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# MCX Documentation

  1. Command Line Options
    1.1. Required option
    1.2. MC options
    1.3. GPU options
    1.4. Output options
    1.5. User IO options
    1.6. Debug options
    1.7. Additional options
  2. Input File Format
    2.1. JSON input file format
    2.2. inp input file format
    2.3. JSON shape file format

## 1. Command Line Options

### 1.1. Required option

#### -f / --input

Format
  A string
Default value
  None, user must specify
Restriction
  parameter can not be longer than 1024 characters (including the string ending mark '\0' )
Description

Use -f to specify an input file. Format:

        -f filename.json

or

        -f filename.inp

filename may contain absolute or relative path. If the filename has a '.json' extension, this input file is in the JSON format. If the input file name ends with '.inp', it is in the legacy format. If the file name contains space, one must use quotation marks to quote the full file name. In Windows, the quotation must be double quote (""). In Linux or Mac, either single or double quotation mark works.

If this flag is followed by an inline JSON string, such as -f '{...}', the parameter is parsed as an "inline JSON input file".

### 1.2. MC options

#### -n / --photon

Format
  An integer or floating point number
Default value
  0
Restriction
  the number can not exceed 2^63-1
Description

This flag specifies the total number of photons to be simulated. The format is

        -n N

where N can be either an integer, such as 100000 or a floating point number, such as 1e5. If N is a floating point number, it will be rounded to the largest integer below the current value.

#### -r / --repeat

Format
  A positive or negative integer
Default value
  1
Max Value
  the number can not exceed 2^32-1
Description

Use -r to repeat the simulation so that the total number of photon is multiplied by the repetition count. Format:

        -r N

where N is a positive integer. When use this option, mcx runs 'nphoton*N' photons with nphoton per batch.

If N is a negative integer, this option splits the total simulation into abs(N) number of smaller batches; in each batch, only nphoton/abs(N) number of photons are simulated.

This option is useful when one does not have access to a dedicated GPU, i.e. the GPU is used for both display and computation. In such case, the mcx computation time is limited to only 5 to 10 seconds due to the graphics driver. One can use the -r option to make the execution time per batch under the time limit set by the driver.

#### -b / --reflect

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

Use -b to enable or disable reflections at the boundaries. Format:

        '-b' or '-b 1' to enable (default) or '-b 0' to disable

When enabled, mcx will either reflect or transmit the photon at the boundaries based on the Fresnel's equation.

By default, mcx considers refractive index mismatch at the boundaries.

#### -B / --reflectin (*depreciated)

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

1 to reflect photons at int. boundary; 0 do not

#### -u / --unitinmm

Format
  a floating point number
Default value
  1.0
Restriction
  can not be less than or equal to 0
Description

Use -u to set the voxel size in mm. Format:

        -u S

where S is a floating point number, denoting the edge length, in mm, of a voxel in the volume. For example, if the volume used in the simulation is 0.1x0.1x0.1 mm^3, then, one should use '-u 0.1' in the command line.

Only isotrpic voxels are currently supported by mcx.

#### -U / --normalize

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

Use -U to enable or disable solution normalization. Format:

        '-U' or '-U 1' to enable (default) or '-U 0' to disable

For fluence, the normalization aims to generate a 'Green's function'. For other qualities, the normalization produces a stable solution by removing the dependency due to total photon numbers, voxel sizes, and time-gate settings. The detailed equations are explained in the MCX paper.

#### -E / --seed

Format
  An integer or a string
Default value
  -1
Restriction
  must be an integer; if is a string, the specified file must exist
Description

Use -E to set the seed of the CPU random number generator (RNG). The CPU RNG in turn initializes the seeds for each GPU thread. Format:

        -E -1    // let MCX to automatically seed the CPU-RNG using system clock
        -E n     // n is a large positive integer, set the CPU-RNG's seed to n
        -E filename.mch // replay detected photons using the seeds saved in the mch file

Setting a fixed RNG seed is expected to create reproducible results on NVIDIA cards if the thread/block size are kept the same.

#### -z / --srcfrom0

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use -z to define the coordinate origin mode of the volume. Format:

        -z 0 (default)// assumes the lower-bottom corner of the first voxel as [1 1 1]
        -z or -z 1    // assumes the lower-bottom corner of the first voxel as [0 0 0]

All source and detector positions are referenced from the origin, determined by this flag.

