Background pass — bg_hue, bg_bright, bg_vignette, the ramp and the band

An audio-tintable gradient + vignette backdrop drawn before the scene, engine- wide. bg_bright = 0 (the default) is a black backdrop; raise it to reveal the gradient. bg_hue picks the tint out of the preset’s palette; bg_vignette (0..1) darkens the corners.

bg_hue is a coordinate in your [palette] (ADR-0086). The pass samples the same baked LUT every scene samples, so an ember preset gets an ember sky and a custom gradient tints its own backdrop — and saturation / palette_mix move the backdrop with the figure, so an A/B crossfade carries the whole frame. It is cyclic, with the same wrap trap color_center has: -0.1 and 0.9 are the same place, and on a stop-list palette that seam is the sharpest transition in the gradient. With no [palette] declared the gradient is spectrum, and the twenty-row swatch table in Colour and palettes is that ramp — so bg_hue 0.30 is cornflower blue, 0.45 aqua, 0.85 amber until you declare stops of your own, at which point read the value off your own gradient instead. A bg_hue copied from another preset arrives at whatever colour your gradient holds there, not the colour it was in the preset you took it from. Visible wherever the scene leaves the frame unpainted: behind the sparse scenes (lines, swarm, attractor) where the gaps show through, and in the reaction-diffusion field’s voids (both composite over the backdrop instead of presenting opaque). The fragment field is the one full-screen scene that still draws opaquely, so bg_* has no visible effect there — including the whole ramp and the band below. A dusk ground under a fragment_field preset is not a dim ground, it is an absent one.

In this section

• The directional ramp — bg_angle, bg_hue_span, bg_shade, bg_shade_end, bg_ramp_gamma
• The ramp can be measured around a point — bg_coord_mode, bg_center_x, bg_center_y
• The curved band — bg_band_amount, bg_band_angle, bg_band_pos, bg_band_width, bg_band_curve, bg_band_hue, bg_band_hue_span
• Paper — bg_paper