In this tutorial we show you how to use polar coordinate conversion to fix a common problem in 3D or in effects boxes such as Inferno and Flame.

This tutorial is based on one of the more popular or funnier tips given at the 2004 Fxguide Live show in Las Vegas: fixing “cat’s arse”.

Polar Coordinates and polar transforms are just a different mathematical way of looking at the world. In a way it flattens out circular images into straight lines and it lets you paint textures that will work when warped around a sphere.

 

 

 

 

 

 

 

 

If we warp a flat rectangular texture around a sphere there is a pinching at each end of the texture mapping. This pinching is a dead give away but it can be easily fixed using Polar coordinates. The common nickname for this pinching is “cat’s arse”. The example below assumes that we want to warp a mountain range around a sphere in Action and then place our camera inside the sphere. This is a favourite trick at fxguide to simulate wide angle lens displacement and sell a flat image as an interactive matte painting.

This solution uses Photoshop since there is not polar conversion and inverse polar conversion in FFI or any of the common Sparks.

The steps are very simple and similar to the part one cog trick. Like the cog trick it is ideal to use a significantly higher resolution image that your final output. For example for a PAL job – a minimum file size would be HD 1920x 1080.

 

 

 

 

 

 

 

First you use Filter:Distort:Polar Coordinates to polar convert the image. Once it has been converted it is very easy to clone or patch out the “cat’s arse ” pinching. Once the file is repaired – perform the inverse operation and then export the image to Inferno or Flame.

 

 

 

 

 

 

 

 

Even with the Cat’s arse removed, there remains a seam problem in the textured sphere. This occurs because the texture from the left doesn’t meet or match the texture on the right of the image when it warps around.

 

 

 

 

 

 

 

 

This is easily repaired in either photoshop or flame by offsetting the image – cloning and patching and then undoing the offset.

 

Submitter: Mike Seymour