Re: [AD] clipping line algorithm

[Sven Sandberg <svens@xxxxxxxxxx>]

Eric Botcazou wrote:
> > With is_linear_bitmap(), according to its own documentation? See
> > http://alleg.sourceforge.net/onlinedocs/en/index009.html#is_linear_bitmap
>
> Are you sure that we would not get a bug report from someone who has wired 
> its own putpixel method to a linear bitmap? I think that Allegro has 
> guaranteed up to now that a user-set vtable will be used instead of the 
> default vtable in all cases. This would break the rule.

We break the rule in rotate.c, the function 
_parallelogram_map_standard(). OK, after looking up the definition of 
is_linear_bitmap() in include/allegro/inline/gfx.inl, I think I 
understand the problem: any bitmaps that Allegro knows about are either 
linear or planar, so there is only one flag for bmp->id that says which 
of them holds (the flag is called BMP_ID_PLANAR). But an add-on may 
provide a bitmap type that is not planar (so it would not be correct to 
set the flag) but does not support _putpixel (so it would not be correct 
for is_linear_bitmap() to return true). Here's a suggestion to fix this: 
Introduce a new flag for bmp->id, say BMP_ID_LINEAR. Allegro must set 
this flag if and only if it does not set BMP_ID_PLANAR. Add-ons that 
create non-planar non-linear bitmaps must set neither flag. Seems OK?

> If we decided to optimize the drawing functions, I think the test could only 
> be something like:
>
>     if (bmp->vtable->putpixel == _linear_vtable->putpixel)
>        use _putpixel
>     else
>        use bmp->vtable->putpixel
>
> _putpixel being the degenerated case (no clipping, no drawing mode) of 
> _linear_vtable->putpixel.

I suppose that would be fine too, although I don't like to force add-ons 
to break the specification of is_linear_bitmap(). (The advantage of this 
workaround is of course that it requires less modifications.)

Sven