Cryptomatte

Cryptomatte provides a way to isolate specific objects in the scene by ingesting user-specified object ids and generating pixel coverages.

In MoonRay, a single geometry (or part) covering a pixel will be represented by an {id, weight} pair, where the id represents some hash value assigned to the geometry by the user, and the weight represents the “coverage” amount, or the fractional amount the associated geometry contributes to the final value of the pixel. To understand coverage amount, you might consider the case where you have an object with presence, where there might be multiple geometries contributing to the final value of a pixel.

These {id, weight} pairs will be stored in the R, G and B, A channels of each .exr layer, and the cryptomatte_depth user attribute determines how many layers will be generated (labeled like so: Cryptomatte00, Cryptomatte01, Cryptomatte02, etc). The {id, weight} pairs are sorted by max coverage, so the geometry with the most pixel coverage will always be the first entry.

Extensions

There are several options to output additional information.

Positions, Normals, Beauty, RefP, RefN, UV

The RenderOutput object contains the additional toggles:

• cryptomatte_output_positions
• cryptomatte_output_normals
• cryptomatte_output_beauty
• cryptomatte_output_refp
• cryptomatte_output_refn
• cryptomatte_output_uv

When turned on, each id/weight pair will also have the associated position/normal/beauty value output to a separate layer. If there are multiple pixel samples, the values from all the samples will be averaged together. The output layers are as follows:

• CryptoP: world position (x, y, z) is stored in (r, g, b), respectively
• CryptoN: shading normal (x, y, z) is stored in (r, g, b), respectively
• CryptoB: beauty (r, g, b, presence) is stored in (r, g, b, a)
• CryptoRefP: reference position (x, y, z) is stored in (r, g, b), respectively
• CryptoRefN: reference normal (x, y, z) is stored in (r, g, b), respectively
• CryptoUV: uv (u, v) is stored in (r, g), respectively

Refractive Cryptomattes

In the RenderOutput, you can enable refractive crypotomattes with the toggle cryptomatte_enable_refract.

The refracted cryptomatte data contains the first surface intersection that isn’t considered “refracted”. This skips any surfaces traversed by the primary ray that the camera can see through, allowing for cryptomatte data for surfaces that are e.g. behind glass.

You must tag “refractive” surfaces (materials) by setting the invisible refractive cryptomatte attribute to true on the surface’s material.

The refracted cryptomatte output is written to a separate set of render output channels that are named the same as the regular cryptomatte channels except with “Refract” appended. E.g. if there is a CryptoP channel, there will also be a CryptoPRefract channel.

Multiple Presence Bounces

In SceneVariables, you can toggle on cryptomatte_multi_presence to count each presence bounce as a separate cryptomatte entry.

Resume/Checkpoint Rendering

In order to support resume/checkpoint rendering for the cryptomatte extensions, use the attribute cryptomatte_support_resume_render. When on, this will output the presence depth and fragment samples to a layer called CryptoS, which allows us to accurately restore the cryptomatte fragment data upon resuming rendering.

Examples

Assigning IDs to Different Geometries

SceneVariables {
    ["deep_id_attribute_names"] = {"prim_id"}
}

local id0 = UserData("id0") {
    ["float_key"] = "prim_id",  -- cryptomatte attribute key
    ["float_values"] = 0.1      -- object id
}

local id1 = UserData("id1") {
    ["float_key"] = "prim_id",  -- cryptomatte attribute key
    ["float_values"] = 0.2      -- object id
}

local geom0 = AbcGeometry("geom0") {
    ["primitive_attributes"] = {id0}
    ...
}

local geom1 = AbcGeometry("geom1") {
    ["primitive_attributes"] = {id1}
    ...
}

RenderOutput("cryptomatte") {
    ["result"] = "cryptomatte",
    ["file_name"] = "result0.exr",
    ["cryptomatte_depth"] = 2       -- max possible number of {id, weight} pairs
                                    -- limits number of generated exr layers 
                                    -- (in this case, only 1 layer needed)
}

RenderOutput("dummy0") {                  -- this extra layer is always needed
    ["file_name"] = "ignore_this_file",   -- so that cryptomatte can access
    ["primitive_attribute"] = "prim_id",  -- the id primitive attribute
    ["result"] = "primitive attribute",
}

Assigning IDs to Geometry Parts

SceneVariables {
    ["deep_id_attribute_names"] = {"prim_id"}
}

local id0 = UserData("id0") {
    ["float_key"] = "prim_id",                   -- cryptomatte attribute key
    ["float_values"] = {0.1, 0.2, 0.3, 0.4, 0.5} -- object part ids
}

local geom0 = AbcGeometry("geom0") {
    ["part list"] = {"face1", "face2", "face3", "face4", "face5"},
    ["primitive_attributes"] = {id0}
    ...
}

RenderOutput("cryptomatte") {
    ["result"] = "cryptomatte",
    ["file_name"] = "result0.exr",
    ["cryptomatte_depth"] = 2,      -- max possible number of {id, weight} pairs
                                    -- limits number of generated exr layers 
                                    -- (in this case, only 1 layer needed)
    ["cryptomatte_output_positions"] = true,
    ["cryptomatte_output_beauty"] = true
}

RenderOutput("dummy0") {
    ["file_name"] = "ignore_this_file",
    ["primitive_attribute"] = "prim_id",
    ["result"] = "primitive attribute",
}

Since this example includes cryptomatte extensions, it will output these named layers: Cryptomatte00, CryptoP00, CryptoP01, CryptoB00, CryptoB01.