#### -R / --skipradius

Format
  An integer
Default value
  -2
Restriction
  must be 0 or 1
Description

Use -R to specify the scope within which to use atomic operations. The possible options include

         -R -2  // this enables full atomic opertions in the entire volume (default)
         -R n   // when n is a positive integer, mcx uses atomic operations in the
                // shared memory for a n x n x n sub-cubic domain centered at the source.
         -R 0   // disable all atomic operations, data racing may exist, particularly
                // near the source.
         -R -1  //use crop0/crop1 to determine atomic zone

Using full atomic operations was very slow in very early CUDA devices, but for most later NVIDIA GPUs, the use of atomic operations is as efficient as the non-atomic version.

#### -k / --voidtime

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use the -k option to tell MCX whether to count the time-of-flight when a photon is launched outside of the volume. Format:

        -k 1 (default)  // the time-of-flight of the photon starts at the launch time
        -k 0            // the time-of-flight starts when a photon enters the first non-zero voxel.

#### -V / --specular

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

Use -V flag to tell MCX whether to consider the specular reflection at the initial entry of the photon to the domain (entry from a 0-voxel to a non-zero voxel). By default, the initial specular reflection is considered (thus, photon loses a small fraction of energy, but enters the domain). The reflected energy is no longer modeled. Please be aware that the "absorption fraction" number printed at the end of the mcx simulation session should include this energy loss due to specular reflection.

If one sets "-V 0", all launched photon energy preserves after enters the domain.

#### -Y / --replaydet

Format
  A positive integer
Default value
  0
Restriction
  can not be negative
Description

Use the -Y option to specify the ID of the detector for the 'replay' calculations. Format:

        -Y n   // n is a positive integer, denoting the index of the detectors to be replayed
        -Y 0   // all detected photons will be replayed regardless of detector

If -Y is not specified, MCX replays all detected photons; otherwise, MCX only replays the detected photons from the specified detector. See -E for more details.

#### -P / --shapes

Format
  A string
Default value
  None, user must specify
Restriction
  parameter can not be longer than 1024 characters (including the string ending mark '\0' )
Description

Use the -P option to dynamically define heterogeneities from the command line. The -P flag is followed by a JSON-formatted string. For example

        -P '{"Shapes":[{"ZLayers":[[1,10,1],[11,30,2],[31,60,3]]}]}'

This defines a 3-layer medium: z slices 1-10 is filled with tissue label 1, z slices 11-30 is filled with label 2, and 31-60 is filled with label 3.

The shape definition always starts an array object named 'Shapes'. Each element in the 'Shapes' object defines a primitive object. The supported primitives include Name/Origin/Grid/Sphere/Box/Subgrid/{XYZ}Layers/{XYZ}Slabs/Cylinder/UpperSpace. Most objects have a sub-field 'Tag', specifying the tissue label (index to the property list). The details of the Shapes objects can be found in this link

#### -N / --reseed (*depreciated)

Format
  An integer
Default value
  0
Restriction
  can not be negative
Description

number of scattering events before reseeding RNG

#### -F / --faststep (*depreciated)

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

1-use fast 1mm stepping, [0]-precise ray-tracing

#### -e / --minenergy

Format
  A floating point number
Default value
  0
Restriction
  must be positive or 0.0
Description

Use -e to set the min photon packet weight to trigger Russian Roulette. Format:

        -e f    // where f is a floating point number between 0 and 1.

#### -g / --gategroup

Format
  A positive integer
Default value
  100
Restriction
  can not be negative
Description

Use -g to split a simulation containing many time gates into smaller but multiple sequential simulations. Format:

        -g n    // n is a positive integer

Only use this option when the GPU global memory can not hold the data for all required time gates, which only happens when one trys to simulate a very large domain with very dense time gates (very rare). If one's GPU can only hold the data for n time gates, while one has to simulate a total of N > n time gates, use -g n to split the total simulations into multiple runs: in the first run, MCX will record the results for 1~n time gates, in the second run, MCX will launch a new kernel to simulate photons for 1~2*n time gates, but only records the photon fluence for n+1 ~ 2*n time gates and so on. As one can see, this method introduces overhead in the later simulations. Therefore, it is recommended to avoid.

