UUCP> <5988@utzoo.UUCP>
Reply-To: ken@turtlevax.UUCP (Ken Turkowski)
Organization: CADLINC, Inc. @ Menlo Park, CA
Lines: 61
Keywords: Rasterops for multi-bit systems
Summary:
In article <5988@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>> Henry, how about a succinct explanation/definition of rasterops?.
>
>RasterOp (also known as BitBlt) is the basic primitive operation for
>modern bit-map raster graphics. It combines a source rectangle of bits
>with a destination rectangle of bits, with a number of variations on the
>combining function. Using C notation, D=S gives straight copying, D|=S
>gives "overprinting", D^=S gives reversible overprinting that is very
>useful for things like highlighting and cursors, D&=~S gives erasing, a
>few other combinations give useful results. Most RasterOp implementations
>also support replication of a "texture" over a rectangle, again potentially
>using any of the combining functions. There are various other complexities
>that some implementations add: clipping, combining two source rectangles
>before working on the destination, etc. Extensions to handle more than
>one bit per pixel are straightforward although the combining functions
>become much more complicated.
I would like to clarify the application of RasterOp to multi-bit
systems.
Four functions are readily extended to multi-bit and color systems:
OVERWRITE, FLIP, MIX, and ERASE. For overwriting, one simply replaces
the destination by the source (D=S).
FLIPping is used mostly for cursors and rubber-band lines; its only
useful definition is that there is no net change after doing a FLIP
with both source and destinations the same. This is usually
implemented by performing a generalized exclusive-OR of the source with
the destination.
MIX is a function that merges the features of the source with the
destination. Determination of what constitutes "feature" is simplified
if the source is of type "alpha", where alpha is a measure of opacity
(somewhere between 0 and 1, inclusive). Otherwise, an alpha mask has
to be somehow extracted from the source. The defining equation for
each pixel is:
D' = (1-alpha) * D + alpha * S
= D + alpha * (S - D)
where D' is the new value to be plugged into the frame store, D is the
old value in the framestore, and S is the value in the source. The
multiplication and addition should be appropriately interpreted in the
algebra of the color space. Logical-OR, saturating-add, and average
are some operations which have been used to implement the MIX function
when the source is not alpha.
ERASE is defined to be the pseudo-inverse of MIX. Theoretically, the
original values in the frame store can be recovered as long as
alpha!=1:
D = (D' - alpha * S) / (1 - alpha)
Quantization may prevent perfect erasure. Some useful convention is
used when alpha==0, because the above equation yields 0/0.
As Henry indicates above, it is also useful to modulate the above
functions with a periodic texture.
Any other functions applied to either single- or multi-bit RasterOps
are difficult to understand and have limited utility.
--
Ken Turkowski @ CADLINC, Menlo Park, CA
UUCP: {amd,decwrl,hplabs,seismo,spar}!turtlevax!ken
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