A few examples of paint brushes

CS 248 - Introduction to Computer Graphics
Autumn Quarter, 2001
Marc Levoy
Handout #4

Option 1 might be called "interpolation in RGB space." It is the default interpolation method using in most commercial paint programs that offer an "airbrush" tool.

Option 2 might be called "interpolation in HSV space." Since straight lines in RGB space are not straight in an HSV space, interpolating in HSV produces a different effect than interpolating in RGB. The exact difference depends on which HSV space is chosen. For example, linearly interpolating halfway from pure red, which is (R,G,B) = (255,0,0) (assuming 8-bit values), to pure green, which is (0,255,0), yields (128,128,0), which is a dark yellow. Using the single-hexcone HSV space described in section 13.3.4 of the textbook, pure red is (H,S,V) = (0^o,1,1), and pure green is (120^o,1,1). Interpolating halfway between them yields (60^o,1,1), or (255,255,0) in RGB space, which is pure yellow. This is the interpolation method we are asking you to implement in project #2.

Option 3 might be called "interpolation in RGB space with per-pixel replacement of HSV components." It is used by some commercial paint programs that support hue-only, saturation-only, or value-only airbrushing. (Photoshop calls these components "hue", "saturation", and "luminosity".) Implementing this option (in addition to option 2) will earn you extra credit in project #2; see the project handout.

Of couse, your mask will be a sampled approximation of the smooth functions shown above.
Note: C⁰ means means C-zero continuity.

levoy@cs.stanford.edu