#### -a / --array

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use -a to tell mcx if the input volume data is a MATLAB-like data (column-major) or a C-like data (row-major).Format

        -a 0 (default) // the input volume is a MATLAB-array
        -a 1           // the input volume is a C-array

If an MATLAB array is used, the fastest loop index is the left-most index of an array; while for a C array, it is the right-most index.

### 1.3. GPU options

#### -L / --listgpu

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use the -L flag to list all available GPUs on your system without running the simulation. For example

    fangq@wazu:~/space/git/Project/mcx/example/quicktest$  ../../bin/mcx -L
    =============================   GPU Infomation  ================================
    Device 1 of 2:		GeForce GTX 980 Ti
    Compute Capability:	5.2
    Global Memory:		2147287040 B
    Constant Memory:	65536 B
    Shared Memory:		49152 B
    Registers:		65536
    Clock Speed:		1.19 GHz
    Number of MPs:		22
    Number of Cores:	2816
    SMX count:		22
    =============================   GPU Infomation  ================================
    Device 2 of 2:		GeForce GT 730
    Compute Capability:	3.5
    Global Memory:		1073545216 B
    Constant Memory:	65536 B
    Shared Memory:		49152 B
    Registers:		65536
    Clock Speed:		0.90 GHz
    Number of MPs:		2
    Number of Cores:	384
    SMX count:		2

#### -t / --thread

Format
  An integer
Default value
  2^14=16384
Restriction
  can not be negative
Description

Use -t to specify the number of threads. Format:

        -t N

where N is a positive integer. The thread number N must be a multiple of 32 - the size of a warp - on all CUDA devices. If N is not a multiple of 32, mcx will round it to the nearest multiple less than N.

     To achieve the best efficiency, it is recommended to launch a large number of threads to mazimize the utility of the GPU resources; a thread number larger than 10000 is generally sufficient.

You should not manually specify the thread number using this option when you use the autopilot mode (-A).

#### -T / --blocksize

Format
  An integer
Default value
  64
Restriction
  can not be negative, must be multiples of 32
Description

Use -T to specify the size of a block. Format:

        -T N

where N is a positive integer. The block size N must be a multiple of 32 - the size of a warp - on all CUDA devices. If N is not a multiple of 32, mcx will round it to the nearest multiple less than N.

     Because mcx does not need inter-thread communication, a small block size, such as 32 or 64, is generally recommended.

You should not manually specify the block size using this option when you use the autopilot mode (-A).

#### -A / --autopilot

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

Use -A to enable automatic thread/block configuration (i.e. autopilot). Format:

        -A 1

When the autopilot mode is enabled, mcx will compute the 'optimal' thread number and block size using a heuristic algorithm.

#### -G / --gpu

Format
  An integer or a string made of '0's and '1's
Default value
  1
Restriction
  If integer, must be positive (>0); if string, can only have '0's and '1's.
Description

Use -G to specify one or multiple GPUs to run the simulation. Format

        -G 1    // use only the first GPU device (device orders based on 'mcx -L' output)
        -G n    // n is a positive integer, use only the n-th GPU device
        -G 1110 // when -G is followed by a string made of only 0s and 1s, it specifies
                // a mask for active GPUs, for example, 1110 means to use GPU 1-3
                // together while the 4th GPU is not used.

When multiple GPU devices are specified, one need to use the -W/--workload flag to optimally partition the total photons to be simulated. By default, photons will be evenly distributed among multiple GPU devices.

#### -W / --workload

Format
  A list of floating point/integer values, separated by commas
Default value
  100
Restriction
  all values must be positive; no space is allowed; length must be equal or longer than number of activated GPUs (i.e. count of all '1's in -G)
Description

Use -W to partition the total simulated photon numbers between multiple devices. Format

        -W w1,w2,w3,...  // w_i is a numerical value, corresponding to the relative portions
                         // of the workload of the i-th GPU device. The total load
                         // is proportional to the sum of w_i.

For example, -W 10,20,20 indicates a 1:2:2 workload split between 3 active GPUs.If one needs to simulate 1e6 photons, the 1st GPU will run 2e5 photons, while the 2nd and the 3rd GPUs will run 4e5 photons each.

#### -I / --printgpu

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

The -I option lists all available GPU, same as -L, but it also run the actual simulation.

