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It's useful to calculate sphere projection in clip space. One usage is to determine LOD of a model. As you might have known, sphere projection results in ellipse instead of sphere. In this post, a way to approximate radius of projected sphere is given. Assume the sphere is in the center of the screen and we know the \(FOV\) of the camera, sphere radius \(r\) and distance from camera \(z\). Looking at the diagram below: The approximate projection of sphere radius in clip space is given by: $$ approx\_projected\_radius = \frac{y}{y_{max}} = \frac{r}{z \times \tan (\frac{FOV}{2})} $$

Here are my growing links to GDC 2016 slides/presentations: Programming Advanced VR Rendering Performance  - Alex Vlachos An Excursion in Temporal Supersampling - Marco Salvi An Introduction to SPIR-V  - Neil Henning Building a Better Jump - J. Kyle Pittman Developing the Northlight Engine: Lessons Learned  - Ville Timonen Enlighten feature sets for large world games  - Geomerics Fast, Flexible, Physically-Based Volumetric Light Scattering – Nathan Hoobler Fixed Point Iteration  - Huw Bowles The runtime asset format forGL-based applications - Patrick Cozzi, Tony Parisi Improving Geometry Culling for ‘Deus Ex: Mankind Divided – Nicolas Trudel More Explosions, More Chaos, and Definitely More Blowing Stuff Up: Optimizations and New DirectX Features in Just Cause 3 - Antoine Cohade & Emil Persson Object Space Lighting  - Dan Baker Optimizing the Graphics Pipeline with Compute  - Graham Wihlidal Real-Time BC6H Compression on GPU  - Krzysztof Narkowicz Stable Spec