The BumpTool filter is for generating a fake bump map from diffuse color. It’s not a substitute for hand sculpted or 3D scanned data but can be helpful when we just want a somewhat reasonable approximation fairly quickly.
Here are the descriptions of each parameter:
Source channel
The channel extracted from the image.
Source blur
The radius of the Gaussian blur on the source channel.
A small (0.5, 1) value is good for removing high frequency noise. A large number is usually used to create bump maps from black and white images. This blur is applied before any other operation.
Invert
Inverts the source channel.
Offset of hills
he white point of the bump map.
The higher the value, the higher the hills become, increasing contrast. If AutoContrast is on, then any value higher than 0 will introduce clipping.
Offset of valleys
The black point of the bump map.
The higher the value, the deeper the valleys become, increasing contrast. If AutoContrast is on, then any value higher than 0 will introduce clipping.
Offset of slopes
The gamma of the bump map.
A value less than 50 makes more valleys, while a value more than 50 makes more hilltops. Keeping the slider on 50 doesn’t change the balance of the image.
Smoothness max
The general smoothness of the surface.
Higher values produce a smoother surface.
The filter combines multiple layers of then differently blurred source channels. This value is defines the maximal radius of the blur passes.
Smoothness weight
The influence of the smoothed, low frequency bump data.
Higher values make small, high frequency details less visible.
Preview mode
The way the generated data is presented to the user.
Preview mode 1 shows the cross section of the image. It is useful for checking the profile of slopes. The processed pixel column can be changed with the CrossSectionPos slider. The CrossSectionHoriz checkbox let us view a horizontal slice.
Preview mode 2 is just an approximation of the actual bump map: The bump map is used as diffuse color. This means that a white pixel in the bump map might not be white in this preview mode, if there is not enough light reaching the surface. So while it gives us as an idea how the bump map is looking, for the technically correct bump map, go to the Filter/Render maps/Bump map menu.
Cross section horiz
Makes the cross section preview mode (1) sample a horizontal slice.
Cross section pos
Changes the location where the cross section preview mode (1) samples from. 0 means the top (or left) side of the image.
Elevation profile
Changes the profile of the slopes.
The default profile means the unprocessed image. The curve on the picture is an approximation of the curvature on a blurred image.
The noise profile’s random seed can be changed with the Variation slider. That slider is always present at the bottom of the control list.
Elevation noise
Adds noise to the chosen profile.
The higher the value is the more the noise affects the base profile.
Tone triangle curve
Changes the source channel’s tonal curve to “triangle”.
Black pixels remain blacks, 20% grays become 50% grays, 50% grays become white, 75% grays become 50%, whites become blacks.
It is usually used with the hue channel. When the hue data is translated into grayscale, colors close to red will have a big difference in brightness: oranges will be very dark while purples will be very bright. On a bump map these sudden height changes seldom look good. The triangle shaped tone map brings the extremes together.
Tone phase shift
Shifts the tonal range around.
It’s like palette rotation back in the day. The higher the value is the more the tone curve is shifted. Blacks become dark grays, dark drays become light grays, light grays become whites and whites become blacks.
I wrote above at the ToneTriangleCurve parameter that if that is turned on then 50% grays become whites. With Tone phase shift we can change what gray value will turn to white.
Auto contrast
It automatically stretches the histogram so it covers the whole brightness range. On extremely low contrast input images (usually “a” and “b” channels) it might not reach maximum contrast. In those cases the Offset of hills and Offset of valleys sliders can help.
Reflectivity
The reflectivity of the preview surface (Affects both specular highlights and reflections).
Reflection blur
The roughness of the surface.
Metallic
The amount of mirror like reflections.
I’d like to thank the following people for their help:
Sphinx for the CMYK extractor
Mike Blackney for his pixel color filter
uberzev for the Curve to shape filter
ThreeDee for the UnGauss and AutoLevels snippet