Re: [AD] clipping line algorithm

[Eric Botcazou <ebotcazou@xxxxxxxxxx>]

> We break the rule in rotate.c, the function
> _parallelogram_map_standard().

Hmm... yes, you're right, this is unfortunate.

> 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).

This semantics sounds very DOS-ish. Moreover, the "linear" qualifier is used 
in a broad acceptation since banked SVGA modes are "linear" for Allegro. It 
really means "which can be accessed linearly within each scanline".

I commited the attached patch to clarify the meaning of "linear".

> 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).

Yes, I think this could be the case for AllegroGL.

Bob, does the screen bitmap of AllegroGL support linear access within each 
scanline? If no, do you set BMP_ID_PLANAR for it?

> 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?

Unfortunately, I think this would break datafile compatibility, because 
bitmaps loaded from old datafiles would not satisfy is_linear_bitmap().

-- 
Eric Botcazou

Index: docs/src/allegro._tx
===================================================================
RCS file: /cvsroot/alleg/allegro/docs/src/allegro._tx,v
retrieving revision 1.178
diff -u -r1.178 allegro._tx
--- docs/src/allegro._tx	8 Sep 2003 20:34:20 -0000	1.178
+++ docs/src/allegro._tx	17 Sep 2003 11:20:06 -0000
@@ -3416,15 +3416,19 @@
    the pointers are equal, one is a sub-bitmap of the other, or they are 
    both sub-bitmaps of a common parent.
 
-@domain.hid @is_linear_bitmap(BITMAP *bmp);
-@domain.hid is_planar_bitmap, is_memory_bitmap
-   Returns TRUE if bmp is a linear bitmap, ie. a memory bitmap, mode 13h 
-   screen, or SVGA screen. Linear bitmaps can be used with the _putpixel(), 
-   _getpixel(), bmp_write_line(), and bmp_read_line() functions.
-
 @@int @is_planar_bitmap(BITMAP *bmp);
 @xref is_linear_bitmap, is_memory_bitmap
    Returns TRUE if bmp is a planar (mode-X or Xtended mode) screen bitmap.
+
+@@int @is_linear_bitmap(BITMAP *bmp);
+@xref is_planar_bitmap, is_memory_bitmap
+   Returns TRUE if bmp is a linear bitmap, i.e. a bitmap that can be accessed
+   linearly within each scanline (for example a memory bitmap, the DOS VGA
+   or SVGA screen, Windows bitmaps, etc). Linear bitmaps can be used with the
+   _putpixel(), _getpixel(), bmp_write_line(), and bmp_read_line() functions.
+
+   Historically there were only linear and planar bitmaps for Allegro, so
+   is_linear_bitmap is actually an alias for !is_planar_bitmap.
 
 @@int @is_memory_bitmap(BITMAP *bmp);
 @xref is_linear_bitmap, is_planar_bitmap