### 1.4. Output options

#### -s / --session

Format
  A string
Default value
  same as the input file name followed by the -f flag (without the path); it can not contain <>:"/|?*
Restriction
  must be 0 or 1
Description

Use -s to specify a session ID for the simulation. Format:

        -s session_name // session_name is a string, it can not contain <>:"/|?*

If -s is set, the output fluence file (.mc2), detected photon file (.mch) and the log file (.log) will be named as session_name.{mc2,mch,log}. Otherwise, the input file name following the -f option will be used in the place of session_name.

#### -d / --savedet

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

Use -d to enable or disable saving the detailed path data for all detected photons. Format:

        '-d' or '-d 1' to enable (default) or
        '-d 0' to disable

When this option is enabled, a binary file, with a suffix of .mch, will be produced in additional to the '.mc2' output. The .mch file contains the partial path data for all photons enters the aperatures of the detectors.

#### -x / --saveexit

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

1 to save photon exit positions and directions setting -x to 1 also implies setting '-d' to 1

#### -X / --saveref

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

1 to save diffuse reflectance at the air-voxels right outside of the domain; if non-zero voxels appear at the boundary, pad 0s before using -X

#### -q / --saveseed

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

1 to save photon RNG seed for replay; 0 not save

#### -M / --dumpmask

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use -M to dump the modified volume data for debugging purposes. The dumped volume is saved in a binary file with Nx x Ny x Nz bytes, each byte contains the tissue label as the input volume, and the highest bit of each byte denotes whether the voxel is next to a detector.

#### -m / --momentum

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use -m to save the momentum transfer for all detected photons. One can use this output for diffuse correlation spectroscopy (DCS) simulations. This information is stored in the .mch file. One can load the data using `loadmch.m` and process the saved data using the `mcxdcsg1.m` script, both matlab scripts can be found under `mcx/utils`.

#### -H / --maxdetphoton

Format
  An integer
Default value
  1000000
Restriction
  can not be negative
Description

Use -H to specify the maximum number of detected photons. Format:

        -H n    // n is a positive integer, signifying mcx to allocate a buffer to hold n detected photons

By default, mcx can save up to 1e6 detected photons. If the detected photons exceed this limit, mcx will show a warning. Users may use the -H option to rerun the simulation and use the number in the warning to reallocate the buffer.

#### -S / --save2pt

Format
  A boolean, 0 or 1
Default value
  1
Restriction
  must be 0 or 1
Description

Use -S to enable or disable saving volumetric fluence distributions (or the requested output specified by the -X flag).

#### -O / --outputtype

Format
  A single character (case insensitive)
Default value
  'X'
Restriction
  must be one of 'X,F,E,J,P', case insensitive
Description

Use -O to specify the type of data to be saved in the volumetric output. The supported formats include

     'X' - output time-resolved fluence rate (1/mm^2), i.e. TPSF
     'F' - output time-resolved fluence rate integrated in each time-gate,
     'E' - energy deposit at each voxel (normalized or unnormalized, depends on -n)
     'J' - Jacobian (replay mode),
     'P' - scattering event counts at each voxel (replay mode only)

#### -F / --outputformat

Format
  A string
Default value
  'mc2'
Restriction
  must be one of 'mc2', 'nii','hdr','ubj', case insensitive
Description

Use -F to specify the volumetric data output format:

        mc2 - MCX mc2 format (binary 32bit float) (default)
        nii - Nifti format (fluence after taking log10())
        hdr - Analyze 7.5 hdr/img format
        ubj - universal binary JSON format (currently not supported yet)

### 1.5. User IO options

#### -h / --help

Format
  no follow-up parameter needed
Default value
Restriction
  None
Description

Use the -h flag to print the built-in help of all supported command line flags

#### -v / --version

Format
  no follow-up parameter needed
Default value
Restriction
  None
Description

Use the -v flag to print the version of MCX

#### -l / --log

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use the -l flag to save the MCX's command line output into a log file. Format

        '-l' or '-l 1' to enable (default) or
        '-l 0' to disable

The output file name is 'session_name.log' where 'session_name' is the string specified by the -s flag. If -l is used, no message will be printed in the command line.

#### -i / --interactive

Format
  A boolean, 0 or 1
Default value
  0
Restriction
  must be 0 or 1
Description

Use -i when one wants to type in the domain settings in an item-by-item prompt mode. The -i option can not be used together with -f. When -i is used, one can use the redirect operator to include an input file, i.e.

        mcx -i < input.inp
        mcx -f input.inp

can produce the same answer.

