objective-c - 苹果机，OpenGL 4 : drawing on FrameBuffer does not work

Question

我尝试将简单的 OpenGL 绘图实现为 Framebuffer 对象，如 described here 。我的代码编译得很好，但是没有绘制几个 2D 三角形的形状，并且输出图片没有从片段着色器获取颜色。我花了很多时间，但没有找到解决方案。请帮助我。

执行期间没有异常或 OpenGL 错误。使用断点，我看到缓冲区、着色器程序、属性的所有索引都获得了适当的值。当我更改 glClearColor 时，我会得到具有指定大小和背景颜色的输出图像。我完全不知道问题出在哪里......我的代码在这里。

一般变量初始化:

GLuint framebuffer, renderbuffer;
GLuint program, uiVBO[1], uiVAO[1];
float ver[8];

_width = 512; _height = 512;
framebuffer = 0; renderbuffer = 0; program = 0;
uiVAO[0] = 0; uiVBO[0] = 0;

float pos = 1.0f; float neg = -1.0f;
ver[0] = pos; ver[1] = neg;
ver[2] = neg; ver[3] = neg;
ver[4] = pos; ver[5] = pos;
ver[6] = neg; ver[7] = pos;

创建帧和渲染缓冲区:

glGenFramebuffersEXT(1, &framebuffer);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer);
glGenRenderbuffersEXT(1, &renderbuffer);
glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderbuffer);
glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_RGBA8, _width, _height);
glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_RENDERBUFFER_EXT, renderbuffer);

if (glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT) != GL_FRAMEBUFFER_COMPLETE_EXT) {
    NSLog(@"Bad Framebuffer status!");
    assert(0);
}

绘图本身:

glClearColor(0.3, 0.3, 0.3, 0);
glClear(GL_COLOR_BUFFER_BIT);
GetError();

glUseProgram(program);

GLint atr = glGetAttribLocation(program, "inPosition");
glBindVertexArray(uiVAO[0]);
glEnableVertexAttribArray(atr);
GetError();

glBindBuffer(GL_ARRAY_BUFFER, uiVBO[0]);
glBufferData(GL_ARRAY_BUFFER, 4*2*sizeof(float), ver, GL_STATIC_DRAW);
glVertexAttribPointer(atr, 2, GL_FLOAT, GL_FALSE, 0, 0);
GetError();

// I also tried GL_ELEMENT_ARRAY_BUFFER and glDrawElements
// But no result is visible anyway... 
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
GetError();

glDisableVertexAttribArray(atr);
glBindVertexArray(0);

// Is it needed?
glFlush();
GetError();

// It just calls "glReadPixels"
[self saveResult];

其他东西，不是那么重要或只是明显正确:

/// Setting up OpenGL Context

 CGLContextObj context = NULL;
 CGLPixelFormatAttribute attributes[13] = {
     kCGLPFAOpenGLProfile,
     (CGLPixelFormatAttribute) kCGLOGLPVersion_GL4_Core,
     kCGLPFAAccelerated,
     kCGLPFAColorSize, (CGLPixelFormatAttribute)24,
     kCGLPFAAlphaSize, (CGLPixelFormatAttribute)8,
     kCGLPFADoubleBuffer,
     kCGLPFASampleBuffers, (CGLPixelFormatAttribute)1,
     kCGLPFASamples,  (CGLPixelFormatAttribute)4,
     (CGLPixelFormatAttribute) 0
 };
 CGLPixelFormatObj pix; CGLError errorCode; GLint num;
 errorCode = CGLChoosePixelFormat( attributes, &pix, &num );
 errorCode = CGLCreateContext(pix, NULL, &context);
 CGLDestroyPixelFormat( pix );
 errorCode = CGLSetCurrentContext( context );



/// Shaders creation

GLuint vertexShader = makeShader(GL_VERTEX_SHADER, loadFile(@"main", @"vsh"));
GLuint fragmentShader = makeShader(GL_FRAGMENT_SHADER, loadFile(@"main", @"fsh"));

program = glCreateProgram();
glAttachShader(program, vertexShader);
glAttachShader(program, fragmentShader);
glLinkProgram(program);
GLint status;
glGetProgramiv(program, GL_LINK_STATUS, &status);
if (0 == status) {
    NSLog(@"Failed to link shader program!\n");
    assert( 0 );
}


/// VAO, VBO creation

glGenVertexArrays(1, &uiVAO[0]);
glGenBuffers(1, &uiVBO[0]);
GetError();



/// Vertex shader:

    #version 410

    layout (location = 1) in vec2 inPosition;

    void main() {
        gl_Position = vec4(inPosition, 0.0, 1.0);
    }

/// Fragment shader:

    #version 410

    out vec4 outputColor;

    void main() {
        outputColor = vec4(1.0, 1.0, 0.5, 1.0);
    }



/// Extracts OpenGL error if it exists

#define GetError( )\
{\
for ( GLenum Error = glGetError( ); ( GL_NO_ERROR != Error ); Error = glGetError( ) )\
{\
switch ( Error )\
{\
case GL_INVALID_ENUM:      printf( "\n%s\n\n", "GL_INVALID_ENUM"      ); assert( 0 ); break;\
case GL_INVALID_VALUE:     printf( "\n%s\n\n", "GL_INVALID_VALUE"     ); assert( 0 ); break;\
case GL_INVALID_OPERATION: printf( "\n%s\n\n", "GL_INVALID_OPERATION" ); assert( 0 ); break;\
case GL_OUT_OF_MEMORY:     printf( "\n%s\n\n", "GL_OUT_OF_MEMORY"     ); assert( 0 ); break;\
default:                                                                              break;\
}\
}\
}


/// Creates shader by string

GLuint makeShader(GLenum type, NSString *cont) {
    const GLchar *source = (GLchar *)[cont cStringUsingEncoding:NSASCIIStringEncoding];
    GLuint shader;

    shader = glCreateShader(type);
    GetError();
    glShaderSource(shader, 1, &source, NULL);
    GetError();
    glCompileShader(shader);
    GetError();

    GLint logLength;

    glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &logLength);
    GetError();
    if (logLength > 0) {
        GLchar *log = (GLchar *)malloc((size_t)logLength);
        glGetShaderInfoLog(shader, logLength, &logLength, log);
        GetError();
        NSLog(@"Shader compilation failed with error:\n%s", log);
        free(log);
    }

    GLint status;
    glGetShaderiv(shader, GL_COMPILE_STATUS, &status);
    GetError();
    if (0 == status) {
        glDeleteShader(shader);
        GetError();
        NSLog(@"Shader compilation failed from code!");
        assert(0);
    }

    return shader;
}


/// Loads file to string

NSString* loadFile(NSString *name, NSString *ext) {
    NSString *absPath = [[NSBundle mainBundle] pathForResource:name ofType:ext];
    return [NSString stringWithContentsOfFile:absPath encoding:NSASCIIStringEncoding error:nil];
}

Answer 1

我会检查 glViewport 是否设置正确。