Radial gradient increase effect - can OpenGL-ES improve?

Question

I am working on an image processing application that applies a radial gradient to an image downloaded from a photo library.

On the screen, I have a slider for dynamically increasing / decreasing the radius of the radial gradient. I found that the performance on the simulator is very good, but on the iPhone 3G or 3GS it is much slower to redraw when moving the slider.

I am currently using CGContextDrawRadialGradient for drawing. The steps that I follow for each redraw:

• Create a graphics context: UIGraphicsBeginImageContext(size) ;
• Create Gradient Object: CGGradientCreateWithColorComponents
• Draw an image (photo downloaded from the photo library) into the screen scale: drawInRect
• Set the blend mode of the blend: CGContextSetBlendMode
• Draw a gradient: CGContextDrawRadialGradient
• Create a UIimage using UIGraphicsGetImageFromCurrentImageContext ();
• UIGraphicsEndImageContext();
• Draw the final image on the screen: drawInRect .

Is there a faster way to draw? Perhaps using OpenGL?

Any suggestions / code examples would be appreciated.

Thanks.

Answer 1

Here are some ideas that can improve performance.
(assuming you are using a gradient to add vignetting to the image):

• cache your drawing image in CGLayer , which is the same size as the view, this avoids scaling every time you need to draw it.
• snap a radial gradient drawing to another CGLayer .
• when you should show the result :
  • draw an image layer
  • and then set up overlay overlay
  • followed by a gradient layer
  • all in the context of a drawing view.
• when the user moves the slider , you can:
  • redraw the gradient layer , then the composition.
  • redraw the composition while scaling the gradient layer, and when use pushes the finger away from the slider, redraw the gradient layer, then draw it again.