Apple OSX OpenGL offline rendering with CoreGL

Question

I'm trying to render on OSX using CoreGL and a Framebuffer: it seems that the triangle rendering is not working at all, while for example a glClear with a color will be shown in the resulting PPM image.

I've tried several variation of the code below, but right now it seems to me that this is an impossible task on OSX.

#include <stdio.h>
#include <OpenGL/OpenGL.h>
#include <Opengl/glext.h>
#include <stdlib.h>
#include <assert.h>
#include <opengl/gl3.h>

int main(int argc, const char * argv[])
{

CGLContextObj context;
CGLPixelFormatAttribute attributes[13] = {
    kCGLPFAOpenGLProfile,
    (CGLPixelFormatAttribute) kCGLOGLPVersion_3_2_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 );
// -------
GLuint VertexArrayID;
glGenVertexArrays(1, &VertexArrayID);
glBindVertexArray(VertexArrayID);
//--------
GLuint fboid, rboid, dboid;
glGenFramebuffers( 1, &fboid );
glBindFramebuffer( GL_FRAMEBUFFER, fboid );

glGenRenderbuffers( 1, &rboid );
glBindRenderbuffer( GL_RENDERBUFFER, rboid );
glRenderbufferStorage( GL_RENDERBUFFER, GL_RGBA, 1440, 900);
glFramebufferRenderbuffer( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rboid );
glBindRenderbuffer( GL_RENDERBUFFER, 0 );

glGenRenderbuffers( 1, &dboid );
glBindRenderbuffer( GL_RENDERBUFFER, dboid );
glRenderbufferStorage( GL_RENDERBUFFER, GL_DEPTH_COMPONENT, 1440, 900);
glFramebufferRenderbuffer( GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, dboid );
glBindRenderbuffer( GL_RENDERBUFFER, 0 );

glBindFramebuffer(GL_FRAMEBUFFER, 0);
// --------

glBindFramebuffer( GL_FRAMEBUFFER, fboid );

static const GLfloat g_vertex_buffer_data[] = {-1.0f, -1.0f, 0.0f, 1.0f, -1.0f, 0.0f, 0.0f,  1.0f, 0.0f};
GLuint vertexbuffer;
glGenBuffers(1, &vertexbuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);
assert(glGetError()==0);
// -------
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0 );
glDrawArrays(GL_TRIANGLES, 0, 3); // Starting from vertex 0; 3 vertices total -> 1 triangle
glDisableVertexAttribArray(0);
// -------
errorCode = CGLFlushDrawable(context);

char data[1440*900*3];
glReadPixels(0, 0, 1440, 900, GL_BGR, GL_UNSIGNED_BYTE, &data[0]);
FILE* ff = fopen("/Users/invernizzi/pippo.ppm", "w");
char* header = "P6 1440 900 255 ";
size_t i = fwrite(header, sizeof(char), 16, ff);
i = fwrite(&data[0], sizeof(char), 1440*900*3, ff);
fclose(ff);

return 0;
}

Answer 1

If you have a 3.2 context, you have to create and bind a program with vertex and fragment programs. The fact that you can see a clear color shows the context is created correctly.