Table of Contents Glide Programming Guide
Download 6.22 Mb.
|
4Chroma-keyingWhen chroma-keying is enabled, color values are compared to a global chroma-key reference value set by grChromakeyValue(). If the pixel’s color is the same as the chroma-key reference value, the pixel is discarded. The chroma-key comparison takes place before the color combine function; the other color selected by color combine function is the one compared (see grColorCombine() in Chapter Chapter 5. ). By default, chroma-keying is disabled. Chroma-keying is useful for certain types of sprite animation or blue-screening of textures. Only one color value is reserved for chroma-keyed transparency, while alpha blending reserves a variable number of color bits for transparency. void grChromakeyMode( GrChromakeyMode_t mode ) Use grChromakeyMode() to enable or disable chroma-keying. The argument, mode, specifies whether chroma-keying should enabled or disabled. Valid values are GR_CHROMAKEY_ENABLE and GR_CHROMAKEY_DISABLE. void grChromakeyValue( GrColor_t value ) The function grChromakeyValue() sets the global chroma-key reference value as a packed RGBA value in the format specified in the cFormat parameter to grSstWinOpen() (see Chapter Chapter 3. ). Example Special Effects.7 Simulating a blue-screen with chroma-keying. A blue screen is a compositing mechanism used in live video where a second scene overlays all the “blue” pixels in the first scene. This technique is used to stand a weathercaster in front of a weather map, for example, and explains why they don’t wear blue suits or ties! With chroma-keying, pixels of any one specific color can be discarded, not just blue. /* draw the background */ draw_weather_map(); /* enable chroma-keying */ grChromakeyMode(GR_CHROMAKEY_ENABLE); /*set the reference color - assumes ARGB format */ grChromakeyValue(0xFF); /* draw the inserted scene - most of it is blue */ draw_weatherman(); Download 6.22 Mb. Share with your friends:
|