iOS：渲染到depthbuffer并从中读取，用于屏幕外处理 问题

我的主要目标是获取设备上触摸的模型坐标，以便检查您触摸的内容。我和一个大模型一起工作，必须画很多东西，这些东西也必须是可触摸的

为了实现这一点，我知道两种可能的方法：一方面，我们可以进行光线投射，并将相机的指向向量与模型相交，我们必须将其存储在内存中的某个位置。另一方面，这正是我想做的，我们可以用旧的方式来做：

function gluUnProject(winx, winy, winz: TGLdouble; 
                  const modelMatrix: TGLMatrixd4; 
                  const projMatrix: TGLMatrixd4; 
                  const viewport: TGLVectori4; 
                  objx, objy, objz

并将屏幕坐标转换回模型坐标。在这里之前我是对的吗？您知道opengl应用程序中触摸处理的其他方法吗？ 正如您所看到的，函数将winz作为参数，这将是屏幕坐标处片段的高度，该信息通常来自深度缓冲区。我已经意识到OpenGLES2.0并没有像在“普通”opengl中那样提供对其内部使用的深度缓冲区的访问。那么我怎样才能得到这些信息呢

提供两种可能性。使用深度附件创建屏幕外帧缓冲区，或者将深度信息渲染到纹理中。遗憾的是，该手册没有显示将信息读回iOS的方法。我想我必须使用glReadPixels并从中读取。我实现了我能找到的一切，但无论我如何设置，我都没有从屏幕外帧缓冲区或纹理中得到正确的高度返回结果。我希望得到一个带有z值的GL_浮点

z:28550323

---

r:72G:235B:191[3]：1井。。。使用深度缓冲区很棘手。它不是一个计算深度值的线性函数（它在接近近平截头体平面时具有更好的精度，而在接近远平截头体平面时会降低精度）。您将需要此函数并将其反转以能够转换。此外，我认为深度缓冲区是16位还是24位，我不确定它是否表示为浮点。

-(GLKVector4)unprojectScreenPoint:(GLKVector2)screen {

//get active viewport
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
NSLog(@"viewport [0]:%d [1]:%d [2]:%d [3]:%d", viewport[0], viewport[1], viewport[2], viewport[3]);

//get matrices
GLKMatrix4 projectionModelViewMatrix = GLKMatrix4Multiply(_modelViewMatrix, _projectionMatrix);
projectionModelViewMatrix = GLKMatrix4Invert(projectionModelViewMatrix, NULL);

//in ios Y is inverse
screen.v[1] = viewport[3]-screen.v[1];
NSLog(@"screen: [0]:%.2f [1]:%.2f", screen.v[0], screen.v[1]);

//read from the depth component of the last rendererd offscreen framebuffer
/*
GLubyte z;
glBindFramebuffer(GL_FRAMEBUFFER, _depthFramebuffer);
glReadPixels(screen.v[0], screen.v[1], 1, 1, GL_DEPTH_COMPONENT16, GL_UNSIGNED_BYTE, &z);
NSLog(@"z:%c", z);
*/

//read from the last rendererd depth texture
Byte rgb[4];
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, _depthTexture);
glReadPixels(screen.v[0], screen.v[1], 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &z);
glBindTexture(GL_TEXTURE_2D, 0);
NSLog(@"r:%d g:%d b:%d [3]:%d", rgb[0], rgb[1], rgb[2], rgb[3]);

GLKVector4 in = GLKVector4Make(screen.v[0], screen.v[1], 1, 1.0);

/* Map x and y from window coordinates */
in.v[0] = (in.v[0] - viewport[0]) / viewport[2];
in.v[1] = (in.v[1] - viewport[1]) / viewport[3];

/* Map to range -1 to 1 */
in.v[0] = in.v[0] * 2.0 - 1.0;
in.v[1] = in.v[1] * 2.0 - 1.0;
in.v[2] = in.v[2] * 2.0 - 1.0;

GLKVector4 out = GLKMatrix4MultiplyVector4(projectionModelViewMatrix, in);
if(out.v[3]==0.0) {
    NSLog(@"out.v[3]==0.0");
    return GLKVector4Make(0.0, 0.0, 0.0, 0.0);
}
out.v[0] /= out.v[3];
out.v[1] /= out.v[3];
out.v[2] /= out.v[3];

return out;

-(void)glkView:(GLKView *)view drawInRect:(CGRect)rect {
    
    glUseProgram(_program);
    
    //http://stackoverflow.com/questions/10761902/ios-glkit-and-back-to-default-framebuffer
    GLint defaultFBO;
    glGetIntegerv(GL_FRAMEBUFFER_BINDING_OES, &defaultFBO);
    GLint defaultRBO;
    glGetIntegerv(GL_RENDERBUFFER_BINDING_OES, &defaultRBO);
    //GLint defaultDepthRenderBuffer;
    //glGetIntegerv(GL_Depth_B, &defaultRBO);
    
    GLuint width, height;
    //width = height = 512;
    width = self.view.frame.size.width;
    height = self.view.frame.size.height;
    
    
    GLuint framebuffer;
    glGenFramebuffers(1, &framebuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
    
    /* method offscreen framebuffer
    GLuint depthRenderbuffer;
    glGenRenderbuffers(1, &depthRenderbuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, width, height);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);
    */
    
    //method render to texture
    glActiveTexture(GL_TEXTURE1);
    //https://github.com/rmaz/Shadow-Mapping        
    //http://developer.apple.com/library/ios/#documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/WorkingwithEAGLContexts/WorkingwithEAGLContexts.html
    GLuint depthTexture;
    glGenTextures(1, &depthTexture);
    glBindTexture(GL_TEXTURE_2D, depthTexture);
    
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    // we do not want to wrap, this will cause incorrect shadows to be rendered
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    // set up the depth compare function to check the shadow depth
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC_EXT, GL_LEQUAL);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE_EXT, GL_COMPARE_REF_TO_TEXTURE_EXT);
    
    //glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8_OES,  width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, width, height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_INT, 0);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depthTexture, 0);
    
    
    GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER) ;
    if(status != GL_FRAMEBUFFER_COMPLETE) {
        NSLog(@"failed to make complete framebuffer object %x", status);
    }
    
    GLenum glError = glGetError();
    if(GL_NO_ERROR != glError) {
        NSLog(@"Offscreen OpenGL Error: %d", glError);
    }
    
    glClear(GL_DEPTH_BUFFER_BIT);
    //glCullFace(GL_FRONT);
    glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
    glUniform1i(_uniforms.renderMode, 1);
    
    //        
    //Drawing calls
    //

    _depthTexture = depthTexture;
    //_depthFramebuffer = depthRenderbuffer;
    
    // Revert to the default framebuffer for now
    glBindFramebuffer(GL_FRAMEBUFFER, defaultFBO);
    glBindRenderbuffer(GL_RENDERBUFFER, defaultRBO);
    
    
    // Render normally
    glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
    glClearColor(0.316f, 0.50f, 0.86f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    //glCullFace(GL_BACK);
    
    glUniform1i(_uniforms.renderMode, 0);
    [self update];

    //        
    //Drawing calls
    //        
}

screen: [0]:604.00 [1]:348.00
r:102 g:102 b:102 [3]:255

screen: [0]:330.00 [1]:566.00
r:73 g:48 b:32 [3]:255

screen: [0]:330.00 [1]:156.00
r:182 g:182 b:182 [3]:255

screen: [0]:266.00 [1]:790.00
r:80 g:127 b:219 [3]:255

screen: [0]:548.00 [1]:748.00
r:80 g:127 b:219 [3]:255