A question about the Alpha Values of images

  // Make half-transparent grey, the background color for the layer
   UIColor *Light_Grey 
      = [UIColor colorWithRed : 110/255.0 
                        green : 110/255.0 
                        blue  : 110/255.0 
                        alpha : 0.5];

  // Get a CGColor object with the same color values
   CGColorRef cgLight_Grey = [Light_Grey CGColor];
   [boxLayer setBackgroundColor : cgLight_Grey];

  // Create a UIImage
   UIImage *layerImage = [UIImage imageNamed : @"Test.png"];

  // Get the underlying CGImage
   CGImageRef image = [layerImage CGImage];

  // Put the CGImage on the layer
   [boxLayer setContents : (id) image];

Consider the above sample code segment.

UIColor *Light_Grey is set with an alpha value of 0.开发者_JS百科5. My question is : Is there anyway I can set the alpha value of CGImageRef image?

The reason of my asking this is even though the alpha value of boxLayer is 0.5, any images set on top of BoxLayer seem to have an alpha default value of 1, which would cover up anything lying directly underneath the images.

Hope that somebody knowledgable on this can help.

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It looks you can make a copy using CGImageCreate and use the decode array to rescale the alpha (e.g. 0.0-0.5)

decode

The decode array for the image. If you do not want to allow remapping of the image’s color values, pass NULL for the decode array. For each color component in the image’s color space (including the alpha component), a decode array provides a pair of values denoting the upper and lower limits of a range. For example, the decode array for a source image in the RGB color space would contain six entries total, consisting of one pair each for red, green, and blue. When the image is rendered, Quartz uses a linear transform to map the original component value into a relative number within your designated range that is appropriate for the destination color space.