[Mesa-dev] [PATCH 1/2] mesa : MESA_framebuffer_flip_y extension

[Chad Versace]

On Fri 15 Jun 2018, Fritz Koenig wrote:
> On Tue, Jun 12, 2018 at 12:28 PM Chad Versace <chadversary at chromium.org> wrote:
> >
> > On Thu 07 Jun 2018, Fritz Koenig wrote:

> > > --- a/src/mesa/main/extensions_table.h
> > > +++ b/src/mesa/main/extensions_table.h
> > > @@ -322,6 +322,7 @@ EXT(KHR_texture_compression_astc_hdr        , KHR_texture_compression_astc_hdr
> > >  EXT(KHR_texture_compression_astc_ldr        , KHR_texture_compression_astc_ldr       , GLL, GLC,  x , ES2, 2012)
> > >  EXT(KHR_texture_compression_astc_sliced_3d  , KHR_texture_compression_astc_sliced_3d , GLL, GLC,  x , ES2, 2015)
> > >
> > > +EXT(MESA_framebuffer_flip_y                 , MESA_framebuffer_flip_y                ,   x,   x,  x , ES2, 2018)
> >
> > Since the extension requires GLES 3.1, this row should have s/ES2/31/.
> >
> 
> I don't see the option for an ES31 extension, only ES2 in that file.

It's non-obvious, but placing a literal 31 in that column does the right
thing. Search the table, and you will find entries in this column with
literal values 30 and 31 in addition to ES2.

A little bit of poorly documented tribal knowledge... In the taxonomy of
OpenGL (and ES) contexts, there currently exist four top-level branches.
It took multiple years of API specification experiments, some good and
other mistakes, to create the top-level taxonomy. Surprisingly, old man
GLX is the only API to get this correct, in extension
GLX_EXT_create_context_es2_profile. (GLX got it right probably because
GLX_EXT_create_context_es2_profile was the last extension to be written
in the group of "context taxonomy" extensions, and so was written after
everyone learned from the past mistakes).

The term used by GLX_EXT_create_context_es2_profile for this hiearchy in
the taxonomy is "profile". Here are the 4:

  ES1 := { OpenGL ES x.y where x = 1 }
       = { OpenGL ES 1.0
         , OpenGL ES 1.1,
         , and minor variants of which no one remembers the name
         }

  ES2 := { OpenGL ES x.y where 2 <= x <= 3 }

  CORE := { OpenGL x.y where x.y >= 3.2
                         and glGetIntegerv(GL_CONTEXT_PROFILE_MASK) contains GL_CONTEXT_PROFILE_CORE_BIT
          }

  COMPATIBILITY := { OpenGL x.y where x.y <= 3.0
                                   or (x.y >= 3.2
                                       and glGetIntegerv(GL_CONTEXT_PROFILE_MASK) contains GL_CONTEXT_PROFILE_COMPATIBILITY_BIT)
                   }


That's why the file defines no token for ES31. It's a subclass of ES2.


The file defines ES2 = 0, and x = ~0.

When Mesa examines this
table in reponse glGetString(GL_EXTENSIONS), it scans the column for the
current context's profile. For each row it compares
(context_version >= column_value). That's why inserting 31 in the column
magically works. Instead of the macro ES2, we could equivalently insert
20 into the column, or even 0, since no context versions < 2.0 exist in
the ES2 class. (In fact, ES2 is 0).