Drawing into OpenGL ES framebuffer and getting UIImage from it on iPhone - iphone

Question

I'm trying to do offscreen rendering of some primitives in OpenGL ES on iOS. The code is as follows:
// context and neccesary buffers
#interface RendererGL
{
EAGLContext* myContext;
GLuint framebuffer;
GLuint colorRenderbuffer;
GLuint depthRenderbuffer;
}
.m file:
- (id) init
{
self = [super init];
if (self)
{
// initializing context
myContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
[EAGLContext setCurrentContext:myContext];
[self setupOpenGL]; // creating buffers
}
return self;
}
-(void) setupOpenGL
{
int width = 256;
int height = 256;
// generating buffers and binding them as Apple,s tutorial says
glGenFramebuffersOES(1, &framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glGenRenderbuffersOES(1, &colorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA8_OES, width, height);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, colorRenderbuffer);
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, width, height);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);
GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) ;
if(status != GL_FRAMEBUFFER_COMPLETE_OES) {
NSLog(#"failed to make complete framebuffer object %x", status);
}
}
- (UIImage *) renderImage
{
int width = 256;
int height = 256;
[EAGLContext setCurrentContext:myContext];
glEnable(GL_DEPTH_TEST);
// clear color - cyan
glClearColor(0.0f, 1.0f, 1.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// trying to draw some primitive - red line
float line[] = {-0.5f, -0.5f, 0.5f, 0.5f};
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer); //should I do this?
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT, 0, line);
glColor4f(1.0, 0.0, 0.0, 1.0);
glLineWidth(10);
glDrawArrays(GL_LINES, 0, 2); // draw line with two points
[myContext presentRenderbuffer:GL_RENDERBUFFER_OES]; // and this?
// then I grab image from _frameBuffer and return it as UIImage - this part is working
NSInteger x = 0, y = 0;
NSInteger dataLength = width * height * 4;
GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));
glPixelStorei(GL_PACK_ALIGNMENT, 4);
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
ref, NULL, true, kCGRenderingIntentDefault);
UIGraphicsBeginImageContext(CGSizeMake(width, height));
CGContextRef cgcontext = UIGraphicsGetCurrentContext();
CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, width, height), iref);
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
free(data);
CFRelease(ref);
CFRelease(colorspace);
CGImageRelease(iref);
return image;
}
the problem is that -renderImage returning completely cyan image, without red line. What can it be? Did I missed some actions before drawing line?

Answer 1

the first problem is that I was creating EAGLContext for OpenGL ES 2.0, but functions which I used were for OpenGL ES 1.1. The solution is to set constant to kEAGLRenderingAPIOpenGLES1
the second - I did not set model and projection matricies (if I use 1.1)
glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
after this all works)