QuickTime 4 Reference

Result Color Limits

The clampMinimum and clampMaximum fields in a color component's gxTransferComponent structure define the allowable range of values for the result color in that component. Color values outside of the range are pinned to the nearest clamp limit. If clampMaximum is less than clampMinimum , the range allowed consists of values less than clampMaximum or greater than clampMinimum . Figure 54 shows the effect of clampMinimum and clampMaximum on drawing using blend mode.

Figure 54 The effect of result color limits on drawing

Note in Figure 54 that, when clampMinimum is less than clampMaximum , extreme color values cannot occur in the result. The portions of the "A" outside of the cloud are darker than they would normally be with blend mode, and the portions of the cloud outside of the letter are lighter than they would normally be. Conversely, when clampMaximum is less than clampMinimum , midrange values are not possible in the result. The background in the result is lighter than it would normally be with blend mode, and the portions of the cloud outside of the "A" are darker than they would normally be.

Note
Pinning restricts the value of the computation, not necessarily the value allowed for the actual pixel. The pixel value is the closest found to the computation, which may be outside of the range of clampMinimum and clampMaximum .