Question

XCode has the ability to capture Opengl ES frames from the iPad, and that’s great! I would like to extend this functionality and capture an entire Opengl ES movie of my application. Is there a way for that?
if it’s not possible using XCode, how can i do it without much effort and big changes on my code? thank you very much!

Answer 1

I use a very simple technique, which requires just a few lines of code.

You can capture each OGL frame into UIImage using this code:

- (UIImage*)captureScreen {

    NSInteger dataLength = framebufferWidth * framebufferHeight * 4;

    // Allocate array.
    GLuint *buffer = (GLuint *) malloc(dataLength);
    GLuint *resultsBuffer = (GLuint *)malloc(dataLength);
    // Read data
    glReadPixels(0, 0, framebufferWidth, framebufferHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    // Flip vertical
    for(int y = 0; y < framebufferHeight; y++) {
        for(int x = 0; x < framebufferWidth; x++) {
            resultsBuffer[x + y * framebufferWidth] = buffer[x + (framebufferHeight - 1 - y) * framebufferWidth];
        }
    }

    free(buffer);

    // make data provider with data.
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, resultsBuffer, dataLength, releaseScreenshotData);

    // prep the ingredients
    const int bitsPerComponent = 8;
    const int bitsPerPixel = 4 * bitsPerComponent;
    const int bytesPerRow = 4 * framebufferWidth;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

    // make the cgimage
    CGImageRef imageRef = CGImageCreate(framebufferWidth, framebufferHeight, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
    CGColorSpaceRelease(colorSpaceRef);
    CGDataProviderRelease(provider);

    // then make the UIImage from that
    UIImage *image = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);

    return image;
}

Then you will capture each frame in your main loop:

- (void)onTimer {

    // Compute and render new frame
    [self update];

    // Recording
    if (recordingMode == RecordingModeMovie) {

        recordingFrameNum++;

        // Save frame
        UIImage *image = [self captureScreen];
        NSString *fileName = [NSString stringWithFormat:@"%d.jpg", (int)recordingFrameNum];
        [UIImageJPEGRepresentation(image, 1.0) writeToFile:[basePath stringByAppendingPathComponent:fileName] atomically:NO];
    }
}

At the end you will have tons of JPEG files which can be easily converted into a movie by
Time Lapse Assembler

If you want to have nice 30FPS movie, hard fix your calc steps to 1 / 30.0 sec per frame.