### 1.6. Debug options

#### -D / --debug

Format
  An integer or a string
Default value
  0
Restriction
  If integer, must be positive; if string, must be made of any combinations of 'R','M','P' (case insensitive).
Description

Use -D to print debug information (you can use an integer or a string by combining the following flags)

      1 R  debug RNG
      2 M  store photon trajectory info (saved in a .mct file, can be loaded with loadmch.m)
      4 P  print progress bar

combine multiple items by using a string, or add selected numbers together

### 1.7. Additional options

#### --gscatter

Format
  An integer
Default value
  -1
Restriction
  1e9
Description

after a photon completes the specified number of scattering events, mcx then ignores anisotropy g and only performs isotropic scattering for speed

#### --maxvoidstep

Format
  An integer
Default value
  1000
Restriction
  can not be negative
Description

maximum distance (in voxel unit) of a photon that can travel before entering the domain, if launched outside (i.e. a widefield source)

#### --maxjumpdebug

Format
  An integer
Default value
  10000000
Restriction
  can not be negative
Description

when trajectory is requested (i.e. -D M), use this parameter to set the maximum positions stored (default: 1e7)

## 2. Input File Format

### 2.1. JSON input file format

Starting from version 0.7.9, MCX accepts a JSON-formatted input file in addition to the conventional tMCimg-like input format. JSON (JavaScript Object Notation) is a portable, human-readable and "fat-free" text format to represent complex and hierarchical data. Using the JSON format makes a input file self-explanatory, extensible and easy-to-interface with other applications (like MATLAB).

A sample JSON input file can be found under the examples/quicktest folder. The same file, qtest.json, is also shown below:

     {
        "Help": {
          "[en]": {
            "Domain::VolumeFile": "file full path to the volume description file, can be a binary or JSON file",
            "Domain::Dim": "dimension of the data array stored in the volume file",
            "Domain::OriginType": "similar to --srcfrom0, 1 if the origin is [0 0 0], 0 if it is [1.0,1.0,1.0]",
    	"Domain::LengthUnit": "define the voxel length in mm, similar to --unitinmm",
            "Domain::Media": "the first medium is always assigned to voxels with a value of 0 or outside of
                             the volume, the second row is for medium type 1, and so on. mua and mus must
                             be in 1/mm unit",
            "Session::Photons": "if -n is not specified in the command line, this defines the total photon number",
            "Session::ID": "if -s is not specified in the command line, this defines the output file name stub",
            "Forward::T0": "the start time of the simulation, in seconds",
            "Forward::T1": "the end time of the simulation, in seconds",
            "Forward::Dt": "the width of each time window, in seconds",
            "Optode::Source::Pos": "the grid position of the source, can be non-integers, in grid unit",
            "Optode::Detector::Pos": "the grid position of a detector, can be non-integers, in grid unit",
            "Optode::Source::Dir": "the unitary directional vector of the photon at launch",
            "Optode::Source::Type": "source types, must be one of the following:
                       pencil,isotropic,cone,gaussian,planar,pattern,fourier,arcsine,disk,fourierx,fourierx2d",
            "Optode::Source::Param1": "source parameters, 4 floating-point numbers",
            "Optode::Source::Param2": "additional source parameters, 4 floating-point numbers"
          }
        },
        "Domain": {
    	"VolumeFile": "semi60x60x60.bin",
            "Dim":    [60,60,60],
            "OriginType": 1,
    	"LengthUnit": 1,
            "Media": [
                 {"mua": 0.00, "mus": 0.0, "g": 1.00, "n": 1.0},
                 {"mua": 0.005,"mus": 1.0, "g": 0.01, "n": 1.0}
            ]
        },
        "Session": {
    	"Photons":  1000000,
    	"RNGSeed":  29012392,
    	"ID":       "qtest"
        },
        "Forward": {
    	"T0": 0.0e+00,
    	"T1": 5.0e-09,
    	"Dt": 5.0e-09
        },
        "Optode": {
    	"Source": {
    	    "Pos": [29.0, 29.0, 0.0],
    	    "Dir": [0.0, 0.0, 1.0],
    	    "Type": "pencil",
    	    "Param1": [0.0, 0.0, 0.0, 0.0],
    	    "Param2": [0.0, 0.0, 0.0, 0.0]
    	},
    	"Detector": [
    	    {
    		"Pos": [29.0,  19.0,  0.0],
    		"R": 1.0
    	    },
                {
                    "Pos": [29.0,  39.0,  0.0],
                    "R": 1.0
                },
                {
                    "Pos": [19.0,  29.0,  0.0],
                    "R": 1.0
                },
                {
                    "Pos": [39.0,  29.0,  0.0],
                    "R": 1.0
                }
    	]
        }
     }

