This is a programming issue, but the question is really about geometry. Let me 'splain. I am making a page control in HTML/CSS/JS *no wait - come back! I swear it's not about programming!!* that allows the user to play with the opacities of three overlapping images (eg. imageA = opacity 60, imageB=39, imageC opacity=1 ). The final 3 values must always sum to 100 (i.e. you cannot have 33,33,0). This means the space for the user to play in is not a simple 3D cube with full freedom along all 3 axes (such as many RGB controls have). So the user can freely play with the "joystick" control by moving their mouse through the space. Note that what the user does, and what the values do - do not have be the exact same "space", as long as the user can manipulate the numbers intuitively. My first solution is an equilateral triangle. No matter where they place the pointer in the triangle, we can measure the distance from each of the three apexes (or, to flip it over, we can measure the altitude from each of the three baselines). This works pretty well. The midpoint of a side will be some combination of 0,50,50. The dead centre will be 33,33,33. It's not a completely linear - or 1:1 - correlation between user movement and values - though it's close enough. I just wonder if I'm missing some geometrical shape that's tailor-made for such a circumstance. I wonder if a circle would make more sense. But a circle doesn't have three apexes ... apex's ... apeces. Ideas?