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I have come across this image that's labelled a bump map to be used with a 3D model. It looks like this:

enter image description here

In this map, which is an RGB map, it seems we have pixels whose normals face upwards shaded lighter, and normals pointing downward shaded darker. I have isolated just one part of this map, a little mound with a whole in the middle:

enter image description here

I've tried to recreate this shape in an image editing program just using white colour for height, it's essentially a heightmap, or bumpmap. It looks like this:

enter image description here

It's basically a circular mound that rises gradually, and then falls off steeply into a hole.

So I know that this is one way normals and height can be stored. This is essentially a heightmap, but some people and programs call this a bumpmap. In 3DS Max the bumpmap essentially expects a heightmap.

Then there's the normal map, which looks like this:

enter image description here

Inside the red channel the information about how flat or inclined the surface is from left to right:

enter image description here

And same as in the green, except from up to down:

enter image description here

So I understand this, I think. However in the original image, this strange "bumpmap", it only seems to contain the information of the flatness of the surface from up to down, not left to right. What I've shown is how it appears as an RGB image. Here is each of the channels:

enter image description here

enter image description here

enter image description here

They all contain the same information, that is, facing of the surface from up to down.

How is this map supposed work? I've come across a few maps like these but I don't see how they're supposed to work. Can someone explain if this is possible?

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Yeah, that doesn’t look like any bump, height, or normal map I’ve ever seen—as you’ve identified, there’s only information about the surface contour along a single axis. If anything, it looks like it’s meant to be overlaid on a texture as a cheap form of fake bump-mapping in an era where it was too expensive to do it for real.

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  • $\begingroup$ Looks to me like that contains the results of "shaded" a bump map. I suspect you'd need to (a) assume an incoming light direction (b) determine a height map (or a normal map) that then produces that shading given the assumed light direction. That would appear to be a rather underconstrained problem, but I guess you could start by assuming that the largest contiguous area of "constant" colour (well anything in the range 123~127), was of "middle height", and try "flood-filling" outwards and use weighted sums and multiple passes to get it to converge to a solution. $\endgroup$
    – Simon F
    Commented May 13, 2019 at 10:14

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