Legacy Document

Inside Macintosh: Imaging With QuickDraw /

Chapter 3 - QuickDraw Drawing / QuickDraw Drawing Reference

Routines / Drawing Ovals

InvertOval

To invert the pixels enclosed by an oval, use the InvertOval procedure.

PROCEDURE InvertOval (r:\xDDRect);

r

The rectangle that defines the oval's boundary.

DESCRIPTION

The InvertOval procedure inverts the pixels enclosed by an oval just inside the bounding rectangle that you specify in the r parameter. Every white pixel becomes black and every black pixel becomes white. The pen location does not change.

SPECIAL CONSIDERATIONS

The InvertOval procedure was designed for 1-bit images in basic graphics ports. This procedure operates on color pixels in color graphics ports, but the results are predictable only with direct devices or 1-bit pixel maps. For indexed pixels, Color QuickDraw performs the inversion on the pixel indexes, which means the results depend entirely on the contents of the CLUT (which is described in the chapter "Color QuickDraw"). The eight colors used in basic QuickDraw are stored in a color table represented by the global variable QDColors. To display those eight basic QuickDraw colors on an indexed device, Color QuickDraw uses the Color Manager to obtain indexes to the colors in the CLUT that best map to the colors in the QDColors color table. Because the index, not the color value, is inverted, the results are unpredictable.

Inversion works better for direct pixels. Inverting a pure green, for example, that has red, green, and blue component values of $0000, $FFFF, and $0000 results in magenta, which has component values of $FFFF, $0000, and $FFFF.

The InvertOval procedure may move or purge memory blocks in the application heap. Your application should not call this procedure at interrupt time.