5. Segments¶
5.1. Concepts¶
Multi-segment datasets: In the Basic Concepts section we assumed that data could be modelled simply by splitting into two ‘segments’ - background (‘off’) and fossil (‘on’); these are called single-segment datasets in SPIERSedit (background is a special case, so not counted as a segment). In some datasets however the situation is more complex; for instance in Figure 11A two different types of material are present in the fossil (light and dark grey). This is best modelled as a multi-segment dataset – there are two types of material (recall that background does not count), so it they requires two segments in SPIERSedit, rather than the default one. In a multi-segment dataset each source image has more than one corresponding working image, one for each segment; each segment has independently specified generation rules. The threshold image no longer consists of pixels simply ‘on’ or ‘off’; instead each pixel is assigned to one of the segments (or none, background). The threshold image is calculated from the working images using the following simple rules: if a pixel is below threshold level for all segments, it is off (black); If it is above threshold level in only one segment it is assigned to that segment; if it is above threshold level in more than one working image it is assigned to the segment in with the lightest shade of grey. Figure 11B-D shows two working images and the corresponding combined threshold image.
Segments and Masks: The segment and mask systems are independent and complementary. Each pixel in each slice assigned to one segment (or none, i.e. background, ‘off’) AND one mask. Segments are primarily used to pick out different types of material, masks to pick out regions. The way in which masks and segments interact to produce renderable objects is discussed in the Output section below.
Figure 11. Multi-segment dataset. A; source image (starfish arm section comprising two distinct materials). B; working image for first segment (lighter material round edge of fossil). C; working image for second segment (darker material in core of fossil). D; Threshold image showing pixels identified as light segment (white), dark segment (purple) or neither (black).
Display colour: Segment colour is visible in all modes except mask mode, where mask colour is used instead, If however the Always Show Segments option in the Mode menu is turned on, you will see segments (in a washed-out form) in mask mode too. As with mask colour, segment colour is simply the colour that segments appear in SPIERSedit, and has no influence on output colour.
Inactive and locked segments: Like masks, segments can be locked or hidden, though showing/hiding is termed activation/inactivation for segments to emphasise the difference in function. Making segments inactive essentially deletes them; SPIERSedit treats inactive segments as though they do not exist. Unlike true segment deletion of course, a hidden segment can be restored with a single operation. Locking a segment stops all segment or brighten mode brush operations from affecting any pixels currently assigned to that segment.
5.2. Working with segments¶
Segment selection: The upper of the two drop-downs in the ‘segments’ section of the main toolbox is used to choose the segment you are working with at the moment. This affects the operation of slice generation, brighten mode and segmentation mode. The lower of the two drop-downs only affects the right mouse button in segmentation mode. As with masks, these choices are displayed as L and R (or B) in the leftmost column of the Segments panel, and left/right clicking in here will select them. Note that the R drop-down also has a ‘delete all’ option – if set to this, the right mouse will remove pixels from all segments (i.e. turn them off).
Brush in segment mode: In segment mode the brush is used to assign pixels directly to the selected segment (left mouse button) or segment specified in the ‘R’ drop-down (right mouse button). Note that the R drop-down also has a ‘delete all’ option – if set to this, the right mouse will remove pixels from all segments (i.e. turn them off).
Brush in brighten mode: The brighten brush only affects the selected segment. Note that brightening in a multi-segment dataset can not only bring pixels over the threshold level so they turn on (as before), but can also make them brighter than the corresponding pixels in other segments so that the pixel changes to the selected segment.
Brush in recalc mode: This only affects the selected segment.
Generation and segments: Generation rules are stored on a per-segment basis. If using linear generation for instance, one segment can use a rule with Invert on to pick out dark pixels, while the other uses a rule with Invert off to pick out light pixels. Selecting a different segment (see above) will flick the settings of the Generation toolbox to those setup for that segment for Linear and Polynomial generation. Range generation works in a different way, specifying a single set of rules applied to all segments (see below).
5.3. Advice on use of Segments¶
Generation method: For datasets based on colour source images, carefully chosen Linear generation settings and/or Polynomial generation are often the best way to generate working images for multi-segment datasets. For datasets based on monochrome source images however (e.g. CT data), multi-segment datasets are best handled with Range generation (see below).
Using segments to fill holes: A common task with fossil datasets is to generate a model of voids in a certain part of the model, for example inside a fossil otherwise reconstructed with a single segment. The segments system allows a simple trick to be used to handle this. First, the normal segment is created and properly edited (if desired). Once prepared, it is locked and a second ‘fill’ segment created. No slice-based generation is performed on this second segment – slices are left blank. To fill the holes, the first segment is locked, and the ‘fill’ segment then brushed roughly over the holes using the segment brush.
Number of segments: There is no formal limit on the number of segments, but the speed of many operations in SPIERSedit is inversely proportional to the number of segments in use; furthermore the disk-space used by the dataset is roughly proportional to the number of segments. In most datasets only two or three segments will be required; users are advised to avoid going beyond three unless they are sure they know what they are doing.
5.4. Segment Manipulation¶
Segment manipulation is primarily performed through the Segments panel, which is very similar to the Masks panel both visually and in operation. Some operations are performed instead through the Segments Menu.
Creating segments: The New button in the Segments panel (or the New Segment command in the Segments menu) creates a new segment; this will take an appreciable amount of time as new working images will need to be created for each source image. These are initially created blank (as are new working images for a new dataset).
Renaming segments: Double click on the Segment Name in the Segments panel to edit it.
Changing colours: Double-click the colour block to change it.
Selecting segments in the panel: One or more segments can be selected by left clicking on any column of the Segments panel. To select multiple segments use Ctrl-click or Shift-click. Selection is indicated by an underlined segment name. Note that segment selection in the panel and selection in the drop-down in the Main Toolbox panel are not the same thing. Selection of segments in the panel is used for bulk locking or activating/deactivation, bulk deleting, segment copying, and the creation of output objects.
Segment activation: Double-clicking a mask’s ‘eye’ icon toggles its activation status (visibility); this can be done to segments in bulk by selecting them (see above) and using the Activate Selected Segments or Deactivate Selected Masks commands on the Segments menu.
Segment locking: Double-clicking a segment’s ‘padlock’ icon toggles its lock status; this can be done to segments in bulk by selecting them (see above) and using the Lock Selected Segments or Unlock Selected Segments commands on the Segments menu.
Re-ordering segments list: Segments can be moved up and down the list by selecting a single mask and using the Up and Down buttons on the Segments panel. This reordering only affects how the segments appear in this list; it has no effect on output or images.
Deleting segments: It is not normal to delete a segment unless it has been created in error; normally segments that are not required are simply deactivated. Deleting a segment removes its associated files and cannot be undone. To delete a segment or segments first select them (see above), then click delete or use the Delete Selected Segment(s) command from the Segments menu.