[cairo] Simplifying the operator set

[Bill Spitzak]

On Thursday 06 January 2005 10:45 pm, Carl Worth wrote:

>         typedef enum cairo_operator {
>             CAIRO_OPERATOR_SOURCE
>             CAIRO_OPERATOR_OVER,
>             CAIRO_OPERATOR_IN,
>             CAIRO_OPERATOR_OUT,
>
>             CAIRO_OPERATOR_DEST,
>             CAIRO_OPERATOR_DEST_OVER,
>             CAIRO_OPERATOR_DEST_IN,
>             CAIRO_OPERATOR_DEST_OUT,
>
>             CAIRO_OPERATOR_ADD,
>             CAIRO_OPERATOR_SATURATE
>         } cairo_operator_t;

I think this is very much the right approach. Main comments are:

"DEST" is in fact a no-op, right? Maybe it should be eliminated too?

"DEST_OVER" can be called "UNDER" which might be easier to read.

I have seen "DEST_IN" called "MASK" and "DEST_OUT" called "STENCIL", but I 
never thought of that as being too clear. However the user will probably see 
these operators as "change an area in the dest to black" so "DEST_OUT" might 
be called "ERASE". I can't think of anything better for "DEST_IN" than 
"INVERSE_ERASE"

"ADD" is really an addition. The alphas have to be added together. It is a 
unique operation in that it is the only one that needs to clamp it's output 
even if the inputs are clamped. This is in fact an entire subset of it's own.

A useful operator we call "SCRN" (but some other software calls "UNION") is to 
do A+B-AB for all the channels. This looks a lot like a double exposure on a 
gamma 2.2 display.

I'm not sure what "SATURATE" does, but there is a whole group of operators I 
call the "photoshop operators". The common thing about them is that the alpha 
is always a+b-ab. The colors are then various calculations to produce 
photographic effects. Technically "OVER" and "SCRN" fall into this catagory. 
As far as I can tell, SVG requires a large set of these, and I see no way of 
doing this correctly excepty by implementing them all in Cairo.

You mentioned getting rid of "CLEAR", and I agree. However there does need to 
be another drawing method along with fill and stroke, that ignores the path 
and thus performs the composition operation to the entire area inside the 
clip region. Though perhaps clearing the path and doing a fill with the 
INVERSE type should do this.

-- 
                   ,~,~,~,~ ~ ~ ~ ~
     /\_       _|_========___         Bill Spitzak
 ~~~/\/\\~~~~~~\____________/~~~~~~~~ spitzak at d2.com