# Parameter Keywords

GSnap can read various settings from a text file containing keyword-value pairs. The location of the file can be specified either using the command line switch --parfile or by setting the environment variable GSNAP_PARS. Alternately, parameters can be specified on the command line using the --param flag. The following is a list of the recognized keywords, their meanings, and their default values [in square brackets].

ImageWidth [600]
The width of the output images, measured in pixels.

ImageHeight [600]
The height of the output images, measured in pixels.

ViewWidth [125]
The width of the field of view, measured in kiloparsecs. The height of the field of view is scaled based on the value of the ImageHeight parameter.

ViewDepth [125]
The depth of the volume rendering region, measured in kiloparsecs.

MinNeighbors [20]
The minimum number of overlapping SPH kernels used in the volume rendering scheme.

MaxNeighbors [40]
The maximum number of overlapping SPH kernels used in the volume rendering scheme.

GasOpacity [100]
Specifies the opacity of the gas. A value of 0 means fully transparent, higher indicate higher opacity.

GasGamma [0.33]
The value of gamma ($\gamma$) used when rendering images of the gas component. This works along with the GasBeta parameter to set the scaling of the pixel brightness in the image. Initially, the pixel array is scaled linearly and is normalized so that all pixel values fall between 0 and 1. Given the limited dynamic range of computer monitors and shallow bit depth of common digital image formats, the brightness of each pixel, $f_{ij}$, must be scaled non-linearly in order for the image to fit into the limited dynamic range. GSnap currently scales $f_{ij}$ according to $$F_{ij} = \left[\frac{1-\exp(-\beta f_{ij})}{1-\exp(-\beta)}\right]^\gamma$$
where $F_{ij}$ are the new pixel values, which are stored in the image file (actually, it is first multiplied by 255 in the case of an image format with 8 bits per pixel, then saved).

GasBeta [0.2]

Sets the value of $\beta$, which was introduced in the description of the GasGamma parameter. This allows the user to fine-tune the gradual transition to black.

GasClipMin [5e-5]
Pixels dimmer than this will be clipped. This is the value of the dimmest pixels in the pixel array when using the --view gas option.

GasClipMaxNorm [0.95]
The brightest pixels in the final image will be set to this fraction of white. For instance a value of 0.95 means that the largest sub-pixel values in the image will be 255*0.95 = 242 (i.e., {R,G,B} = {242,242,242})

GasClipMaxVal [0.05]
Pixels brighter than this will be clipped. (used with --view gas)

StarGamma [0.33]
Same as GasGamma, but applies to the stellar rendering, rather than gas.

StarBeta [0.2]

Same as GasBeta, but applies to the stellar rendering, rather than gas.

StarClipMin [0.0]
Same as GasClipMin, but applies to the stellar rendering, rather than gas.

StarClipMaxVal [0.066]
Same as GasClipMaxVal, but applies to the stellar rendering, rather than gas.

StarClipMaxNorm [1.0]
Same as GasClipMaxNorm, but applies to the stellar rendering, rather than gas.

SlitWidth [1.0]
Set the width of the slit—just like the -w option.

SlitLength [5.0]
Set the length of the slit— like the -l option.

Projections [1000]
Set the number of projections used in the velocity dispersion statistics calculations.

ColorMap
Specifies the location and name of the colormap that you wish to use for indicating gas temperatures.