Threaded resize of ofImage

advanced
#1

I’m trying to use the gif encoder to save gifs of my app over a ~20 second period. While the saving is already multi-threaded in the library, I also need to resize the frames from a large (2160 x 1920) size, to something appropriate for a gif (540 x 480 or so). The problem is that the resize is intensive enough that it slows the framerate to a crawl. I’ve tried to use the multithreading example, but keep getting bad access exceptions. Does anyone have any experience (or a library) running resizing in a separate thread?

Thanks!

#2

the problem is that resize also updates the texture, you can’t do opengl calls in a different thread, what you can do is just disable the texture in the image while you are resizing it:

img.setUseTexture(false)

and then once it’s resized in the main thread, in update for example, enable it again:

img.setUseTexture(true)
img.update();

#3

Ah, perfect. Thanks so much!

#4

So I tried disabling the texture, and I know avoid having a bad access exception. That being said, I can’t seem to clone the image of the current frame (in the main thread) to the image to be resized (in the secondary thread). When I try to capture a frame I do the following:

  
   
        currentFrame.grabScreen(0,0,ofGetWidth(),ofGetHeight());  
        // lock access to the resource  
        thread.lock();  
        // copy image  
        currentFrame.setUseTexture(false);  
        currentFrame.clone(thread.toResize);  
        currentFrame.setUseTexture(true);  
        thread.imageFresh = true;  
        thread.resizeComplete = false;  
          
        // done with the resource  
        thread.unlock();

Where toResize is an ofImage in the thread, and currentFrame is an ofImage in the main thread. Current frame is fine, and gets an image of the correct dimensions (2160 x 1920) but the clone never seems to take, and the thread simply has a height and width of zero.

Would love to hear any thoughts!

#5

Might be easier to just use an ofPixels object so that you’re really only locking around and resizing just the pixels themselves.

#6

Good call: I did this and it simplified things considerably, since I ended up just working with ofPixels objects as shared thread resources.

Just to wrap up the thread: I was also having a problem where my screen captures were all black. This is apparently the result of known bug since 2010(!) that requires setBackgroundAuto to be true.

Thanks again!