3 ms·

I have a bit of a complaint. I've used something similar to the "derivative based model" (DBM) for 3D shading before, and tested them on moonscapes. The angle-

by tabtab 8y ago

I have a bit of a complaint. I've used something similar to the "derivative based model" (DBM) for 3D shading before, and tested them on moonscapes. The angle-to-brightness curve can be adjusted as needed to avoid under or over-emphasizing mountain slopes. The "Andes" example is probably a case of exaggerating topology to illustrate relative differences, and NOT in inherent fault of slope-to-brightness maps (DBM-like). I can't say which one is more practical without going to the actual slopes to check the look and feel of the terrain and "feel" of the slopes. For technical accuracy, color coding is probably better (color-per-height), but slope-shaded maps are easier for regular folks to relate to. Ray tracing is probably better when dealing with direct shadows and reflections, such as found with buildings. But for high level maps with mountains and rivers, I believe DBM is just fine if tuned right, and won't differ noticeably from comparable ray tracing. To avoid misinterpretations, the large-scale shading should more or less mirror a semi-cloudy day such that direct shadows and reflections would play an extremely minor role anyhow. I should point out there is an element of subjectivity here. Illustrations often exaggerate to emphasize various attributes of the illustration's target. Often there is a trade-off in terms of aiding the quick mental digestion of information versus "native" accuracy. Abstractions lie on purpose, or we wouldn't call them "abstractions".

tylermw 8y ago
I do address all of your concerns in the article. For the angle-to-brightness curve, one of my footnotes says: "I believe his code only lacks a step that implements a minimum threshold on the slope before it gets colored--a step that would remove the problems seen in Figure 1 in flat areas." With regards to misinterpretations: "There's a tradeoff between realism and actual topographic information--real shadows can tell you about the relative height of two objects, but nothing about the slope of the land in the shadow itself." (I also mention how you can adapt the rayshading technique for a local model that doesn't have that problem) And later on in that paragraph, about subjectivity: "Thirdly, cartography has an artistic and practical component, and it’s true what reliefshading.com said in the non-computational shade it threw at global models–sometimes, you need to tweak the direction of the light hitting a particular mountain or valley because it looks better."
- tabtab 8y ago
  I still have to conclude the Andes example as given is misleading. It strongly implies ray-tracing "fixes" the problems shown. A footnote alone can't fix that (or is the wrong way to fix it). This clause under the picture is particularly problematic: "but it's disconnected from the underlying physics of what makes real shadows occur: rays of light not hitting a surface due to another surface in the way." -- As I mentioned, you often DON'T want that effect for the type of map shown, and that alone is not the main cause of the differences in the illustration. I'd bet money I can tweak the brightness-mapping curve to make the left look very similar to the right. (As somebody else mentioned, the actual curve depends on weather and local textures etc. such that both techniques are ultimately rough approximations.)
voidmain 8y ago
It looks to me like the original gradient based shading code is just wrong. Maybe there are numerical problems due to its frivolous use of trig functions, or more likely the way it calculates gradients is just broken. But shading based on slope should obviously make all zero slope areas (like water) the same color, and the fact that it doesn't means it isn't working. The raycasting code in this article, on the other hand, seems to be missing the Lambert term! Just because a surface is not in shadow doesn't mean that it will appear at uniform brightness. If it is diffuse/matte, which seems like a reasonable starting point for most terrain, each light ray L should have a contribution to the brightness of the point proportional to L dot N, where N is the normal vector of the surface (and to find that you need, sorry, the gradient). In short, light almost tangent to the surface doesn't illuminate it much. Other kinds of surfaces will have different BRDFs (e.g. they can be shiny) but none of them will have the completely unshaded behavior shown. Of course, it's fine to do totally nonphysical things if it helps emphasize some aspect of the data, but it doesn't seem to be done knowingly and I think it produces a misleadingly flat look.
- ffwacom 8y ago
  His dataset includes underwater topography
civility 8y ago
I have the same complaint. I couldn't think of a polite way to put it, so this won't be polite, but whenever someone compares their algorithm with a poor version of a competing technique, it makes me question their integrity and results.

by tabtab 8y ago

tylermw 8y ago

I do address all of your concerns in the article. For the angle-to-brightness curve, one of my footnotes says: "I believe his code only lacks a step that implements a minimum threshold on the slope before it gets colored--a step that would remove the problems seen in Figure 1 in flat areas." With regards to misinterpretations: "There's a tradeoff between realism and actual topographic information--real shadows can tell you about the relative height of two objects, but nothing about the slope of the land in the shadow itself." (I also mention how you can adapt the rayshading technique for a local model that doesn't have that problem) And later on in that paragraph, about subjectivity: "Thirdly, cartography has an artistic and practical component, and it’s true what reliefshading.com said in the non-computational shade it threw at global models–sometimes, you need to tweak the direction of the light hitting a particular mountain or valley because it looks better."

tabtab 8y ago
I still have to conclude the Andes example as given is misleading. It strongly implies ray-tracing "fixes" the problems shown. A footnote alone can't fix that (or is the wrong way to fix it). This clause under the picture is particularly problematic: "but it's disconnected from the underlying physics of what makes real shadows occur: rays of light not hitting a surface due to another surface in the way." -- As I mentioned, you often DON'T want that effect for the type of map shown, and that alone is not the main cause of the differences in the illustration. I'd bet money I can tweak the brightness-mapping curve to make the left look very similar to the right. (As somebody else mentioned, the actual curve depends on weather and local textures etc. such that both techniques are ultimately rough approximations.)

voidmain 8y ago

It looks to me like the original gradient based shading code is just wrong. Maybe there are numerical problems due to its frivolous use of trig functions, or more likely the way it calculates gradients is just broken. But shading based on slope should obviously make all zero slope areas (like water) the same color, and the fact that it doesn't means it isn't working. The raycasting code in this article, on the other hand, seems to be missing the Lambert term! Just because a surface is not in shadow doesn't mean that it will appear at uniform brightness. If it is diffuse/matte, which seems like a reasonable starting point for most terrain, each light ray L should have a contribution to the brightness of the point proportional to L dot N, where N is the normal vector of the surface (and to find that you need, sorry, the gradient). In short, light almost tangent to the surface doesn't illuminate it much. Other kinds of surfaces will have different BRDFs (e.g. they can be shiny) but none of them will have the completely unshaded behavior shown. Of course, it's fine to do totally nonphysical things if it helps emphasize some aspect of the data, but it doesn't seem to be done knowingly and I think it produces a misleadingly flat look.

ffwacom 8y ago
His dataset includes underwater topography

civility 8y ago

I have the same complaint. I couldn't think of a polite way to put it, so this won't be polite, but whenever someone compares their algorithm with a poor version of a competing technique, it makes me question their integrity and results.