Re: [9fans] new versions of graphics programs?

["Russ Cox" <rsc@plan9.bell-labs.com>]

It's hard to know which one is really right.
We decided that the most common use of DTransparent
was when allocating bitmaps, so it should be 0x00000000
(that is, the alpha channel should be 0, indicating
that the pixels are completely see-through).
This means that the transparent pixels, once put
into an image, have no effect when doing other
draw operations with those images.  Hence the comment
	DTransparent	= 0x00000000,		/* only useful for allocimage, memfillcolor */

We used to use 0x000000FF (that is, an alpha channel
of 255, indicating that the pixels are completely
opaque).  The reason for this was that then if you
were trying to create or manipulate masks, you'd want
the "transparent" color to have some substance to it,
so it could be copied from place to place (see for 
example /sys/src/cmd/page/rotate.c).  Thus I gave it
a solid alpha.

When you are creating images without an alpha 
channel, the lack of an alpha channel means the
distinction is lost (indeed, this is why
page/rotate.c still works with DTransparent = 0).

DTransparent really should be 0, so that when you
create an image filled with DTransparent and then draw
it on top of another image, nothing happens.
Before, you'd have blacked out the whole image.

This minor error hints at a bigger problems.  
Aside from creation, there is no way to do things
like fill a bitmap with a partially transparent
color.  And so on.  This is because the draw operator only
uses the Porter-Duff "over" operator.  I'm starting
to rework some of these things, and it looks like
being able to specify the operator as an argument
to gendraw will solve this and other problems.

Russ