A JSON input file requiers several root objects, namely "Domain", "Session", "Forward" and "Optode". Other root sections, like "Help", will be ignored. Each object is a data structure providing information indicated by its name. Each object can contain various sub-fields. The orders of the fields in the same level are flexible. For each field, you can always find the equivalent fields in the *.inp input files. For example, The "VolumeFile" field under the "Domain" object is the same as Line#6 in qtest.inp; the "RNGSeed" under "Session" is the same as Line#2; the "Optode.Source.Pos" is the same as the triplet in Line#3; the "Forward.T0" is the same as the first number in Line#5, etc.

An MCX JSON input file must be a valid JSON text file. You can validate your input file by running a JSON validator, for example http://jsonlint.com/ You should always use "" to quote a "name" and separate parallel items by ",".

MCX accepts an alternative form of JSON input, but using it is not recommended. In the alternative format, you can use

     "rootobj_name.field_name": value

to represent any parameter directly in the root level. For example

     {
        "Domain.VolumeFile": "semi60x60x60.json",
        "Session.Photons": 10000000,
        ...
     }

You can even mix the alternative format with the standard format. If any input parameter has values in both formats in a single input file, the standard-formatted value has higher priority.

To invoke the JSON-formatted input file in your simulations, you can use the "-f" command line option with MCX, just like using an .inp file. For example:

      mcx -A -n 20 -f onecube.json -s onecubejson

The input file must have a ".json" suffix in order for MCX to recognize. If the input information is set in both command line, and input file, the command line value has higher priority (this is the same for .inp input files). For example, when using "-n 20", the value set in "Session"/"Photons" is overwritten to 20; when using "-s onecubejson", the "Session"/"ID" value is modified. If your JSON input file is invalid, MCX will quit and point out where the format is incorrect.

### 2.2. inp input file format

Historically, MCX supports an extended version of the input file format used by tMCimg. The difference is that MCX allows comments in the input file. A typical MCX input file looks like this:

    1000000              # total photon, use -n to overwrite in the command line
    29012392             # RNG seed, negative to generate
    30.0 30.0 0.0 1      # source position (in grid unit), the last num (optional) sets srcfrom0 (-z)
    0 0 1                # initial directional vector
    0.e+00 1.e-09 1.e-10 # time-gates(s): start, end, step
    semi60x60x60.bin     # volume ('unsigned char' binary format)
    1 60 1 60            # x voxel size in mm (isotropic only), dim, start/end indices
    1 60 1 60            # y voxel size, must be same as x, dim, start/end indices
    1 60 1 60            # y voxel size, must be same as x, dim, start/end indices
    1                    # num of media
    1.010101 0.01 0.005 1.37  # scat. mus (1/mm), g, mua (1/mm), n
    4       1.0          # detector number and default radius (in grid unit)
    30.0  20.0  0.0  2.0 # detector 1 position (real numbers in grid unit) and individual radius (optional)
    30.0  40.0  0.0      # ..., if individual radius is ignored, MCX will use the default radius
    20.0  30.0  0.0      #
    40.0  30.0  0.0      #
    pencil               # source type (optional)
    0 0 0 0              # parameters (4 floats) for the selected source
    0 0 0 0              # additional source parameters

Note that the scattering coefficient mus=musp/(1-g).

### 2.3. JSON shape file format

Starting from v0.7.9, MCX can also use a shape description file in the place of the volume file. Using a shape-description file can save you from making a binary .bin volume. A shape file uses more descriptive syntax and can be easily understood and shared with others.

Samples on how to use the shape files are included under the example/shapetest folder.

