Skip to main content

WebGL doesn't have query objects.


WebGL doesn't support query objects yet. There's not even an extension for them. It's a bit of an oversight, but stems from the fact that they aren't in OpenGL ES 2.0 (which WebGL is based on) either.

Queries are useful for a number of things. Timer queries can be used to extract detailed profiling information about your rendering. Occlusion queries can be used for culling objects before they're drawn (especially useful if you have an expensive fragment shader). And so on.

This is a nuisance for me because I was using an occlusion query to perform fast pixel-accurate collision detection for my c++ game Schroedinger's Cat. Now that I'm attempting to port the game to WebGL, I need to find an alternative approach.

Update:
I've written a follow-up to this post: OpenGL ES and occlusion queries.

Comments

  1. Hi Vilya

    Wish I'd realised this sooner! I was trying to get OpenCSG ported to Javascript/WebGL https://groups.google.com/forum/?fromgroups=#!topic/emscripten-discuss/COOHsqZ6mQI

    But without boolean queries, that's not possible. Did you ever find a work around?

    Thanks

    Marcos

    ReplyDelete
    Replies
    1. I haven't been able to find a *fast* workaround, no.

      For a slow workaround, you can read back the region of the framebuffer that you drew the player sprite to & compare it pixel-by-pixel to the original (unoccluded) player sprite. That would be _very_ slow though and I doubt it would help with your OpenCSG port. Sorry!

      Delete

Post a Comment

Popular posts from this blog

Assert no lock required

This is a technique I learnt about from Jason Gregory's excellent book, Game Engine Architecture (3rd Edition) . If you have a shared resource accessed by multiple threads, where you're fairly certain that it's only ever accessed by one thread at a time, you can use an assert() to check for this at debug time without having to pay the runtime cost of locking a mutex. The implementation is fairly straightforward: class UnnecessaryMutex { public: void lock() { assert(!_locked); _locked = true; } void unlock() { assert(_locked); _locked = false; } private: volatile bool _locked = false; }; #ifdef ENABLE_LOCK_ASSERTS #define BEGIN_ASSERT_LOCK_NOT_REQUIRED(mutex) (mutex).lock() #define END_ASSERT_LOCK_NOT_REQUIRED(mutex) (mutex).unlock() #else #define BEGIN_ASSERT_LOCK_NOT_REQUIRED(mutex) #define END_ASSERT_LOCK_NOT_REQUIRED(mutex) #endif Usage is equally straightforward: UnnecessaryMutex gMutex; void PossiblyOverlappingFunction

Triangle bounding boxes in a single byte

Just thought of a way to store the bounding box for a single triangle in only one byte. It's not really practical or something you'd ever really want to use, but what the hell. Assume we have some kind of indexed mesh structure with a list of vertex positions and a list of triangle indices:   struct Mesh {     std::vector<vec3> verts;     std::vector<uvec3> triangles;   }; We can find the bounding box of a triangle by taking the min and max of all three vertices:   vec3 Mesh::lowerBound(uint32_t tri) const {     vec3 v0 = verts[triangles[tri].x];     vec3 v1 = verts[triangles[tri].y];     vec3 v2 = verts[triangles[tri].z];     return min(min(v0, v1), v2);   }   vec3 Mesh::upperBound(uint32_t tri) const {     vec3 v0 = verts[triangles[tri].x];     vec3 v1 = verts[triangles[tri].y];     vec3 v2 = verts[triangles[tri].z];     return max(max(v0, v1), v2);   } This is nice and simple and probably way better than what I'm about to suggest. W

LD_DEBUG

Posting this mainly as a reminder to myself... If you ever find yourself needing to figure out a dynamic library loading problem on Linux, LD_DEBUG can be a massive help. This is an environment variable you can set to make the dynamic linker print out a ton of useful diagnostic info. There are a number of different values which control the amount and type of diagnostics printed. One of the values is help; if you set LD_DEBUG to this and run executable it will print out a list of all the available options along with brief descriptions. For example, on my Linux workstation at the office: > LD_DEBUG=help cat Valid options for the LD_DEBUG environment variable are: libs display library search paths reloc display relocation processing files display progress for input file symbols display symbol table processing bindings display information about symbol binding versions display version dependencies all all previous options combi