绘制到OpenGL ES帧缓冲区并在iPhone上从中获取UIImage [英] Drawing into OpenGL ES framebuffer and getting UIImage from it on iPhone

问题描述

我正试图在iOS上的OpenGL ES中对一些基元进行屏幕外渲染。代码如下：

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文件：

- (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;
    }

问题是-renderImage返回完全青色的图像，没有红线。它能是什么？我在绘制线之前是否错过了一些操作？

the problem is that -renderImage returning completely cyan image, without red line. What can it be? Did I missed some actions before drawing line?

推荐答案

第一个问题是我正在为OpenGL ES 2.0创建EAGLContext，但是功能我使用的是OpenGL ES 1.1。解决方案是将常量设置为kEAGLRenderingAPIOpenGLES1

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

第二个 - 我没有设置模型和投影基质（如果我使用1.1）

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);

此后全部有效）

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