Render pass (render elements)

A render pass (called render elements in some software) is a component of the final image saved as its own separate image. Direct light, indirect light, reflection, refraction, ambient occlusion, depth (Z-depth) and object or material masks are typical examples. Stacked together in a compositing or image-editing program, they rebuild the main image, but each one can be adjusted on its own.

What render passes are for

Their main benefit is flexibility. When a client asks for slightly weaker reflections on the glazing or a darker timber floor, a few minutes on the right pass can replace hours of re-rendering. Mask passes, usually flat-colored IDs for objects or materials, make it quick to select one surface precisely. A depth pass lets you add depth of field or haze afterwards.

Practical points

Each pass adds a little to file size and render time, so it pays to choose only the ones you will actually use. When recombining them, color space and bit depth have to be consistent, otherwise the layers will not add back up to the original image. High bit depth formats such as EXR are well suited to this kind of work. For other ways to lift quality, see our tips for high-quality architectural renders.