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Crosshairs

Scope​

This specification describes the crosshairs control from the point of view of the user. It covers the intersection lines, the control points, and the slab thickness of the plane. It does not cover the reference point marker, which is a separate control.

Change rule:

  • A user requirement changes in two cases only: the requirement describes the behaviour wrongly, or the intended user-facing behaviour itself changes. An implementation that is inconvenient is never a reason to change a user requirement.
  • An implementation requirement changes freely when a better approach appears.

Description​

The crosshairs show the user where the other views cut the image of the current view. Each other view contributes one line. The colour of a line names the plane of that view: red for axial, yellow for sagittal, green for coronal.

The lines are a control, and not only an indicator. The user drags a line to move the plane that the line represents. The user rotates the plane with the round control points. The user changes the slab thickness of the plane with the hollow control points.

A plane does not need to stay parallel to an anatomical axis. The user rotates a plane, and the plane becomes oblique. Every requirement in this specification holds for an oblique plane.

User requirements​

Lines (CROSSHAIRS-LINE)​

  • CROSSHAIRS-LINE-1 The user sees one line for each other view of the same data. The user does not see a line for the view under the cursor, and the user does not see a line for a view of the same plane.
  • CROSSHAIRS-LINE-2 The user drags a line, and every view of that plane moves to the new position. No other view moves.
  • CROSSHAIRS-LINE-3 The user sees the colour of a line, and the user knows which plane the line represents.

Control points on hover (CROSSHAIRS-HOVER)​

  • CROSSHAIRS-HOVER-1 The lines carry no control point until the user moves the cursor onto a line. The image stays clear while the user reads it.
  • CROSSHAIRS-HOVER-2 The user moves the cursor onto a line, and the control points of that line appear: the round points for the rotation, and the hollow points for the slab thickness.
  • CROSSHAIRS-HOVER-3 The user moves the cursor from the line to a control point of that same line, and the control points stay visible for the whole travel. The control points never disappear under the cursor.
  • CROSSHAIRS-HOVER-4 The user reaches every control point that the user sees. A control point that the user cannot reach is a defect.
  • CROSSHAIRS-HOVER-5 Two lines cross each other, and the user hovers near the point where the lines cross. The control points of the closest line appear.
  • CROSSHAIRS-HOVER-6 The application removes a control, and the user gets no hover response for that control. The user never selects something that does nothing.

Slab thickness (CROSSHAIRS-SLAB)​

  • CROSSHAIRS-SLAB-1 The slab of a plane is thicker than the minimum, and the user sees two dashed lines, one on each side of the centre line. The two dashed lines show the two boundaries of the slab.
  • CROSSHAIRS-SLAB-2 The hollow control points sit on the dashed boundary lines. The user reads the position of a hollow control point, and the user knows the boundary that the control point moves.
  • CROSSHAIRS-SLAB-3 The user hovers a dashed boundary line, and the control points of that plane appear. The dashed line is part of the control.
  • CROSSHAIRS-SLAB-4 The user hovers the space between the centre line and a dashed boundary line, and nothing appears. That space belongs to the image, and not to the control.
  • CROSSHAIRS-SLAB-5 The user drags a hollow control point, and the dashed boundary line follows the cursor. The control point stays under the cursor.
  • CROSSHAIRS-SLAB-6 The slab is at the minimum, and the user sees no dashed line. The hollow control points still appear near the line, because the user must be able to make the slab thicker again.

Oblique planes (CROSSHAIRS-OBLIQUE)​

  • CROSSHAIRS-OBLIQUE-1 The user rotates a plane in one view, and the line of that plane in a third view becomes oblique. All of the requirements above hold in that third view.
  • CROSSHAIRS-OBLIQUE-2 The plane is oblique, and the hollow control points stay on the dashed boundary lines. The user does not search for a control point that left the line.
  • CROSSHAIRS-OBLIQUE-3 The angle of the plane changes the position of the dashed lines, and the control points move with those lines. The user needs no knowledge of the angle.

Implementation requirements​

  • CROSSHAIRS-IMPL-1 The SliceIntersectionTool implements this control on the generic planar viewports.
  • CROSSHAIRS-IMPL-2 The hollow control points take the position from the dashed boundary lines that the tool draws. A projection of the slab offset gives the wrong position on an oblique plane, so the tool does not use one.
  • CROSSHAIRS-IMPL-3 The sliceIntersections example carries a slab thickness slider and a toggle for the slab thickness control points. A developer reproduces every requirement above with that example.

Past defects​

This section records the defects that the requirements above prevent. A new defect of the same kind gets a new line here.

  • The control points disappeared before the cursor reached them, because the hover region held the centre line only. Fixed in PR 2920. Requirement: CROSSHAIRS-HOVER-3.
  • The hollow control points left the dashed boundary lines on an oblique plane, because the tool computed the position from the slab offset instead of the drawn line. Fixed in PR 2920. Requirement: CROSSHAIRS-OBLIQUE-2.
  • A hover on a dashed boundary line selected a plane although the application had removed the slab control. Fixed in PR 2920. Requirement: CROSSHAIRS-HOVER-6.
  • A hover near the point where two dashed boundary lines cross selected the wrong plane. Fixed in PR 2920. Requirement: CROSSHAIRS-HOVER-5.