Lasso Selection

Core Feature

This feature is available in all Portrai Explorer deployments.

Lasso selection allows you to interactively draw regions on the map or scatter plot to select specific data points. Selected points can be used for subsetting and analysis.

Activating the Lasso

From the Mode Switcher

1. Locate the mode switcher at the bottom-center of the map viewer
2. Click the Lasso button, or press S
3. The button highlights to indicate Lasso mode is active

Mode Indicator

When Lasso mode is active:

• The Lasso icon appears highlighted/selected
• The pointer keeps its normal arrow; the lasso line appears once you press and drag
• The other tool buttons appear unselected

Refining a lasso

A lasso is one outline and cannot be edited once drawn. To adjust the boundary, switch to the Brush or Eraser and paint: the outline becomes an editable filled region. See Brush Selection.

Drawing a Lasso Selection

Basic Drawing

1. Press - Press the mouse button on the map where you want to begin
2. Drag - Keep the button held and drag around your region of interest
3. Release - Release the button to close the selection

While you drag, a solid blue line follows the pointer and a fainter straight line joins the pointer back to the start, showing how the shape will close. The map does not pan or zoom until you release. You can drag past the edge of the map; the stroke continues, and the part drawn outside still counts.

On release the shape closes along that straight line, the outline turns dashed, and the points outside it are dimmed.

Tips for Drawing

• Draw a closed shape - The selection automatically closes when you release
• Smooth movements - Draw steadily for accurate selections
• Encompass points - Ensure desired points are inside the boundary
• Do not cross your own line - Where the outline loops over itself, the area it encloses twice is left out of the selection
• Redo if needed - Draw a new selection to replace the previous one

Abandoning a Stroke

A lasso is discarded instead of completed if, while the mouse button is still down, you:

• Press Esc (which also returns the map to Pan) or one of the tool keys V, B or E
• Press Space
• Switch away, so the browser window loses focus

Releasing without moving (a plain click) discards it too.

A discarded stroke also clears the previous selection, so no lasso is left. To keep a selection, add it to the subset before you draw again.

On a scatter plot the keys Esc, V, B and E do not affect a stroke; the other cases apply there as well.

Selection Results

Point Count

About half a second after you release, the mode switcher displays the count of selected points:

Click the Items dropdown in the mode switcher to see available actions for selected points.

Visual Feedback

Selected points are drawn at full strength. Everything else that passes your filters stays on screen, dimmed: same color, lower opacity. Nothing is removed, so you can see where the selection sits inside the tissue or the embedding instead of losing its surroundings.

• Points inside the lasso keep their color at full opacity
• Points outside it are dimmed rather than hidden, and remain hoverable
• Points a Subset condition or a cutoff excludes are hidden, not dimmed
• Hovering a category or a histogram bar in the Detail Panel hides the dimmed layer while the pointer rests on it, leaving only the highlighted points that are also selected

Very large datasets colored by a continuous feature or a Signature score are the exception: there the map and the scatter plots draw a blended picture, and the points outside the lasso disappear instead of dimming. See Very Large Datasets.

On the map the dashed outline stays after you release, also once you add the selection to the subset. It goes away when you start another lasso, when you switch projection (see Switch Projection), and once neither the selection nor a map lasso condition is left. On a scatter plot the outline stays only while that plot holds the active selection, so adding it to the subset, clearing it or drawing a lasso elsewhere removes it.

Using Selections

Add Selection to Subset

Save your lasso selection as a subset condition:

1. Complete your lasso selection
2. Click the Items dropdown in the mode switcher
3. Select Add selection to subset
4. The selection is added to the Subset

Once added, the lasso condition:

• Appears in the Subset Editor
• Can be combined with other conditions using AND/OR
• Persists even after drawing new selections
• Can be named for easy identification
• Hides the points outside the lasso instead of dimming them, because the lasso now narrows the subset (see Subsets Narrow, Selections Mark)

Selection in Different Views

Spatial Coordinates

Lasso selections in spatial view:

• Select points based on physical tissue location
• Useful for defining anatomical regions
• Saved with spatial coordinate information

Embedding Views (UMAP/t-SNE)

