Color spaces for color adjustment of digital images

Lightening or darkening images is a fundamental adjustment used to correct exposure or to improve aesthetics. However, these operations in regular RGB or YCC/CI ELAB spaces will create artifacts, like clipping, over or under saturation, or hue shift. We describe a new lightness adjustment method in lightness-chrominance color spaces. The problem is formulated geometrically, as the traversal of colors along curves in lightness-saturation planes. Saturation is adjusted concurrently when lightness is modified. This approach reduces clipping, it avoids insufficient saturation when dark colors are lightened and when light colors are darkened, and it also avoids over saturation when dark colors are darkened and light colors are lightened. Evaluation with test images shows good subjective results, and clipping is significantly reduced to about 5% of a prior approach.