Creating an Image Mask
A Quartz bitmap image mask is used the same way an artist uses a silkscreen. A bitmap image mask determines how color is transferred, not which colors are used. Each sample value in the image mask specifies the amount that the current fill color is masked at a specific location. The sample value specifies the opacity of the mask. Larger values represent greater opacity and specify locations where Quartz paints less color. You can think of the sample value as an inverse alpha value. A value of 1 is transparent and 0 is opaque.
Image masks are 1, 2, 4, or 8 bits per component. For a 1-bit mask, a sample value of 1 specifies sections of the mask that block the current fill color. A sample value of 0 specifies sections of the mask that show the current fill color of the graphics state when the mask is painted. You can think of a 1-bit mask as black and white; samples either completely block paint or completely allow paint.
Image masks that have are 2, 4, or 8 bits per component represent grayscale values. Each component maps to a range of 0 to 1 using the following formula:
For example, a 4-bit mask has values that range from 0 to 1 in increments of 1/15 . Component values that are 0 or 1 represent the extremes—completely block paint and completely allow paint. Values between 0 and 1 allow partial painting using the formula 1 – MaskSampleValue. For example, if the sample value of an 8-bit mask scales to 0.7, color is painted as if it has an alpha value of (1 – 0.7), which is 0.3.
The function CGImageMaskCreate creates a Quartz image mask from bitmap image information that you supply and that is discussed in “Bitmap Image Information.” The information you supply to create an image mask is the same as what you supply to create an image, except that you do not supply colorspace information, a bitmap information constant, or a rendering intent, as you can see by looking at the function prototype in Listing 11-3.
Listing 11-3 The prototype for the function CGImageMaskCreate
CGImageRef CGImageMaskCreate ( |
size_t width, |
size_t height, |
size_t bitsPerComponent, |
size_t bitsPerPixel, |
size_t bytesPerRow, |
CGDataProviderRef provider, |
const float decode[], |
int shouldInterpolate |
); |
Quartz image masks provide one way to control where and how Quartz paints color. “Masking Images” shows how to use a Quartz image mask to mask an image or to clip a graphics context. The section also shows how to use color and grayscale images to achieve masking effects.
Last updated: 2007-12-11
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