Lasso selections in embedding views:

• Select points based on transcriptional similarity
• Groups cells that cluster together in reduced dimensions
• Saved with embedding column reference

Scatter Plot

Lasso selections on scatter plots:

• Select points based on X/Y axis values
• Useful for selecting specific value ranges
• Saved with axis column references
• Workspace-global — the selection is shared across every scatter plot in the dashboard, with only one active at a time (drawing a new lasso in any plot clears the previous plot's outline)

Clearing Selections

Clear the Selection

Open the Subset Panel. The selection preview banner at the top carries a Clear ✕ that discards the selection without adding it to the subset.

Draw New Selection

Drawing a new lasso automatically replaces the previous selection. On the Plot dashboard the lasso is shared across every scatter plot — only one selection is active at a time, so drawing a new lasso in any plot replaces the previous one and clears the earlier plot's outline. The map and the scatter plots share that one selection as well: a lasso drawn on the map replaces a scatter-plot lasso, and the other way round. A scatter-plot lasso also applies to the map, where the selection preview banner names it after the plot's two axes.

Switch Projection

Switching the map to another projection clears a map lasso that has not been added to the subset, and a notification titled Lasso selection cleared says so. To keep the selection, add it to the subset before you switch: a lasso condition keeps selecting by the projection it was drawn on, whichever projection is shown. The switch also removes the outline, even for a lasso already in the subset, and the outline does not come back when you return. A lasso drawn on a scatter plot is not affected.

A pending brush region is cleared the same way, with a notification titled Brush selection cleared (see Brush Selection).

Leaving the Explorer for another extension and coming back returns the map to the default projection, which clears a pending lasso or brush region made on another projection the same way. See Per-Projection View.

Return to Pan Mode

Click the Pan button (on the map you can also press V or Esc) to leave the Lasso. Leaving the tool does not clear the selection: it stays until you draw a new one, discard it from the banner, or add it to the subset. On the map, switching projection also clears it, and painting over it with the Brush or Eraser turns it into a brush region. Leaving the tool in the middle of a stroke is the exception: see Abandoning a Stroke.

Lasso Conditions in Subsets

When a lasso selection is added to the subset, it becomes a Lasso Condition:

Lasso Condition Properties

Property	Description
Name	Auto-generated or custom name
Type	Source view (Spatial, UMAP, t-SNE, Scatter Plot)
Polygon	Stored coordinate boundary

Combining Lasso Conditions

Multiple lasso conditions can be combined:

• AND - Points must be in all selected regions
• OR - Points can be in any selected region

This enables complex spatial queries like "cells in Region A AND Region B" or "cells in Region A OR Region B".

Best Practices

Accurate Selections

1. Zoom in for precise selection of small regions
2. Zoom out to select larger areas efficiently
3. Use embeddings to select transcriptionally similar cells regardless of location

Workflow Integration

1. Explore first - Use Pan mode to understand your data
2. Select purposefully - Switch to the Lasso with a specific goal
3. Save important selections - Add meaningful selections to subsets
4. Combine with other conditions - Use lasso with feature-based conditions

Performance

• The map and the scatter plots dim the points outside a lasso as soon as you release
• The item count, the Detail Panel, the plots other than the scatter plot and Download Selected Data follow within about half a second
• When you draw lassos one after another, the item count, the other plots and Download Selected Data wait until you pause, so the next stroke stays smooth

Troubleshooting

Selection Not Working

• Ensure the Lasso tool is active (its button highlighted in the mode switcher)
• Check that you're clicking and dragging on the map area
• Drag rather than click: a click without movement draws no lasso and clears the one you had
• Verify data points are loaded and visible

Wrong Points Selected

• Zoom in for more precise selection
• Check the current projection mode (Spatial vs. Embedding)
• Verify you're selecting in the intended coordinate space

Count Shows Zero

• The lasso may not contain any data points
• Check if subsets are excluding all points
• Check that the outline does not loop over itself; an area it encloses twice is left out

Related Topics

• Subset - Work with saved lasso conditions
• Map Viewer - Map viewer interaction modes
• Scatter Plot - Lasso selection on plots