Okay, a relatively simple solution, but how can we create our projection map? With our scene already set up, we can utilise the reflective properties of the glass object, in this case a bottle, to generate our caustic projection map. The key light is focused directly onto the bottle and rendered off with a square 1:1 aspect; if a non-square aspect is used, the projection map will be distorted and will not line up correctly when re-used. Note that there are imperfections in the glass; these are generated using bump mapping to simulate a mass-produced product that has been sealed and contains some heat distortion. These imperfections can add to the effect. The resulting render is of a black background with white highlights generated by the glass material.

With the key light Viewport rendered off, this image can be re-imported into max and used in a clone of the original key light. However, in the original photograph the caustic effect becomes dispersed and more diffused the further away it gets from the glass. To simulate this we can combine two (or more) copies of the rendered images with different blurring intensities. To combine these we can use a Mix map with an intense blur assigned to one copy and a lesser (or none) blur in the other. To control the mixing, a Gradient Ramp map is used to set how far away from the glass (or how far up in the Viewport if viewed from the key light’s Viewport) the maps should change. Due to this blurring, some intensity is lost, therefore the Output amount in the render’s Bitmap map should be significantly increased to generate that high intensity glow around the base of the glass where the light is immediately focussed.