====== Triangle Engine ======

''tri D,S2,S1'' draws a primitive using the fixed Triangle Engine parameter registers. Set ''DrawCtl[3:0] = 0xB'' before writing the parameters. The instruction performs a vector add before launch; it can load XY, ZQ, UV, RA, GB, R2F or G2B2 as part of the operation.

Launch takes one cycle after setup. Engine time is stated as ''4 + scanlines + pixels'' cycles, excluding memory stalls. The RISC continues until it needs a shared resource or writes a busy shadowed register. There is a three-instruction hazard from ''tri'' to a Fill Engine instruction.

===== Register setup =====

^ Locations ^ Parameters ^
| ''a0..a13 / %128..%141'' | XTop, YTop, Z, Q, R, A, G, B, R2, F, G2, B2, U, V |
| ''a14 / %142'' | dXLong/dy |
| ''a16..a29 / %144..%157'' | XMid, YMid and span gradients |
| ''a30 / %158'' | dXTop/dy |
| ''q0 / %160'' | [[registers:risc-shadow-views:shadow_160|dX: direction, swapxy and primitive type]] |
| ''q1 / %161'' | YBot |
| ''q2..q13 / %162..%173'' | Edge gradients |
| ''q14 / %174'' | dXBot/dy |

''a15'', ''a31'' and ''q15'' are not assigned triangle parameters. Full positions and formats are in [[registers:risc-shadow-views:start|the shadow register map]].

Set the DrawCtl mono bit before writing color or color-gradient registers. With mono enabled, writing a component replicates it to the respective R, G and B values. There is no hazard between enabling shadowing and writing shadow registers.

Load all start values after enabling shadowing and before the first ''tri''. Load dX and all edge X gradients. Initialize every iterated parameter before each ''tri''; only gradients for enabled parameters are required. XTop, YTop and XMid always iterate. For a long edge on the left, dX is zero; for a long edge on the right, dX is ''0x80000000''.

===== Edge and span walk =====

Sort vertices by Y. The longest edge is walked in Y; spans go left-to-right when that edge is on the left and right-to-left when it is on the right. For parameter p:

<code>
E1_dpdy = (p2 - p0) / (y2 - y0)
x3 = x0 + E1_dxdy * (y1 - y0)
dpdx = (p1 - p3) / abs(x1 - x3)
</code>

The hardware applies Y subpixel correction to X, X2, Z, U, V and Q, and X subpixel correction to Z, U, V and Q. Color correction in Y may be done by microcode. Coverage includes pixels with X ≥ left edge and X < right edge. The edges must not intersect before YBot.

Q iteration enables perspective-correct U/V. DrawCtl also enables Z, diffuse color, specular color, alpha and fog iteration independently. Non-iterated Pixel Engine parameters retain their current values.

===== Other primitives =====

dX[3:1] selects triangle, line, span, dot or aaedge. Triangle uses edge and span gradients; line and aaedge use edge gradients; span uses span gradients; dot uses neither. ''swapxy'' makes X the major edge direction and applies to all primitive types.

[[pixel:overview|Pixel processing]] · [[system:context|Context save]]

Sources: [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=16|Technical Spec, p.16]], [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=17|Technical Spec, p.17]], [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=18|Technical Spec, p.18]], [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=19|Technical Spec, p.19]], [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=20|Technical Spec, p.20]]