iOS OpenGL ES 2.0: Offscreen render and save the result to an UIImage

Question

I'm using OpenGL ES to render some special effects, I don't want to show this to user, I just want to save the result as an UIImage, can anybody please help me? this is the code I'm using, I can get an image which contains the red clear color I use, but no geometry drawing shown. ``` #import "RendererGL.h" #import #import #import #import #import #import static NSInteger WIDTH_IN_PIXEL = 400; static NSInteger HEIGHT_IN_PIXEL = 300; typedef struct { GLKVector3 positionCoords; } SceneVertex; static const SceneVertex vertices[] = { {{-0.5f, -0.5f, 0.0}}, // lower left corner {{ 0.5f, -0.5f, 0.0}}, // lower right corner {{-0.5f, 0.5f, 0.0}} // upper left corner }; @implementation RendererGL { EAGLContext* _myContext; GLuint _framebuffer; GLuint _colorRenderbuffer; GLuint _depthRenderbuffer; GLuint _vertexBufferID; GLKBaseEffect *_baseEffect; } - (id) init { self = [s…

Accepted Answer

I had a very similar problem - to render some lines and get UIImage. I used OpenGL ES 1.1 and multisampling. I removed some additional code that does not refers to rendering and some OpenGL error checks. You can find the full code here: OSPRendererGL. Also, sorry for my one-for-all method.

```
@interface OSPRendererGL
{
    EAGLContext* myContext;
    GLuint framebuffer;
    GLuint colorRenderbuffer;
    GLuint depthRenderbuffer;
    GLuint _vertexArray;
    GLuint _vertexBuffer;
    GLuint resolveFramebuffer;
    GLuint msaaFramebuffer, msaaRenderbuffer, msaaDepthbuffer;

int width;
    int height;
}

@implementation OSPRendererGL

- (id) init
{
    self = [super init];
    if (self)
    {
        myContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];
        [EAGLContext setCurrentContext:myContext];
        [self setupOpenGL];
        [EAGLContext setCurrentContext:nil];
        width = 256;
        height = 256;
    }
    return self;
}

-(void) setupOpenGL
{
    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);

glGenFramebuffersOES(1, &msaaFramebuffer); 
    glGenRenderbuffersOES(1, &msaaRenderbuffer);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, msaaFramebuffer); 
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, msaaRenderbuffer);

glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_RGBA8_OES, width, height); 
    glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, msaaRenderbuffer);

glGenRenderbuffersOES(1, &msaaDepthbuffer);   
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, msaaDepthbuffer); 
    glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_DEPTH_COMPONENT16_OES, width, height); 
    glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, msaaDepthbuffer);

}

-(UIImage *) renderImageAtZoom:(int)zoom
{
    CGRect b = CGRectMake(0, 0, width, height);
    OSPCoordinateRect r = OSPRectForMapAreaInRect([self mapArea], b);

double_scale = b.size.width / r.size.x;
    double scale = 1.0/_scale;

[EAGLContext setCurrentContext:myContext];

glBindFramebuffer(GL_FRAMEBUFFER_OES, msaaFramebuffer);
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, msaaRenderbuffer);

glViewport(0, 0, width, height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrthof(0.0f, 256.0f, 256.0f, 0.0f, 1.0f, -1.0f);
    glMatrixMode(GL_MODELVIEW);

glPushMatrix();

glScalef(_scale, _scale, 1);
    glTranslatef(-r.origin.x, -r.origin.y, 0);

glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glEnable(GL_LINE_SMOOTH);
    glEnable(GL_POINT_SMOOTH);
    glEnable(GL_BLEND);

glClearColor(1, 1, 1, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// rendering here

glPopMatrix();

// msaa

glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, msaaFramebuffer); 
    glBindFramebufferOES(GL_DRAW_FRAMEBUFFER_APPLE, framebuffer);

glResolveMultisampleFramebufferAPPLE();

glBindFramebuffer(GL_FRAMEBUFFER_OES, framebuffer);
    glBindRenderbufferOES(GL_RENDERBUFFER, colorRenderbuffer);

// grabbing image from FBO

GLint backingWidth, backingHeight;

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

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,…

iOS OpenGL ES 2.0: Offscreen render and save the result to an UIImage

Solution

Problem