Keywords: normal maps

Summary

This demo includes several objects that utilize tangent space normal maps that are defined relative to the surface rather than the object’s local coordinate system. In this example, the same normal map is applied to the sides of a wooden crate, which requires the map to be rotated into the coordinate system relative to each side rather than be interpreted in the coordinate system of the entire object.

Related Materials

The following demos, manuals and tutorials can provide additional information about the topics at the focus of this demo:

Important Files

This section highlights key files important to the simulation.

The Geometry Setup

The geometry/demo.glist file assembles the floor, walls, crate and barrel objects into a scene. For the tangent space normal map option to work correctly, the tangent vector for the crate geometry should be explicitly compiled into the scene data model used by DIRSIG. This tangent data is what alows the normal map to be correctly and unambigously rotated into the plane of each side of the crate object. Requesting the tangent data is accomplished via the explicit tangent data option in the GLIST file:

The GLIST configuration for the crate object.
  <object>
    <basegeometry>
      <obj generatetangents="true">
        <filename>Crate.obj</filename>
      </obj>
    </basegeometry>
    <staticinstance>
      <translation>
        <point><x>-1.562</x><y>-1.648</y><z>0.616</z></point>
      </translation>
      <rotation units="radians" rotationorder="xyz">
        <cartesiantriple><x>0.0</x><y>0.0</y><z>0.0</z></cartesiantriple>
      </rotation>
      <scale>
        <cartesiantriple><x>0.01</x><y>0.01</y><z>0.01</z></cartesiantriple>
      </scale>
    </staticinstance>
  </object>

The Map Setup

The Crate material in the demo.mat file includes the normal map definition. The Crate.obj geometry was created with each side of the crate using it’s own UV space, meaning that the same normal map can be applied to each side of the crate. The normal map RGB image is shown below.

WoodCrate normal
Figure 1. The RGB normal map image for the crate sides.

The Crate material in the demo.mat file includes the normal map configuration using the UV projection method with the SPACE = TANGENT option:

MATERIAL_ENTRY {
    ID           = Crate
    NAME         = Crate (RGB, w/ Normal Map)
    DOUBLE_SIDED = TRUE
    RAD_SOLVER_NAME = Classic
    RAD_SOLVER {
        ENABLE_SAMPLED_DIFFUSE = FALSE
    }

    NORMAL_MAP {
        NAME = WoodCrate_normal.png
        UV_PROJECTOR {
            SPACE = TANGENT
            ORIGIN = IMAGE
            FLIPX = FALSE
            FLIPY = FALSE
            EXTENDX = MIRROR
            EXTENDY = MIRROR
        }
        IMAGE_FILENAME = WoodCrate_normal.png
        OPTIONS {
            SCALE = 4.0
        }
    }

    SURFACE_PROPERTIES {
        REFLECTANCE_PROP_NAME = RgbImage
        REFLECTANCE_PROP {
            IMAGE_FILENAME = WoodCrate_basecolor.png
            SCALE = 0.003921569
            UV_PROJECTOR {
                ORIGIN = IMAGE
                FLIPX = FALSE
                FLIPY = TRUE
                EXTENDX = MIRROR
                EXTENDY = MIRROR
            }
        }
    }
}
MATERIAL_ENTRY {

Without the TANGENT space option, the normal map will be interpreted in the object coordinate system. As a result, the RGB values in the normal map applied to the sides would produce normals that are generally in the +Z direction, which is in the plane of the crate sides rather than generally normal too it. The TANGENT option rotates the normal map into the tangent plane of the sides, so that the resulting normals are generally normal to the sides of the crate.

Simulations and Results

This simulation can be run in the GUI or via the command line:

$ dirsig5 demo.jsim

Load the resulting demo.img radiance file in the DIRSIG image viewer and display the RGB bands using the 2%$ and gamma = 2 range and scaling options.

demo
Figure 2. The RGB image simulation.

The RGB texture image can mask the effects of the normal map. To verify the impact of the normal map, the truth imagery can be used to visualize the surface orientation. The images below show the normal and view angle cosine truth, which confirm the manipulations of the normal across the otherwise flat sides of the crate.

tangent normal truth
Figure 3. The normal truth displayed as an RGB image.
view angle truth
Figure 4. The view angle cosine truth.

If the SPACE option in the map projector is set to OBJECT, the normal map will not be rotated into the coordinate system of each face, and the derived normals will be constant across a complex object. In this case, all the surfaces on the crate will have their normals pointing in the same direction:

object normal truth
Figure 5. The normal truth when using the OBJECT space projection.