bas relief model from a 3d model/scene?

Question

From Wikipedia: https://en.wikipedia.org/wiki/Relief#Bas-relief_or_low_relief

A low relief or bas-relief is a projecting image with a shallow overall depth, for example used on coins, on which all images are in low relief. In the lowest reliefs the relative depth of the elements shown is completely distorted, and if seen from the side the image makes no sense, but from the front the small variations in depth register as a three-dimensional image.

Example image from the same page:

This can obviously be done with sculpting, but I feel like there should be an easier/faster way to make such an effect (for example a coin, medallion, religious icon, etc.) from an existing 3d scene rather than recreating from scratch by sculpting. I'm not sure how, but I think rendering the scene in a specific way via the Compositing Nodes and then applying that rendered image as Displacement modifier on a grid mesh might give such an effect.

Here are some models from Blendswap someone trying to find a solution can use for testing: http://www.blendswap.com/blends/search?keywords=statue&is_fan_art=1&blend_license=&render_engine=&sort=downloads&direction=desc

Answer 1

Generating nice heightmaps for bas reliefs:

1) Add a camera facing the model(s).

2) If there are multiple models, make sure the models are the same distance from the camera and have the same "thickness" (Y scale/size in the camera's local Y axis), make the latter 1 blender unit.

3) Set materials to black, shadeless. Set background and ambient colors to white.

4) Set renderer to Blender Renderer.

5) Enable Mist. This is where the magic happens. Have the mist appear where the closest polygons to the camera are, have it completely fade in when the farthest polygon from the camera is located. If the camera is 4 units away from the closest point of the model, set Mist Start value to 4.0, and if the model has been scaled to 1 Blender unit in the Y axis, set the Mist Depth to 1.

A Bright/Contrast Node can be used in the compositing nodes to further adjust the rendered heightmap.

6) Important! Render in OpenEXR or another 32 bit format, otherwise the depth data stored in pixels won't look smooth when applied as a Displacement modifier later.

A model like this (download link in the question, in Blendswap),

Should render into something like this:

Use power-of-two resolutions for this image to save render time when used as heightmap. Use a high resolution, such as 2048x2048.

Now, add a plane and subdivide it 10 times, or any other flat mesh with many polygons. Add a Displacement modifier, and set its texture to the image you have generated. Change the Strength value until it looks good for you.

You should end up with something like this:

To make the edges of your relief less sharp, blur the edges of the model in the heightmap image, or in Blender Compositor nodes when generating that heightmap.

Done.

There is probably a different way to do this besides using Mist. The point is to just have the closest points to the camera colored completely black and the farthest yet visible completely white, or the opposite way.

Answer 2

A way to achieve the effect is by using bump maps or displacement maps (or combine them).

For either, you need a (grayscale) image of the relief you want to superimpose. You can create it from a 3d rendering, Andrew Price paints his map in his snow landscape tutorial at https://www.youtube.com/watch?v=82zWmOqE0Nc

Depending on the desired scene lighting you can use the resulting image as bump map (doesn't add geometry, useful in distance/face-on views) or as displacement map (adds geometry, useful in closeup/angled view).

If you want to combine the two, use bump map and a displacement modifier (see also Andrew Price's tutorial).

To match the relief to an object with more complex geometry than a coin or medallion, you'd probably have to map the relief onto a UV-unwrapped image of the object.

Hope this helps,

Rob.