How to generate optimal normal maps in RealityCapture on simplified models.

Introduction

Normal maps are an excellent way to fake detail from very high polygon models onto models with simplified or decimated geometry. 

Our Original Model for these examples is a log, with a lot of surface detail of 2.4 million triangles:

Method 1 - Using the Simplification Tool

The easiest way to generate a normal map in RealityCapture is to enable normal reprojection in the simplify tool:

This is the perfect way to generate a normal map if you are only going to do one stage of simplification straight from your highest detailed model to your final simplified model. In this case we generated a model of 20,000 triangles, with a perfect normal map to fake all of the lost surface detail from the original model. You can view your normal map by dragging it onto a 2D view. You can also view it on the model in 3D view by clicking the eye next to it:

There are situations where more than one simplification step is used, and in these situations, the simplify tool might not generate an optimal normal map.

Method 2 - Using the Texture Reprojection Tool

Sometimes more than one simplification step is needed, and because the normal map is generated directly from the surface detail of the model being simplified, you can get a far better normal map by reprojecting your texture from the original model, with all of the fine detail from the scan.

For this example we added an extra stage of simplification to the 20,000 triangle model. We kept the detail from the 20,000 triangle model on the edges and finer parts of the model, but we decimated all of the flatter surfaces of the model using the relative type of simplification set to 10%. This further reduced the total triangle count down to just over 5000 triangles, without losing the most important geometry.

Here is the result of the second round of simplification:

We kept normal reprojection enabled, just so we can compare this normal map, with the one generated using texture reprojection.

We duplicated the model and then created a new unwrap of 4K in the duplicate, to make it ready for texture reprojection. Then we used the texture reprojection tool with these settings:

Now we have 2 models both with 5,100 triangles, and both with a 4K normal map. The first model "Selective simplification 1" had the normal map generated using the simplification tool from the 20,000 triangle model (the left 2D view in the image below).

The second model "selective simplification 2" had the normal map generated from the original 2.4 million triangle using the texture reprojection tool (the right 2D view in the image below).

Here you can see a comparison of the two 4K normal maps.

You can clearly see there is a lot more fine detail in the normal map from the texture reprojection.