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Using a Bitmap Context for Offscreen Drawing

A bitmap graphics context in Quartz is analogous to an offscreen graphics world with user-supplied storage for the pixel map (NewGWorldFromPtr). You can create bitmap contexts with many different pixel formats, including 8-bit gray, 16-bit RGB, and 32 bit RGBA, ARGB, and CMYK.

You create a bitmap context by calling the function CGBitmapContextCreate and passing in a specific set of attributes, including a bitmap into which Quartz renders your drawing. You’re free to use the bitmap for other purposes—for example, you could create a bitmap context and a graphics world that share the same memory.

After drawing in a bitmap context, you can easily transfer the bitmap image to another Quartz context of any type. To maintain device independence, copying an image is a drawing operation and not a blitting operation. There are two steps:

1. Create a Quartz image from the bitmap. In Mac OS X v10.4 and later, you can use the function CGBitmapContextCreateImage.

2. Draw the image in the destination context using the function CGContextDrawImage.

For detailed information about creating and using bitmap contexts, see Graphics Contexts in Quartz 2D Programming Guide.

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