Isophotes

Isophotes are derived by shining parallel rays of light at a surface, then for a grid of points on the surface obtained by faceting, at every point calculating the surface normal and thereby the relative angle between the ray of light and the surface normal. The Isophotes are then bands of equal relative angle. The possible range of relative angles is from 0 to 90 degrees.

To display Isophotes the user should set the material to Isophote. Having selected the required option the user should click the icon to render the view

To increase the number of bands select the No. Of Bands option under Properties, enter the new value and click the Apply button. The user should always calculate the Isophotes for several different directions of parallel light rays. The direction is changed by selecting the Direction option and entering the X, Y and Z components of a vector defining the direction. The Colour Scheme option can be used to alter the number of colours used to shade the bands. In practice it is generally found the default of four colours gives the best result. Having altered any of the settings the user should click the Apply button.

When analysing a model with Isophotes, the user should be aware of the following characteristics. Flat planes will have a constant relative angle therefore they should appear to be all one colour whatever the angle of parallel light. Within an individual curved surface the Isophote bands should form smooth curves. At the boundaries of adjacent patches, the continuity condition can be determined as follows:

• If the Isophotes show distinct breaks then knuckle continuity has been applied.

• If the Isophotes are continuous but there is a sharp change in direction, then tangent continuity has been applied.

• If the Isophotes are continuous and the change in direction is smooth, then curvature continuity has been applied.

The following three figures show Isophotes on two surfaces with first knuckle continuity then tangent and finally curvature.

Isophotes on Two Surfaces with Knuckle Continuity

Isophotes on Two Surfaces with Tangent Continuity

Isophotes on Two Surfaces with Curvature Continuity