nx_paint_blend.nx

nx_paint_blend.nx -- alpha-compositing rect fill into a Framebuffer using the standard "source over" Porter-Duff operator. Phase 4 fourth primitive of NISHI_BROWSER_ROADMAP. Pair to nx_paint_solid_rect (which OVERWRITES pixels regardless of source alpha); this primitive MIXES per pixel. Formulas (Porter-Duff "A over B", premultiplied math executed with straight-alpha 8-bit source + destination): out.a = src.a + dst.a * (255 - src.a) / 255 out.r = (src.r * src.a + dst.r * (255 - src.a) * dst.a / 255) / out.a (compositing) Simplified for the common case where destination is opaque (dst.a = 255), reducing to: out.r = (src.r * src.a + dst.r * (255 - src.a)) / 255 out.g = (src.g * src.a + dst.g * (255 - src.a)) / 255 out.b = (src.b * src.a + dst.b * (255 - src.a)) / 255 out.a = 255 We use the simplified form because the framebuffer's destination is always opaque (no transparency on the final screen surface). When src.a == 0 the pixel is unchanged (early-out). When src.a == 255 the result is equivalent to nx_paint_solid_rect (full overwrite, faster path). What it does NOT handle yet (Phase 4b queued): - non-opaque destination (would need premultiplied-alpha buffer) - non-source-over operators (multiply, screen, overlay, etc.) - per-pixel-alpha source (currently solid color only) - SIMD acceleration Per cardinal feedback-honest-perf-verdict-no-aspirational-claims: gap list is EXACT. genealogy_id: porter_duff_compositing_1984 + w3c_css_compositing_module_1 lineage_id: nishi_browser_paint_blend_v1