What is Multipass compositing?

Here are some common types of passes used in VFX and 3D –

• Beauty pass – A Beauty pass is a final rendered image that includes all lighting, shading, and effects applied to a 3D scene. This pass is an important part of the composting process and allows artists to adjust elements individually to create a polished final image.

• Depth pass – A depth pass is a grayscale image that contains information about the distance of objects from the camera. It is used in compositing to create realistic depth of field effects, fog and volumetric lighting. 

• Normal pass – A normal pass is a type of render pass that shows the surface depth or orientation of 3D objects in a scene. It’s useful for adjusting lighting, shading, and creating effects like bump mapping.

• Object ID pass – An Object ID pass assigns a unique color value to each object in a 3D scene. It helps in compositing to easily select and change specific objects individually.

• Material ID pass –  Material ID pass is a rendering output that assigns a unique color value to each material in a 3D scene so which can be used in compositing to easily select and change specific materials.

• Ambient Occlusion pass -AO helps to create realistic shadows and shading on 3D objects. This pass is used in compositing to make the scene look more realistic by darkening the areas where surfaces come together, creating natural-looking shadows and enhancing the sense of depth.

• Shadow pass – A Shadow pass separates the shadows cast by objects in a 3D scene. It helps in compositing to adjust and enhance the appearance of the shadows in the final image, without affecting other elements in the scene.

• Reflection pass – A Reflection pass creates a grayscale or color image that represents the reflected parts of the 3D scene on reflective surfaces, like mirrors, glass, or water. By combining the Reflection pass with other render passes or elements in compositing, the reflection effect can be adjusted or manipulated to achieve the desired result.

• Refraction pass – A Refraction pass shows where light has been bent or changed direction while passing through transparent or translucent objects like water or glass. It helps in compositing to adjust and enhance the refraction effect, making the final image look realistic. 

• Motion Vector pass – A Motion Vector pass tells us how much and in what direction each object in a 3D scene has moved between two frames. It helps compositors to create motion blur or other visual effects and make 3D elements look more realistic and combine into live-action footage.

• Diffuse pass – A Diffuse pass is an image that shows the basic color and brightness of objects in a 3D scene. It’s like a picture that shows what each object looks like without any fancy lighting effects, reflections or shadows and other properties. It shows the raw color and raw texture. 

• Specular pass – A Specular pass shows only the shiny or glossy parts of objects. It doesn’t show any other types of lighting, just the reflections on the shiny surfaces. In compositing, this pass can be used to make the shiny parts of objects look even better, or to add more realistic reflections to them.

• Alpha pass– An Alpha pass is an image that shows which parts of a 3D scene are visible and which are transparent.  

• Emissive pass – An emissive pass is a method used in compositing to make objects in a scene appear to glow or emit light. 

• Global Illumination pass – A global illumination pass simulates how light bounces and interacts with surfaces in a scene creating realistic shadows, reflections, and color bleeding. This technique allows for more  realistic lighting, which can greatly improve the overall visual quality.

• Caustics pass – A caustics pass is to simulate the way light refracts and focuses through transparent or translucent materials, such as water or glass, creating patterns of light and shadow on other surfaces.

• Transparency pass – A Transparency pass helps to separate see-through objects like glass or water, from solid objects in a scene. This pass creates a black and white image that shows where the transparent parts of the image are. It makes it easier to adjust the transparency effects in the final image.

• SSS (Subsurface Scattering) pass – SSS pass is used to create more realistic images of semi-transparent or translucent objects like skin or leaves. The SSS pass creates a black and white image that shows where light penetrates and scatters within the object. This pass allows greater control over the scattering effects in the final image.

• UV pass – UV pass generates a 2D image that represents the coordinates of a 3D model’s surface. This pass is rendered separately from other passes for better control over the texture mapping of the model in the final image. The UV pass is commonly used in compositing workflows to apply additional textures or effects to 3D model.

• Velocity pass – Both Velocity pass and Motion Vector pass are used to create an image that shows how objects in a 3D scene are moving in terms of both speed and direction.  Velocity pass is used in 3D rendering software while Motion Vector pass is used in compositing software but both passes serve the same purpose.

• Direct and Indirect Illumination passes – Direct Illumination light comes directly from a light source and falls on an object without any interaction with other objects in the scene. Direct illumination always creates sharp shadows and highlights that are visible on an object.

Indirect Illumination refers to the light that has bounced off other surfaces in the scene and then falls on the object. Indirect illumination creates soft shadows. 

• Cryptomatte pass – The Cryptomatte pass is a set of images that store information about which objects and materials are present in each pixel of a rendered image. This pass can be used to automatically generate matte and selection masks for compositing or post-processing purposes. It makes it easier to work with and manipulate specific elements in a rendered image, saving time and improving efficiency in the compositing workflow.

• Cutout Matte pass – A Cutout Matte pass is a special rendering pass that helps separate the foreground objects from the background in a scene where background is black and foreground objects is white.

• Normal Derivative pass – A Normal Derivative pass provides information about the changes in surface orientation across a 3D object in a scene. 

• Object Motion pass – An Object Motion pass is the movement or motion of objects in a scene between two frames of an animation. It is mostly used in compositing to create motion blur. Velocity pass and Object Motion pass are similar in that they both provide information about the movement or motion of objects in a scene. 

• Camera Motion pass –Camera motion pass is a way of creating a separate image layer that represents the movement of the camera during animation.  The Camera motion pass only captures the movement of the camera. It’s a useful tool for creating special effects and gives greater control over camera motion and blur in the final image.

• Specular Roughness pass – Specular Roughness pass shows the roughness or smoothness of the reflective highlights on an object’s surface. This pass provides information about the size and intensity of the specular highlights. 

• Glossiness pass –  The Glossiness pass shows the shininess or glossiness of a surface. This pass provides information about how much the surface reflects light in a concentrated, mirror-like way  and can be adjusted in post-production to control the amount and clarity of surface reflections. 

• Depth of Field pass – The Depth of Field  pass captures the amount of blur in areas of an image that are out of focus. This pass provides grayscale or color coded information that represents the distance from the camera for each pixel in the image.

• Volume pass – A Volume pass is a technique used in 3D rendering that captures the density and color of volumetric effects like smoke, fog etc. 
• Wireframe pass – A wireframe pass is a separate image layer showing only the wireframe model of a 3D object or scene. This pass is useful for examining the topology of the model and also helps in the time of job interview for showreel work.