Can I use OpenGL for off-screen rendering? [duplicate]

Answer 1

I'm assuming that you know how to draw stuff to the screen with OpenGL, and you wrote a function such as drawStuff to do so.

First of all you have to decide how big you want your final render to be; I'm choosing a square here, with size 512x512. You can also use sizes that are not power of two, but to keep things simple let's stick to this format for now. Sometimes OpenGL gets picky about this issue.

const int width = 512;
const int height = 512;

Then you need three objects in order to create an offscreen drawing area; this is called a frame buffer object as user1118321 said.

GLuint color;
GLuint depth;
GLuint fbo;

The FBO stores a color buffer and a depth buffer; also you screen rendering area has these two buffers, but you don't want to use them because you don't want to draw to the screen. To create the FBO, you need to do something like the following only one time for instance at startup:

glGenTextures(1, &color);
glBindTexture(GL_TEXTURE_2D, color);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
glBindTexture(GL_TEXTURE_2D, 0);

glGenRenderbuffers(1, &depth);
glBindRenderbuffer(GL_RENDERBUFFER, depth);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);

glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depth);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

First you create a memory area to store pixel color, than one to store pixel depth (which in computer graphics is used to remove hidden surfaces), and finally you connect them to the FBO, which basically holds a reference to both. Consider as an example the first block, with 6 calls:

• glGenTextures creates a name for a texture; a name in OpenGL is simply an integer, because a string would be too inefficient.
• glBindTexture binds the texture to a target, namely GL_TEXTURE_2D; subsequent calls that specify that same target will operate on that texture.
• The 3rd, 4th and 5th call are specific to the target being manipulated, and you should refer to the OpenGL documentation for further information.
• The last call to glBindTexture unbinds the texture from the target. Since at some point you will hand control to your drawStuff function, which in turn will make its whole lot of OpenGL calls, you need to clear you workspace now, to avoid interference with the object that you have created.

To switch from screen rendering to offscreen rendering you could use a boolean variable somewhere in your program:

if (offscreen)
    glBindFramebuffer(GL_FRAMEBUFFER, fbo);
else
    glBindFramebuffer(GL_FRAMEBUFFER, 0);

drawStuff();

if (offscreen)
    saveToFile();

So, if offscreen is true you actually want drawStuff to interfere with fbo, because you want it to render the scene on it.

Function saveToFile is responsible for loading the result of the rendering and converting it to file. This is heavily dependent on the OS and language that you are using. As an example, on Mac OS X with C it would be something like the following:

void saveImage()
{
    void *imageData = malloc(width * height * 4);

    glBindTexture(GL_TEXTURE_2D, color);
    glGetTexImage(GL_TEXTURE_2D, 0, GL_BGRA, GL_UNSIGNED_BYTE, imageData);

    CGContextRef contextRef = CGBitmapContextCreate(imageData, width, height, 8, 4 * width, CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB), kCGImageAlphaPremultipliedLast);
    CGImageRef imageRef = CGBitmapContextCreateImage(contextRef);
    CFURLRef urlRef = (CFURLRef)[NSURL fileURLWithPath:@"/Users/JohnDoe/Documents/Output.png"];
    CGImageDestinationRef destRef = CGImageDestinationCreateWithURL(urlRef, kUTTypePNG, 1, NULL);
    CGImageDestinationAddImage(destRef, imageRef, nil);        
    CFRelease(destRef);

    glBindTexture(GL_TEXTURE_2D, 0);

    free(imageData);
}