Overview

After Far Cry we changed our early Z pass to output the pixel Z value for alpha tested and fully opaque geometry into a texture so that we could do some features in a deferred way. We started with a R16f texture format which provided good quality but was not supported on all hardware. R16G16f was very well supported but not a native format for NVidia at that time. R32f was supported on all hardware with very good quality and fast as well.

However, if hardware allows lookups into the native Z buffer then this is preferred. That allows faster rendering when generating the data (double Z write performance), less memory requirements and sometimes even better quality. However, the lookup can get a bit more complicated as the z buffer might not return the linear Z directly.

Having the z value per pixel is beneficial for several features and effects:

• Deferred shadow mask generation.
• Soft z-clipped particles.
• Global volumetric fog.
• Fog volumes.
• Depth of field.
• Motion blur.
• Fill lights (used in CryENGINE 2 to locally darken or brighten up ambient).
• Screen-Space Ambient Occlusion.

By using deferred techniques we decouple the geometric scene complexity from the shader. For example, the shadow mask generation before was rendering all opaque scene objects with a shadow mask shader. That shader used the interpolated position in the shadow maps to output the shadow contribution to the shadow mask. This drawcall heavy pass was replaced by a deferred pass that reconstructed the position in the shadow maps from the stored pixel depth values. Instead of the numerous draw calls for all scene objects, just a single draw call for a fullscreen quad was required.

Sample image rendered by engine.

Linear z value of GBuffer.

Shadow mask generated from GBuffer data.

SSAO generated from GBuffer data.

Useful Console Variables

Toggles g-buffer pass.
Usage: r_UseZPass [0/1]
Default is 1 (on). Set to 0 to disable Z-pass. 2 to enable z-prepass for expensive materials