Rock surface

Part 1: Introduction
Part 2: Rock surface
Bump tool reference

In this part we’ll generate a bump map for the following image.

By looking at the rock surface we can guess its topology: the brighter areas are worn away, but in the cracks moss can survive on the accumulated dirt. We have a pretty good idea what part of the bump map should be dark (low) and bright (high). In this case the brighter the surface the higher it is. The moss, dirt and deep cracks are dark, so we’ll use the brightness channel to make the bump map.


Fire up FilterForge, choose the BumpTool filter and let’s see what do the default settings give us:

The bump map, shading only and the final surface

     Source channel     =  6
     Source blur        =  1
     Invert?              [ ]
     Offset of hills    =  0
     Offset of valleys  =  0
     Offset of slopes   = 50
     Smoothness max     = 20
     Smoothness weight  = 40
     Preview mode       =  6
     CrossSectionHoriz    [ ]
     CrossSectionPos    = 50
     Elevation profile  =  1
     Elevation noise    =  0
     ToneTriangleCurve    [ ]
     Tone phase shift   =  0
     AutoContrast         [X]
     Reflectivity       = 50
     Reflection blur    = 30
     Metallic           =  0 

Let’s go through the most important parameters. (Check the BumpTool reference sheet for more details.)

• Source channel: The channel to be extracted from the image, in the following order: Red/Green/Blue, Hue/Saturation/Brightness, Lightness/a/b, Cyan/Magenta/Yellow.

• Source blur: The radius of the gaussian blur applied to the source image. A small value removes high frequency noise without destroying the more important details.

• Offset of hills: The higher the number is the more the hilltops will be stretched upwards. Zero will not change the input image.

• Offset of valleys: The higher the number is the more the valleys will be stretched downwards. Zero will not change the input image.

• Offset of slopes: It adjusts the bumps so there are either more valleys (more to the left) or more hilltops (more to the right). Keeping the center in the middle changes nothing on the input image.

• Smoothness max: The general smoothness of the surface.

• Smoothness weight: How much the surface is smoothed. The lower the number the more the small details are visible.

And now we are going to tweak these parameters to improve the result.

The first thing which is apparent when looking at the bump map is the lack of contrast: the darkest pixel is not black and the brightest is not white. We can fix these with the Offset of hills and Offset of valleys sliders. [NOTE: In versions later than 2009-04-24, there is an AutoContrast feature which automatically fixes this issue, so you don’t have to bother with it.]

Drag them around to get a feel of what they do. You’ll see that dragging them too much will introduce flat areas at the bottom and top areas of the bump map. While this effect can be useful at times, now we would like to avoid them.
The plateaus might be hard to see in the preview window with the diffuse texture visible, so let’s move the Preview mode slider to 4. This mode highlights the flat areas, green at the bottom, red at the top, so it’s easier to see and fix the clipping.

To check if everything is all right, move the Preview mode back to 6.

The surface could use a bit more high frequency detail, so let’s type 0.5 to the source blur input field. The default 1 makes small details too smooth, while 0 introduces too much noise.

Finally move Smoothness weight to 50, so the influence of the big scale bump data increases.
This is how I set this up, but there is no “good” or “bad” settings, only ones you like and ones you don’t.

The tweaked bump map, normal map and the lit surface

When you are happy with the look of the preview, the only thing left is saving the bump map. Go to Filters menu, Render maps and select Bump map or Normal map to get the appropriate map rendered. That’s it.

Part 1: Introduction
Part 2: Rock surface
Bump tool reference