Inside Macintosh: Apple Game Sprockets Guide /

Chapter 2 - DrawSprocket / DrawSprocket Reference

DrawSprocket Functions / Using Alternate Buffers

DSpAltBuffer_RebuildTransparencyMask

Use the DSpAltBuffer_RebuildTransparencyMask function to rebuild the transparency mask associated with an alternate buffer being used as an overlay.

OSStatus DSpAltBuffer_RebuildTransparencyMask (
DSpAltBufferReference inAltBuffer,
UInt32 inTransparencyValue);

inAltBuffer

The buffer whose transparency mask is to be rebuilt.

inTransparencyValue

The pixel values that are to be transparent.

function result

A result code.

DESCRIPTION

You must rebuild the transparency masks associated with an alternate buffer if you are using the alternate buffer as an overlay in the following situations:

• You have just created the alternate buffer and rendered its image for the first time. Any time a context is activated, the transparency mask is built automatically.
• You have modified the overlay image in such a way that you need a different transparency mask.

The DSpAltBuffer_RebuildTransparencyMask function takes a reference to the buffer containing the transparency mask to be rebuilt in the inAltBuffer parameter and instructions as to which pixels to make transparent in the inTransparencyValue parameter.

If you have modified only a subsection of the alternate buffer, you can invalidate the rectangles enclosing the areas you have modified with the DSpAltBuffer_InvalRect function (page 2-65) and then rebuild the transparency mask for those areas only.

CALLING RESTRICTIONS

Do not call this function during an interrupt.