Domain: Humanities and Social Sciences — Arts
Target concept: Gradient Technique: The shading in a sketch transitions smoothly from dark to light, designated as the gradient area, with no distinct color boundaries. The tonal variation conforms to the light and shadow laws of the object.

Video generation prompt: First, sketch a simple sphere. Apply the gradient technique to draw smooth transitional areas of light and shadow on the sphere, keeping the light source direction fixed as the shading is applied.

Evaluation guide:
(i) Target concept: Visual realization of gradient shading as a continuous tonal mapping constrained by spherical geometry and directional lighting.
Minimum mechanism: A static sphere must be illuminated from a fixed direction, generating a predictable highlight-core shadow-midtone arc; tonal interpolation must occur along that arc without violating surface continuity or lighting logic.
(ii)Storyline: Frame 1–2: Clean circular sphere outline appears, centered and intact. Frame 3–6: Shading motion initiates at highlight region and advances continuously along a convex arc toward the opposing shadow region. Frame 7–10: Final tonal band stabilizes: lightest, mid, and darkest regions coexist in correct spatial order, fully contained within the sphere silhouette, with no boundary breaks or tonal inversions. Perspective is fixed frontal; no camera movement; all events occur within a single unoccluded view.

Manual:
Experimental Procedure
1. Draw a clean circular outline to define a sphere.
2. Apply gradual tonal modulation from lightest to darkest region across the sphere surface in one continuous motion.
Observation of Experimental Phenomena
A single sphere appears with smooth, unbroken tonal transition spanning light → midtone → shadow; no hard edges or discrete zones; darkest region is fully contiguous and opposite the lightest region.
Summary of Experimental Principles
Gradient technique requires smooth, continuous tonal variation conforming to the light and shadow laws of a spherical object—no abrupt boundaries, and shading must follow surface curvature.

Prompt Grounding rubric:
level_1: No sphere appears in the frame; or the drawn object is not a single, isolated sphere (e.g., an ellipse, polygon, or multiple shapes); or no shading marks are applied to it at all; or unrelated objects (e.g., a cube, a hand, text) occupy the frame instead of or alongside the sphere.
level_2: A sphere is present, but a second distinct object also appears (e.g., a second sphere, a ruler, a leftover construction line); or shading is applied to an object that is not spherical; or the shading tool/mark-making method (pencil stroke, brush pass, digital blend) cannot be identified from the visible marks.
level_3: A single sphere is drawn and remains the only object in frame; visible shading is applied directly onto its surface using a continuous surface-blending technique (smudged/blended tone) rather than separate hatch lines, dots, or patches; the sphere's outline stays circular and centered while shading proceeds.
level_4: All conditions for score 3 are met, AND the shading is produced in one continuous pass over the sphere's surface: it does not stop, get erased, and restart as a visually disconnected patch of tone elsewhere on the sphere; no extra construction marks (grid lines, cross-hatching guides, stray dots) remain visible once shading begins.
level_5: All conditions for score 4 are met, AND by the final frame the sphere is the sole rendered object, at its original size and position from Frame 1–2, with the shading fully confined to the sphere's silhouette (no tone marks spill outside its circular edge onto the background).

Scientific and Causal Correctness rubric:
level_1: The sphere's surface shows no tonal variation at all (uniform flat color); or the light/dark split is a hard-edged binary zone (e.g., one half solid black, the other solid white) with no intermediate tones; or the light and dark regions are separated by a straight line rather than following the sphere's curved surface (e.g., a vertical line cutting the sphere in half).
level_2: Some tonal variation is visible, but it does not follow the sphere's curvature: for example, the gradient radiates outward from the center like a flat disc rather than wrapping around a rounded form, or the brightest tone sits on the side opposite where the shading logically originates, or the tonal blend is confined to a thin ring at the sphere's edge instead of spreading across its visible face.
level_3: Tonal variation spans the sphere's surface with at least three visually distinct bands: a light region, a mid-tone region, and a dark region; the bands form one continuous curved arc across the surface rather than concentric rings or a straight split; the darkest band occupies one side of the sphere and does not wrap all the way around to meet itself.
level_4: All conditions for score 3 are met, AND the lightest band is a compact area rather than covering half the sphere, the mid-tone band forms a continuous bridge between the light and dark bands with no gaps, and the dark band is a single unbroken region sitting on the side of the sphere opposite the light band (no islands of mid-tone breaking up the dark region).
level_5: All conditions for score 4 are met, AND the light-to-mid-to-dark transition reads as one smooth, unbroken curved band that follows the sphere's rounded surface from edge to edge, with no kinks, no split branches, and no tone bleeding past the sphere's silhouette onto the background.

Spatiotemporal Consistency rubric:
level_1: The sphere visibly deforms (stretches, flattens, or pulses in size) at any point; the sphere's outline vanishes or flickers between frames; the shading tone jumps abruptly from light to dark between two consecutive frames with no intermediate tone ever shown; or the viewing angle changes (pan, zoom, or rotation of the camera).
level_2: The sphere's outline stays intact but shows visible jitter (its edge wobbles slightly frame to frame); the shading advances then visibly pauses or reverses direction more than once; or the lit side of the sphere appears to relocate mid-sequence without any visible cause (the highlight jumps to a new spot rather than sliding there).
level_3: The sphere's circular outline stays stable and in the same position throughout; the shading advances in one direction only, without reversing; there is no flicker and no sudden relocation of the shaded region; the apparent light-source direction stays fixed relative to the sphere for the whole sequence.
level_4: All conditions for score 3 are met, AND the shaded area grows steadily darker/larger along the motion path without ever visibly shrinking or lightening back; no frame shows a tone region suddenly becoming lighter than it was in the previous frame; no part of the shading appears to float in front of or sit behind the sphere's surface.
level_5: All conditions for score 4 are met, AND in every frame the darkening tone stays confined to the same curved path across the sphere's surface: it does not drift sideways off that path, does not appear to sit at a different depth than the sphere's surface, and does not shift out of alignment with the sphere as if seen from a slightly different angle.