Re: Why Should Engineers and Scientists Be Worried About Col...
by jacobolus on Monday November 18, 2013 @04:16AM
(#6753430)
If the goal is to merely map elevation to color in a way that best reveals slope, then both colorings are pretty terrible. What you’d want instead is to have uniform change in lightness [lightness differences are the most salient perceptually].
The big problem with “rainbow” gradients is that they typically have some sections with steep change in lightness, and other sections of nearly uniform lightness, mixed together with nonuniform (in terms of human perception) hue and chroma changes from one part of the gradient to another. It’s very difficult to compare the slopes of a rainbow-gradient-mapped function across different parts of the gradient, and in some sections of the gradient very difficult to visually distinguish salient details.
It would be much better to have uniform changes in lightness. The simplest such map just goes from black to white in a perceptually uniform way. But if more distinctions are needed, then another possible method is to make the lightness attribute sort of sawtooth (or can potentially use more of a triangle wave, in some cases), while keeping chroma relatively constant and letting hue change close to uniformly. This makes a few places where the perceived lightness has either a change of slope or a discontinuity, but since the rate of lightness change is otherwise constant, it is still fairly easy to make comparisons. [I really should make a proper article/essay about this at some point, with some demonstrative images / interactive widgets: it’s much easier to show visually than to describe in text]
In the case of the chart under discussion, if the goal is to show the slope across the shoreline boundary and compare it to the slope elsewhere, I would recommend having uniform lightness change from dark [low elevation] to light [high elevation], combined with a hue discontinuity at the shore boundary. Because lightness is the most perceptually salient color attribute for noticing object boundaries, textures, fine details, motion, etc., it would still be possible to have a good idea of the slope across the shoreline boundary, while being able to perceive the boundary when closely examining the image.
[Note: such schemes will also generally be helpful for colorblind users]
The big problem with “rainbow” gradients is that they typically have some sections with steep change in lightness, and other sections of nearly uniform lightness, mixed together with nonuniform (in terms of human perception) hue and chroma changes from one part of the gradient to another. It’s very difficult to compare the slopes of a rainbow-gradient-mapped function across different parts of the gradient, and in some sections of the gradient very difficult to visually distinguish salient details.
It would be much better to have uniform changes in lightness. The simplest such map just goes from black to white in a perceptually uniform way. But if more distinctions are needed, then another possible method is to make the lightness attribute sort of sawtooth (or can potentially use more of a triangle wave, in some cases), while keeping chroma relatively constant and letting hue change close to uniformly. This makes a few places where the perceived lightness has either a change of slope or a discontinuity, but since the rate of lightness change is otherwise constant, it is still fairly easy to make comparisons. [I really should make a proper article/essay about this at some point, with some demonstrative images / interactive widgets: it’s much easier to show visually than to describe in text]
In the case of the chart under discussion, if the goal is to show the slope across the shoreline boundary and compare it to the slope elsewhere, I would recommend having uniform lightness change from dark [low elevation] to light [high elevation], combined with a hue discontinuity at the shore boundary. Because lightness is the most perceptually salient color attribute for noticing object boundaries, textures, fine details, motion, etc., it would still be possible to have a good idea of the slope across the shoreline boundary, while being able to perceive the boundary when closely examining the image.
[Note: such schemes will also generally be helpful for colorblind users]