Hi all,
I have some issues with the eglMakeCurrent() specification.
What I’m trying to do is to have multiple threads that render to the same surface.
So I’m creating a single EGLSurface form the window native surface and then an EGLContext for each thread.
Each thread calls the eglMakeCurrent function passing the eglSurface (for both read/write) and its own eglContext.
In this way I have a context current only on one thread.
I tried this solution on the OpenGL|ES Java binding on the Android platform and when the second thread call the eglMakeCurrent() i get an EGL_BAD_ACCESS error. The documentation (http://www.khronos.org/opengles/sdk/1.1 … urrent.xml) says:
EGL_BAD_ACCESS is generated if context is current to some other thread.
and I’m sure that the context is a brand new context that cannot be current on any other thread.
So the OpenGL|ES documentation doesn’t specify clearly that my code is invalid.
But to be really sure that the problem is not my implementation that is buggy, I went an read the specification. In the section 3.7.3 of the eglspec.1.1.pdf (November 10, 2004) it’s written:
Only one rendering context may be in use, or current, for a particular thread at a given time, and only one context may be bound to a particular surface at a given time.
Because I’m sure that the OpenGL|ES Java binding on the J2ME platform allows to have multiple context bound to the same surface, I tried a native implementation.
Using the Imagination Technolgies SDK (PCEMULATION build 2.04.24.0811) I saw that I can bound multiple context to the same surface. That is, the multi-thread approch with each thread its own EGLContext works fine.
So here is the final question: who is doing wrong? Me, Android, Java or Imagination Technologies?
As the spec says, “only one context may be bound to a particular surface at a given time”. If an implementation lets you have a surface current in two or more threads at the same time that’s either an extension or a bug. And given that multiple threads accessing the same surface doesn’t exactly lead to a defined rendering result, it’s very likely the latter.
EGL_BAD_ACCESS is generated if context is current to some other thread or if the surface has already a context bound.
Is that right?
P.S.: @Xmas: do you read that using your SDK i was able to acces the same surface form different Threads? I think is something “powerful” and should be enabled. Than, of course, is responsability of the developer to synchronize the threads to have a “well defined” behaviour.
The last question related to the multithreading.
What if I create an EGLContext and an EGLSurface for each thread, but all the EGLSrufaces are created form the same native-window?
If there is already an EGLConfig associated with win (as a result of a previous eglCreateWindowSurface call), then an EGL_BAD_ALLOC error is generated.
