The parameters described on this page are commonly used by almost every assimp command. They specify how the library will postprocess the imported data. This is done by several configurable pipeline stages, called 'post processing steps'. Below you can find a list of all supported steps along with short descriptions of what they're doing.
Programmers: more information can be found in the aiPostProcess.h header.

Parameter	Long parameter	Description
-ptv	–pretransform-vertices	Move all vertices into worldspace and collapse the scene graph. Animation data is lost. This is intended for applications which don't support scenegraph-oriented rendering.
-gsn	–gen-smooth-normals	Computes 'smooth' per-vertex normal vectors if necessary. Mutually exclusive with -gn
-gn	–gen-normals	Computes 'hard' per-face normal vectors if necessary. Mutually exclusive with -gsn
-cts	–calc-tangent-space	If one UV channel and normal vectors are given, compute tangents and bitangents
-jiv	–join-identical-vertices	Optimize the index buffer. If this flag is not specified all vertices are referenced once.
-rrm	–remove-redundant-materials	Remove redundant materials from the imported data.
-fd	–find-degenerates	Find and process degenerates primitives.
-slm	–split-large-meshes	Split large meshes over a specific threshold in smaller sub meshes. The default vertex & face limit is 1000000
-lbw	–limit-bone-weights	Limit the number of bones influencing a single vertex. The default limit is 4.
-vds	–validate-data-structure	Performs a full validation of the imported data structure. Recommended to avoid crashes if an import plugin produces rubbish
-icl	–improve-cache-locality	Improve the cache locality of the vertex buffer by reordering the index buffer to achieve a lower ACMR (average post-transform vertex cache miss ratio)
-sbpt	–sort-by-ptype	Splits meshes which consist of more than one kind of primitives (e.g. lines and triangles mixed up) in 'clean' submeshes.
-lh	–convert-to-lh	Converts the imported data to left-handed coordinate space
-fuv	–flip-uv	Flip UV coordinates from upper-left origin to lower-left origin
-fwo	–flip-winding-order	Flip face winding order from CCW to CW
-ett	–evaluate-texture-transform	Evaluate per-texture UV transformations (e.g scaling, offset) and build pretransformed UV channels
-guv	–gen-uvcoords	Replace abstract mapping descriptions, such as 'spherical' or 'cylindrical' with proper UV channels
-fixn	–fix-normals	Run a heuristic algorithm to detect meshes with wrong face winding order/normals.
-tri	–triangulate	Triangulate poylgons with 4 and more points. Lines, points and triangles are not affected.
-fi	–find-instances	Search the data structure for instanced meshes and replace them by references. This can reduce vertex/face counts but the postprocessing-step takes some time.
-og	–optimize-graph	Simplify and optimize the scenegraph. Use it with care, all hierarchy information could be lost. Animations remain untouched.
-om	–optimize-meshes	Optimize mesh usage. Meshes are merged, if possible. Very effective in combination with –optimize-graph
-tuv	–transform-uv-coords	Will transform uv-coordinates if possible.
-fid	–find-invalid-data	Will look for invalid data in the imported model structure.
-db	–debone	Removes nearly losslessly or according to a configured threshold bones from the model.
-sbc	–split-by-bone-count	Split meshes with too many bones. Necessary for our (limited) hardware skinning shader.

For convenience some default postprocessing configurations are provided. The corresponding command line parameter is -c<name> (or –config=<name>).

Name	Description	List of steps executed
fast	Fast post processing config, performs some essential optimizations and computes tangents	-cts, -gn, -jiv, -tri, -guv, -sbpt
default	Balanced post processing config; performs most optimizations	-cts, -gsn, -jiv, -icl, -lbw, -rrm, -slm, -tri, -guv, -sbpt, -fd, -fiv
full	Full post processing. May take a while but results in best output quality for most purposes	-cts, -gsn, -jiv, -icl, -lbw, -rrm, -slm, -tri, -guv, -sbpt, -fd, -fiv, -fi, -vds -om

The -tuv, -ptv, -og flags always need to be enabled manually.

There are also some common flags to customize Assimp's logging behaviour:

Name	Description
-l or –show-log	Show log file on console window (stderr)
-lo<file> or –log-out=<file>	Streams the log to <file>
-v or –verbose	Enables verbose logging. Debug messages will be produced too. This might decrease loading performance and result in very long logs ... use with caution if you experience strange issues.