The sample shape file, shapes.json, is shown below:

     {
      "MCX_Shape_Command_Help":{
         "Shapes::Common Rules": "Shapes is an array object. The Tag field sets the voxel value for each
             region; if Tag is missing, use 0. Tag must be smaller than the maximum media number in the
             input file.Most parameters are in floating-point (FP). If a parameter is a coordinate, it
             assumes the origin is defined at the lowest corner of the first voxel, unless user overwrite
             with an Origin object. The default origin of all shapes is initialized by user's --srcfrom0
             setting: if srcfrom0=1, the lowest corner of the 1st voxel is [0,0,0]; otherwise, it is [1,1,1]",
         "Shapes::Name": "Just for documentation purposes, not parsed in MCX",
         "Shapes::Origin": "A floating-point (FP) triplet, set coordinate origin for the subsequent objects",
         "Shapes::Grid": "Recreate the background grid with the given dimension (Size) and fill-value (Tag)",
         "Shapes::Sphere": "A 3D sphere, centered at C0 with radius R, both have FP values",
         "Shapes::Box": "A 3D box, with lower corner O and edge length Size, both have FP values",
         "Shapes::SubGrid": "A sub-section of the grid, integer O- and Size-triplet, inclusive of both ends",
         "Shapes::XLayers/YLayers/ZLayers": "Layered structures, defined by an array of integer triples:
              [start,end,tag]. Ends are inclusive in MATLAB array indices. XLayers are perpendicular to x-axis, and so on",
         "Shapes::XSlabs/YSlabs/ZSlabs": "Slab structures, consisted of a list of FP pairs [start,end]
              both ends are inclusive in MATLAB array indices, all XSlabs are perpendicular to x-axis, and so on",
         "Shapes::Cylinder": "A finite cylinder, defined by the two ends, C0 and C1, along the axis and a radius R",
         "Shapes::UpperSpace": "A semi-space defined by inequality A*x+B*y+C*z>D, Coef is required, but not Equ"
      },
      "Shapes": [
         {"Name":     "Test"},
         {"Origin":   [0,0,0]},
         {"Grid":     {"Tag":1, "Size":[40,60,50]}},
         {"Sphere":   {"Tag":2, "O":[30,30,30],"R":20}},
         {"Box":      {"Tag":0, "O":[10,10,10],"Size":[10,10,10]}},
         {"Subgrid":  {"Tag":1, "O":[13,13,13],"Size":[5,5,5]}},
         {"UpperSpace":{"Tag":3,"Coef":[1,-1,0,0],"Equ":"A*x+B*y+C*z>D"}},
         {"XSlabs":   {"Tag":4, "Bound":[[5,15],[35,40]]}},
         {"Cylinder": {"Tag":2, "C0": [0.0,0.0,0.0], "C1": [15.0,8.0,10.0], "R": 4.0}},
         {"ZLayers":  [[1,10,1],[11,30,2],[31,50,3]]}
      ]
     }

A shape file must contain a "Shapes" object in the root level. Other root-level fields are ignored. The "Shapes" object is a JSON array, with each element representing a 3D object or setting. The object-class commands include "Grid", "Sphere", "Box" etc. Each of these object include a number of sub-fields to specify the parameters of the object. For example, the "Sphere" object has 3 subfields, "O", "R" and "Tag". Field "O" has a value of 1x3 array, representing the center of the sphere; "R" is a scalar for the radius; "Tag" is the voxel values. The most useful command is "[XYZ]Layers". It contains a series of integer triplets, specifying the starting index, ending index and voxel value of a layered structure. If multiple objects are included, the subsequent objects always overwrite the overlapping regions covered by the previous objects.

There are a few ways for you to use shape description records in your MCX simulations. You can save it to a JSON shape file, and put the file name in Line#6 of yoru .inp file, or set as the value for Domain.VolumeFile field in a .json input file. In these cases, a shape file must have a suffix of .json.

You can also merge the Shapes section with a .json input file by simply appending the Shapes section to the root-level object. You can find an example, jsonshape_allinone.json, under examples/shapetest. In this case, you no longer need to define the "VolumeFile" field in the input.

Another way to use Shapes is to specify it using the -P (or --shapes) command line flag. For example:

     mcx -f input.json -P '{"Shapes":[{"ZLayers":[[1,10,1],[11,30,2],[31,60,3]]}]}'

This will first initialize a volume based on the settings in the input .json file, and then rasterize new objects to the domain and overwrite regions that are overlapping.

For both JSON-formatted input and shape files, you can use the JSONlab toolbox [4] to load and process in MATLAB.

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