Question

Or this code can be executed in a background thread safely?

    CGImageRef cgImage;
    CGContextRef context;
    CGColorSpaceRef colorSpace;

    // Sets the CoreGraphic Image to work on it.
    cgImage = [uiImage CGImage];

    // Sets the image's size.
    _width = CGImageGetWidth(cgImage);
    _height = CGImageGetHeight(cgImage);

    // Extracts the pixel informations and place it into the data.
    colorSpace = CGColorSpaceCreateDeviceRGB();
    _data = malloc(_width * _height * 4);
    context = CGBitmapContextCreate(_data, _width, _height, 8, 4 * _width, colorSpace,
                                    kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);

    // Adjusts position and invert the image.
    // The OpenGL uses the image data upside-down compared commom image files.
    CGContextTranslateCTM(context, 0, _height);
    CGContextScaleCTM(context, 1.0, -1.0);

    // Clears and ReDraw the image into the context.
    CGContextClearRect(context, CGRectMake(0, 0, _width, _height));
    CGContextDrawImage(context, CGRectMake(0, 0, _width, _height), cgImage);

    // Releases the context.
    CGContextRelease(context);

How to acheive the same result, if not?

(My problem is that I can’t see my OpenGL textures based on the output buffer of this method, if it runs in the background)

Answer 1

I think you might have trouble with running this code on a separate thread from GL’s like this. Even if it would work you might encounter half drawn images/textures. You could avoid this by creating a double buffer:
Your “_data” should be allocated only once and should hold 2 raw image data buffers. Then just create 2 pointers defined as foreground and background buffer (void *fg = _data[0], void *bg = _data[1] to begin with). Now when your method collects data from CGImage to bg just swap the pointers (then void *fg = _data[1], void *bg = _data[0] or the other way around)
Now your GL thread should fill your texture with data on fg (same thread as drawing).

Also you might need some locking mechanisms:

1. Before you push data to texture you should lock “buffer swap” and
   unlock it after the push.

2. You will probably want to know if the
   buffer has been swapped and only push fg data to texture in such
   case.

Also note that if you call GL methods on more then 1 thread you will have trouble in most cases.