Basic_viewer: colour faces by value (distance, cell size, and a drawer value)

[RajdeepKushwaha5]

Small feature

This PR adds a small feature to the public API: a drawer can attach a scalar value to each face, which the Basic_viewer maps to a colour palette, mirroring the existing colour path.

New API:

• Graphics_scene_options: valued_face (does this face have a value), face_value (the value), face_value_name (the legend label).
• Graphics_scene: set_face_value(float), set_value_name(const std::string&).

Purpose: mesh-quality visualisation. The viewer cannot compute a quality measure itself (it only has the boundary triangles); each drawer can. The surface mesh drawer uses this to expose the aspect ratio of each face.

Scope: small and additive. It mirrors colored_face / face_color, defaults to off (no value), and changes no existing behaviour.

Description

This adds an automatic colouring of the faces by a value, mapped to a colour from a palette, discussed with the mentors as a follow-up to the clipping work.

Palette (D key). D turns the colouring on and cycles the palette: off, then a heat, a jet, a grey ramp and viridis (a perceptually uniform map). The palette is evaluated in the face shader, so there is no texture.

Value (Shift+D key). Shift+D picks the value, skipping the ones that do not apply to the scene:

• distance to the plane, as a smooth per-fragment gradient;
• the same distance, one flat colour per cell (needs volumes);
• the cell size, one flat colour per cell (needs volumes);
• a value provided by the drawer, one flat colour per face (see below).

Distance is computed live in the shader from the clipping plane the user places with Ctrl+C. Distance and size are normalised over the range actually present in the scene, so the whole palette is used.

A value from the drawer. Following Guillaume's suggestion, the viewer does not have to compute geometric values itself. A drawer can attach a scalar value to each face, the same way it attaches a colour: the graphics-scene options gain valued_face / face_value / face_value_name, the scene stores the value next to the colour, and the viewer maps it to the palette. As the example, the surface mesh viewer exposes the aspect ratio (longest edge / shortest edge) of each face. This is the general mechanism for mesh-quality colouring (aspect ratio, Jacobian, and so on), with each drawer computing what its mesh allows.

Legend. A small legend is drawn when the colouring is on: a gradient bar in the current palette with the value range, labelled by the drawer's name (or distance / size). It uses the same range and palette as the faces, so the two match, and it is hidden when the value has no range (a uniform field).

The Basic_viewer part is contained in Basic_shaders.h and Basic_viewer.h. The value mechanism also touches Graphics_scene.h, Graphics_scene_options.h, the face-graph drawer (BGL/draw_face_graph.h), and the surface mesh drawer.

Testing: verified on grids of hexahedra (Linear_cell_complex) and on triangle meshes (Surface_mesh). The values and palettes behave as described, the per-cell and per-face modes read as flat blocks, the colours span the full palette with the legend matching the values present (including as the clipping plane is moved and rotated), the drawer-provided aspect ratio colours the surface mesh with a named legend, and the colouring holds in every clipping mode.

Note on next step (Guillaume's suggestion)

The value mechanism generalises the built-in size and enables mesh-quality colouring per data structure. The surface mesh aspect ratio is the first example; values on volumes (set_volume_val) and other measures are natural follow-ups.

[sloriot]

Successfully tested in CGAL-6.3-Ic-52

[sloriot]

@gdamiand I'm waiting for your approval to merge it.

[github-actions]

This pull-request was previously marked with the label Tested, but has been modified with new commits. That label has been removed.