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Tool: Volume Viewer

Volume data is a broad term for density maps, electrostatic potential maps, 3D imaging data, and other data sets with numerical values associated with points on a 3D grid. Volume data can be multichannel and/or time series. Volume data can be opened with the command open (which automatically starts Volume Viewer) and saved to a file with save.

3eebB ESP isosurfaces

Volume Viewer is a graphical interface for adjusting contour levels and other aspects of volume data display. Alternatively, these settings and more can be adjusted with the volume command.

Even without data, Volume Viewer can be started from the Volume Data section of the Tools menu or with the command toolshed show. The Volume Viewer window can be manipulated like other panels in ChimeraX (more...).

See also: Map icons, Right Mouse icons, Marker Placement, show, color, transparency, vseries, molmap, fitmap, surface operations, measurements, ChimeraX DICOM Reference

By default, a data set will be displayed automatically if it does not exceed a certain size (volume command option showOnOpen), although it may be subsampled (volume command option limitVoxelCount).

A histogram of values with moveable thresholds (contour levels) is shown for each data set. Above the histogram are:

The step setting controls subsampling; a value of 1 means all the data points are used to generate the display, while 2 means every other data point is taken along each axis, reducing the number of points used by a factor of eight. Step sizes can be different in the X, Y, and Z directions, although this can only be set with the volume command step option (not with the graphical interface).

The display region is not necessarily the full extent of the data grid; it can be limited with the volume command region option or the mouse mode for crop volume .

The thresholds on the histogram control how data values are mapped to the display:

surface or mesh style:
Each threshold is shown on the histogram as a vertical bar that can be moved horizontally. For unsigned data types, an initial threshold is set so that 1% of voxels (1% of the volume encompassed by the data region) lie above it; for signed data types, positive and negative thresholds are placed symmetrically about zero. Each threshold corresponds to a contour surface, generated as a submodel of the data model.
volume (image) style:
Each threshold is shown as a small square that can be moved horizontally and/or vertically. For unsigned data types, initial thresholds are set to give zero intensity for the 10% of voxels (in the data region) with the lowest values, scaling up to 0.99 intensity for the 1% of voxels with the highest values; for signed data types, positive and negative thresholds are placed symmetrically about zero. The thresholds and connecting lines on each histogram define a transfer function that maps data values to colors and intensities. The rightmost threshold does not have to be at the far right of the histogram, and a threshold at a lower data value can be higher (vertically) than a threshold at a higher data value.

Threshold levels can be adjusted interactively by dragging in the Volume Viewer histogram or by using the mouse mode for contour level or windowing in the graphics window.

The Volume Viewer context menu includes:

Each threshold is shown in the same color as its display. The color and level shown above the histogram apply to the threshold most recently moved or clicked. The color, including opacity, can be changed by clicking the color square and using the resulting color chooser. The level can be changed by entering a different value or by dragging the threshold. Holding the Shift key down reduces the speed (mouse sensitivity) of threshold dragging tenfold, allowing finer control.

Histogram ranges and threshold levels are given in the same units as the data. The volume command also allows specifying threshold levels in units of root-mean-square deviations from zero or standard deviations from the mean.

Multichannel Data

When multiple maps are specified in a single open command (multiple files specified with wild cards or globbing), channel numbers are discerned from the filenames, if possible. Multiple channels can also be read from a single file (OME TIFF and certain other volume formats). Either way, the channels are automatically grouped, in that changing the display style, step size, or plane for one channel in Volume Viewer changes it for all of the channels in the group. The volume command allows changing settings for individual channels.

Color blending of multichannel data in the image display style can be done on the GPU.

See also: the ChimeraX light microscopy highlight

UCSF Resource for Biocomputing, Visualization, and Informatics / June 2019