OpenGL ES Async texture loading - ios

Question

Simple question, is it possible to load texture asynchronously with iOS and OpenGL ES ?
Here is my loading method, called on a separate thread:
//Image size
GLuint width = CGImageGetWidth(image.CGImage);
GLuint height = CGImageGetHeight(image.CGImage);
//Create context
void *imageData = malloc(height * width * 4);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(imageData, width, height, 8, 4 * width, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
//Prepare image
CGContextClearRect(context, CGRectMake(0, 0, width, height));
CGContextDrawImage(context, CGRectMake(0, 0, width, height), image.CGImage);
//Dispatch OpenGL stuff on main thread
dispatch_sync(dispatch_get_main_queue(), ^{
//Bind texture
glBindTexture(GL_TEXTURE_2D, name);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
});
//Release
CGContextRelease(context);
free(imageData);
If I don't dispatch OpenGL calls on the main thread, my textures wont be displayed...
Same question for the glDeleteTextures call...
Any idea ?

Answer 1

You need to use the same context on your background thread that you're using on the main one. For this use setCurrentContext:.
So on main thread create new thread (as an example the simplest way) and pass main context
[self performSelectorInBackground: #selector(loadTextureWithContext:) withObject: [EAGLContext currentContext]];
And the creation code:
-(void) loadTextureWithContext:(EAGLContext*) main_context {
[EAGLContext setCurrentContext: main_context];
//Image size
GLuint width = CGImageGetWidth(image.CGImage);
GLuint height = CGImageGetHeight(image.CGImage);
//Create context
void *imageData = malloc(height * width * 4);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(imageData, width, height, 8, 4 * width, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
//Prepare image
CGContextClearRect(context, CGRectMake(0, 0, width, height));
CGContextDrawImage(context, CGRectMake(0, 0, width, height), image.CGImage);
//Bind texture
glBindTexture(GL_TEXTURE_2D, name);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
//Release
CGContextRelease(context);
free(imageData);
[EAGLContext setCurrentContext: nil];
}
As an option you can also create the new context and share the same EAGLSharegroup with the main one.