Hi Robert, hi J-S
I'll remind the conclusions of our discussion in case someone new to the
topic is interested:
(1) We agreed that we may now remove the code forcing attachment of
color/depth buffers to FBO when user had not attached such buffer. To relax
FBO requirements its neeccessary to refactor DrawBuffer/ReadBuffer
management. Specifically, we need to allow for explicit setting of GL_NONE
for DrawBuffer & ReadBuffer. Current implementation does use GL_NONE flag
but interprets it more like "INHERIT BUFFER SETTING FROM PARENT" value. In
practice glDrawBuffer( GL_NONE ) and glReadBuffer( GL_NONE ) is not called
by OSG.
(2) We also agreed that it would be nice if we could add
setDrawBuffer/setReadBuffer methods to State and change buffers in 'lazy
update' manner.
I promised I would try to come up with solution and post a submission
implementing above proposals. I considered couple options and decided that
best approach to implementing (1) would be adding DRAW_BUFFER / READ_BUFFER
to CullSettings::VariablesMask flags. As we are going to allow for GL_NONE
buffers in their verbatim meaning, we need to specifcally define methods to
allow cameras inherit or override buffer settings from their parents. I
think it will be appropriate to handle this in similar way
Camera::_clearColor is handled. CullSettings::CLEAR_COLOR mask defines
whether ClearColor is inherited or overriden. CullSettings::DRAW_BUFFER and
CullSettings::READ_BUFFER could be introduced and interpreted in the same
way with Camera::_drawBuffer, Camera::_readBuffer variables.
Do you approve such a plan of extending CullSettings masks for use with
DrawBuffer and ReadBuffer ?
I also have a decision problem with feature (2): I wanted to add following
osg::State methods and variables
State::_drawBuffer
void State::setDrawBuffer( GLuint )
GLuint State::getDrawBuffer( void )
State::_readBuffer
void State::setReadBuffer( GLuint )
GLuint State::getReadBuffer( void )
Difficulty here is related to proper setting of default values of
State::_readBuffer and State::_drawBuffer. OpenGL spec says that defaults
depend on context. They are set to GL_BACK for double buffered contexts and
set to GL_FRONT for single buffered contexts. When State is created its not
yet known with which context it would be used.
I came up with 2 possible solutions:
(A) Changing State::_drawBuffer/_readBuffer defaults after associating State
with GraphicsContext. This is usually done in GraphicsWindow(or its
subclass)::init virtual method. This method would have to call
state->setDrawBuffer/setReadBuffer to initialize defaults properly.
Drawbacks:
- many classes deriving from GraphicsWindow - many changes need to be done.
- not sure what defaults should be applied for SharedContexts and external
contexts for windows created by the users.
- I am hesitant to modify all GraphicsWindow subclasses::init because I am
mainly Windows progrmammer. I'am afraid I may break something in Linux or
MacOS.
(B) Read buffer defaults ( glGet(GL_DRAW_BUFFER) ) set in GL Context when
State sees context for the first time. Good place for this would be
State::initializeExtensionProcs method.
Drawbacks:
- initializeExtensionProcs is mainly used for setting OpenGL context /
State capability variables like number of texture stages etc. Setting
DrawBuffer/ReadBuffer defaults would be (dangerous?) precedence of
StateAttributes/StateModes like initialization.
Which option do You prefer here ?
Cheers,
Wojtek Lewandowski
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