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I came across these terms in the Watt?s brothers? book on photorealistic modeling; I thought I?d share, and the terms are fairly meaningless without visual examples. Bias and tension are properties of control points (anchors), and we can use them when a curve is applied to a multi-color fill--in Photoshop (using Curves or Displacement), Xara, and other apps.
Tension determines the attraction of a curve to a control point, as shown in figure 1. When low tension is applied to a gradient fill, the fill?s break point is clearly defined (figure 2); when tension is high, the median tone becomes more prominant in the fill.
Bias refers to how a curve enters and exits a control point. In figure 1, when bias is applied leaving a curve, the curve skews toward the right; the converse is true when bias is applied entering a curve.
Different programs display different curves when bias and tension are adjusted, and Xara has no controls for applying bias or tension to a curve per se, but you manually achieve these math terms by adjusting the control point in the middle of a curve.
I?m just trying to provide terms here for effects we use every day, but might not have words for them!
In Photoshop, bias and tension can be controlled in various ways:
- using bezier control handles on a Path
- using the Curves filter
- using the Levels filter
- using the gradient editor
And in a backhanded way, i believe some specific blend modes can control this also.
Am i correct in this assumption Gare? :B
PS: Gary, did you not get my emails recently asking your permission to use you as a reference for a new project i'm doing fo a client? Could you please let me know if i may or may not do this? Thanks man.
Feel free to use me as a reference, but be forewarned that I'm about the best kept secret in digital graphics
You are correct in your riff on my bias and tension example, on all counts. Also, displacement maps can be explained as vector graphs, and these two can have tension and bias.
Bias and Tension 
Linear Mode
