Question

I’m using GLKit in an OpenGL project. Everything is based on GLKView and GLKBaseEffect (no custom shaders). In my project I have several views that have GLKViews for showing 3D objects, and occasionally several of those view can be “open” at once (i.e. are in the modal view stack).

While until now everything was working great, in a new view I was creating I needed to have a rectangle with texture to simulate a measuring tape for the 3D world of my app. For some unknown reason, in that view only, the texture isn’t loaded right into the opengl context: the texture is loaded right by GLKTextureLoader, but when drawing the rectangle is black, and looking at the OpenGL frame in debug, I can see that an empty texture is loaded (there’s a reference to a texture, but it’s all zeroed out or null).

The shape I’m drawing is defined by: (it was originally a triangle strip, but I switched for triangles to make sure it’s not the issue)

static const GLfloat initTape[] = {
    -TAPE_WIDTH / 2.0f, 0, 0,
    TAPE_WIDTH / 2.0f, 0, 0,
    -TAPE_WIDTH / 2.0f, TAPE_INIT_LENGTH, 0,

    TAPE_WIDTH / 2.0f, 0, 0,
    TAPE_WIDTH / 2.0f, TAPE_INIT_LENGTH, 0,
    -TAPE_WIDTH / 2.0f, TAPE_INIT_LENGTH, 0,
};
static const GLfloat initTapeTex[] = {
    0, 0,
    1, 0,
    0, 1.0,

    1, 0,
    1, 1,
    0, 1,
};

I set the effect variable as:

    effect.transform.modelviewMatrix = modelview;
    effect.light0.enabled = GL_FALSE;

    // Projection setup
    GLfloat ratio = self.view.bounds.size.width/self.view.bounds.size.height;
    GLKMatrix4 projection = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(self.fov), ratio, 0.1f, 1000.0f);
    effect.transform.projectionMatrix = projection;


    // Set the color of the wireframe.
    if (tapeTex == nil) {
        NSError* error;
        tapeTex = [GLKTextureLoader textureWithContentsOfFile:[[[NSBundle mainBundle] URLForResource:@"ruler_texture" withExtension:@"png"] path] options:nil error:&error];
    }
    effect.texture2d0.enabled = GL_TRUE;
    effect.texture2d0.target = GLKTextureTarget2D;
    effect.texture2d0.envMode = GLKTextureEnvModeReplace;
    effect.texture2d0.name = tapeTex.name;

And the rendering loop is:

       [effect prepareToDraw];
        glDisable(GL_DEPTH_TEST);
        glDisable(GL_CULL_FACE);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glEnable(GL_BLEND);
        glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
        glEnableVertexAttribArray(GLKVertexAttribPosition);
        glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
        glVertexAttribPointer(GLKVertexAttribPosition, COORDS, GL_FLOAT, GL_FALSE, 0, tapeVerts);
        glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 0, tapeTexCoord);

        glDrawArrays(GL_TRIANGLES, 0, TAPE_VERTS);

        glDisableVertexAttribArray(GLKVertexAttribPosition);
        glDisableVertexAttribArray(GLKVertexAttribTexCoord0);

I’ve also tested the texture itself in another view with other objects and it works fine, so it’s not the texture file fault.

Any help would be greatly appreciated, as I’m stuck on this issue for over 3 days.

Update: Also, there are no glErrors during the rendering loop.

Answer 1

After many many days I’ve finally found my mistake – When using multiple openGL contexts, it’s important to create a GLKTextureLoader using a shareGroup, or else the textures aren’t necessarily loaded to the right context.

Instead of using the class method textureWithContentOf, every context needs it’s own GLKTextureLoader that is initialized with context.sharegroup, and use only that texture loader for that view. (actually the textures can be saved between different contexts, but I didn’t needed that feature of sharegroups).