Asynchronous texture loading and sharegroups in OpenGL ES 2.0 and iOS - iphone

Question

I am trying to create a GLKView where I add cubes and draw them. The problem is, each cube is of type NSObject and has its own vertex and texture buffers but I want to draw them in a single context. In order to do this, I followed some WWDC videos and created two contexts, one for rendering and one for texture loading, and I put both into the same sharegroup. Code-wise what I did in this respect, was adding a property called renderContext to my GLKView, which I want all cubes to be drawn in and I also set up a loaderContext property, where I want to load textures. However, nothing is drawn at all, I do not see anything, and sometimes I get a crash and GL ERROR 0x0500. It used to work and model view matrix should be setup correctly and everything. The introduction of the asynchronous loading and the two shared contexts caused the problem...
Here is the code:
This is the GLKView: Container(containing the cubes)
- (void)setupGL {
self.renderContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
self.loaderContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2 sharegroup:self.renderContext.sharegroup];
glGenFramebuffers(1, &defaultFrameBuffer);
glBindFramebuffer(GL_FRAMEBUFFER, defaultFrameBuffer);
glGenRenderbuffers(1, &depthBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, depthBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, self.bounds.size.width, self.bounds.size.height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBuffer);
glEnable(GL_DEPTH_TEST);
}
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect {
self.opaque = NO;
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
[EAGLContext setCurrentContext:self.renderContext];
for(Cube *cube in self.cubes){
[cube draw];
}
}
Each individual cube is set up like this:
-(id)init {
self = [super init];
if(self){
self.effect = [[GLKBaseEffect alloc]init];
self.effect.transform.projectionMatrix = GLKMatrix4MakePerspective(45.0f,0.95f, 0.1f, 2.0f);
self.effect.transform.projectionMatrix = GLKMatrix4Translate(self.effect.transform.projectionMatrix, 0, 0.0, 0.0);
self.effect.transform.modelviewMatrix = GLKMatrix4Translate(self.effect.transform.modelviewMatrix,0,0,-1.3);
glGenBuffers(1, &vertexArray);
glBindBuffer(GL_ARRAY_BUFFER, vertexArray);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
glVertexAttribPointer(GLKVertexAttribPosition,3,GL_FLOAT,GL_FALSE,0,0);
glGenBuffers(1, &texArray);
glBindBuffer(GL_ARRAY_BUFFER, texArray);
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
glBufferData(GL_ARRAY_BUFFER, sizeof(TexCoords), TexCoords, GL_STATIC_DRAW);
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 0,0);
}
return self;
}
And has a draw method:
-(void)draw{
[self.effect prepareToDraw];
self.effect.texture2d0.enabled = YES;
for(int i=0;i<6;i++){
if(i==0)glBindTexture(GL_TEXTURE_2D, frontTexture.name);
if(i==1)glBindTexture(GL_TEXTURE_2D, rightTexture.name);
if(i==2)glBindTexture(GL_TEXTURE_2D, backTexture.name);
if(i==3)glBindTexture(GL_TEXTURE_2D, leftTexture.name);
if(i==4)glBindTexture(GL_TEXTURE_2D, bottomTexture.name);
if(i==5)glBindTexture(GL_TEXTURE_2D, topTexture.name);
glDrawArrays(GL_TRIANGLES, i*6, 6);
}
}
Here is how I try to asynchronously load textures:
Note: The GLKView (container) is the parent of each individual cube, whose loaderContext I retrieve, which is in the renderContext's sharegroup, so textures should be drawn correctly, right ?
-(void)loadTextureForTexture:(GLKTextureInfo*)texN withView:(CubeView *)cV{
__block GLKTextureInfo *texName = texN;
EAGLContext *loaderContext = self.parent.loaderContext;
self.textureLoader = [[GLKTextureLoader alloc]initWithSharegroup:loaderContext.sharegroup];
[EAGLContext setCurrentContext:loaderContext];
NSDictionary *options = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:GLKTextureLoaderOriginBottomLeft];
dispatch_queue_t loaderQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t mainQueue = dispatch_get_main_queue();
[self.textureLoader textureWithCGImage:[self imageWithView:cV].CGImage options:options queue:loaderQueue completionHandler:^(GLKTextureInfo *tex, NSError *err){
texName = tex;
if(err)
NSLog(#"%#", err);
else
NSLog(#"no error");
dispatch_async(mainQueue, ^{
[self display];
});
}];
}

Answer 1

It looks like you’re doing more context management than is necessary:
GLKTextureLoader only needs to know which EAGLSharegroup to create its textures in. You don’t have to create an EAGLContext on its behalf, and the one you’re creating in your code already isn’t being passed into any GLKTextureLoader methods.
You should not manually need to manage the current EAGLContext—in fact, the documentation for GLKView specifically mentions that you should not change the current context from inside your drawing methods.
The net result of this is that other than extracting the EAGLSharegroup from your existing context for GLKTextureLoader creation, you should have no new context management code.
Additionally, it looks like the result of loading the texture never makes it out of loadTextureForTexture:withView:. Your texN variable is not passed into the function by reference, so texName is only visible to loadTextureForTexture:withView: and your texture loading completion block. Once loadTextureForTexture:withView: returns and your completion block is invoked, the data is gone. It seems like there should be some kind of CubeView setter that needs to be called with the GLKTextureInfo * you’ve received.

Answer 2

First problem I see in your code is absence of color attachment to the renderbuffer, so you only get depth output which would not draw anything.
glGenRenderbuffers(1, &colorBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, colorBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA4, self.bounds.size.width, self.bounds.size.height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT, GL_RENDERBUFFER, colorBuffer);