Because Cogl defines the origin for texture as top left and offscreen draw
buffers can be used to render to textures, we (internally) force all
offscreen rendering to be upside down. (because OpenGL defines the origin
to be bottom left)
By forcing the users scene to be rendered upside down though we also reverse
the winding order of all the drawn triangles which may interfere with the
users use of backface culling. This patch ensures that we reverse the
winding order for a front face (if culling is in use) while rendering
offscreen so we don't conflict with the users back face culling.
Cogl's support for offscreen rendering was originally written just to support
the clutter_texture_new_from_actor API and due to lack of documentation and
several confusing - non orthogonal - side effects of using the API it wasn't
really possible to use directly.
This commit does a number of things:
- It removes {gl,gles}/cogl-fbo.{c,h} and adds shared cogl-draw-buffer.{c,h}
files instead which should be easier to maintain.
- internally CoglFbo objects are now called CoglDrawBuffers. A
CoglDrawBuffer is an abstract base class that is inherited from to
implement CoglOnscreen and CoglOffscreen draw buffers. CoglOffscreen draw
buffers will initially be used to support the
cogl_offscreen_new_to_texture API, and CoglOnscreen draw buffers will
start to be used internally to represent windows as we aim to migrate some
of Clutter's backend code to Cogl.
- It makes draw buffer objects the owners of the following state:
- viewport
- projection matrix stack
- modelview matrix stack
- clip state
(This means when you switch between draw buffers you will automatically be
switching to their associated viewport, matrix and clip state)
Aside from hopefully making cogl_offscreen_new_to_texture be more useful
short term by having simpler and well defined semantics for
cogl_set_draw_buffer, as mentioned above this is the first step for a couple
of other things:
- Its a step toward moving ownership for windows down from Clutter backends
into Cogl, by (internally at least) introducing the CoglOnscreen draw
buffer. Note: the plan is that cogl_set_draw_buffer will accept on or
offscreen draw buffer handles, and the "target" argument will become
redundant since we will instead query the type of the given draw buffer
handle.
- Because we have a common type for on and offscreen framebuffers we can
provide a unified API for framebuffer management. Things like:
- blitting between buffers
- managing ancillary buffers (e.g. attaching depth and stencil buffers)
- size requisition
- clearing
This relates back to an earlier commitment to stop using the OpenGL matrix
API which is considered deprecated. (ref 54159f5a1d)
The new texture matrix stacks are hung from a list of (internal only)
CoglTextureUnit structures which the CoglMaterial code internally references
via _cogl_get_texure_unit ().
So we would be left with only the cogl-matrix-stack code being responsible
for glMatrixMode, glLoadMatrix and glLoadIdentity this commit updates the
journal code so it now uses the matrix-stack API instead of GL directly.
The Journal can be considered a standalone component, so even though
it's currently only used to log quads, it seems better to split it
out into its own file.
