Part of Family 6 (Colour & effects) — changing a register mid-frame. Normal DMA moves data during VBlank; HDMA hands the PPU a table and fires a transfer at the start of each scanline, so a register holds a different value on every line. That is how the SNES bends backgrounds, gradients the sky, and draws effects that look impossible for a 2D tile machine.

The ladder

Rung	Example	Developer question
6e.2	gradient_colors	How do I gradient the backdrop colour per scanline?
6e.3	hdma_indirect_gradient	How does indirect HDMA (pointer-table addressing) work?
6e.4a	hdma_wave	How do I distort a background with a precomputed HDMA table?
6e.4b	hdma_wave_table	How do I build that table by hand in C and animate the pointer?
6e.5	hdma_helpers	How do I just call the hdma module helpers (wave, ripple, iris)?

hdma_wave (use a precomputed table) and hdma_wave_table (build it by hand) are the "use it" / "understand it" pair — slated to merge into one "build the table, then use the helper" rung (a code-merge left as a reviewed follow-up). On-ramp to author: 6e.1 a minimal single-channel HDMA.

The idea in one screen

An HDMA table is a list of (line count, value) entries; the PPU walks it, writing value to a target register for count scanlines, then moves on. Aim it at the backdrop colour and you get a per-line gradient (gradient_colors); aim it at a BG scroll register with a sine table and the background ripples (hdma_wave). Indirect mode adds a level of pointer indirection so the table entries point at data elsewhere (hdma_indirect_gradient). The hdma module wraps the common cases (hdmaWaveH, hdmaWaterRipple, hdmaIrisWipe) so you rarely hand-roll the table — but building one once (hdma_wave_table) is how you understand what the helpers do.

Gradient Colors -- HDMA Color Gradients HDMA Helpers Demo HDMA Indirect Gradient HDMA Wave HDMA Wave Table