Figure 7

Various alternative transparency functions for volume rendering based on colored image stacks. Different transparency functions offer various options in volume rendering of colored image stacks. (a) A linear transparency function based on grayscale-conversion of the image stack. (b) A threshold-based (threshold = 10) transparency function yielding a very hard volume rendering with completely opaque object voxels and completely transparent background voxels. The Amira Arithmetic module is used to transform the alpha channel of the RGBA stack using the threshold expression "Expr A: 255*(Aa > 10)", while the three color channels remain unchanged ("Expr R: Ar", "Expr G: Ag", "Expr B: Ab"). (c) A quadratic transparency function with decreasing slope of the curve yielding a rendering that is harder than using a linear function, but obviously softer as the threshold-based rendering. In the demonstrated example we again used the Amira Arithmetic module with the alpha channel expression "Expr A: 255-(pow((Aa*-1+255),3)/pow(255,2))". Alternatively one could also use a logarithmic function to obtain similar results. (d) A quadratic transparency function with increasing slope of the curve yielding a rendering that is much softer than using a linear function. In the demonstrated example we used the expression "Expr A: pow(Aa,3)/pow(255,2)". Alternatively one could also use an exponential function to obtain similar results.