"Can you tell me how much work it would require to add the ability to evaluate
vector functions and vector swizzling to povray's function virtual machine?"

Copilot said:

Reasoned for 1s

Estimating effort for additions
Below is a realistic engineering assessment of what it would take to add (1)
true vector‑valued functions and (2) vector swizzling (e.g., .xzy, .yyx)

code‑generation changes needed to surface them cleanly in SDL.


front‑end (scanner → tokenizer → parser) and back‑end

byte‑code/IR executed for user functions defined in SDL (the code path

mankier
+1

Executive summary

generator, VM op set, and runtime math library.
Complexity: Mostly mechanical but non‑trivial because the current pipeline
is strongly scalar‑oriented; vectors are supported in SDL expressions, but
the function VM primarily evaluates scalar functions and relies on inlining or
ad‑hoc paths for vector math. Extending it cleanly implies adding
first‑class vector types and opcodes.
mankier
+1
Estimated effort (for an experienced contributor familiar with the code base):


backwards‑compatibility tests rather than algorithmic difficulty.
(Breakdown below.)
Where you have to make changes (top‑down)
1) Language & parser surface
Goals

Allow function definitions that return a vector and/or take vector arguments.
Extend expression typing so that vector expressions can be compiled to VM
bytecode.
Support swizzle selectors (e.g., V.x, V.xy, V.zxy, V.xxx) as l‑values (if
you want assignment) or at least r‑values.
Files / modules affected

Tokenizer/Reserved words: the dot accessor today is used in various contexts;
ensure tokenization allows chained component selectors on identifiers and
sub‑expressions (e.g., (A+B).xz). (parser_tokenizer.cpp,
reservedwords.cpp)
povray
Parser: add productions for:
function return type = vector (e.g., function { |x,y| <...> } today is scalar;

typed header).
Vector expressions inside function bodies compiled to VM (not just evaluated at
parse time). (parser.cpp, parser_expressions.cpp, parser_functions.cpp)
mankier
Semantic checks: arity/type mismatches; disallow swizzle lengths other than
1..4; enforce read‑only swizzles unless you implement write‑back.
The parser currently builds scene structures while parsing; for functions it

code‑generation path to emit vector instructions. (fncode.cpp and helpers)
mankier

2) Function VM & bytecode
Goals

Introduce a first‑class vector value type in the VM stack model (likely
3‑component float vectors; optionally 4D for color/alpha).
Add opcodes for: LOAD_VEC, BUILD_VEC3, SWZ1/2/3/4, VADD, VSUB, VMUL
(elementwise), VDOT, VCROSS, VLEN, VNORM, VCLAMP, VMIX, etc.
Define type promotion rules: scalar op vector (broadcast scalar), numeric
literals to vector, etc.
Extend the runtime math library to vectorize existing intrinsics where
appropriate (e.g., sin(vec) → component‑wise).
Files / modules affected

VM representation / code emitter: extend structures that describe opcodes and
the stack to accommodate vectors (size and alignment), and update the
interpreter loop to handle them. (fncode.cpp chiefly)
mankier
Parser→VM bridge: in the code that compiles expressions to VM IR/bytecode,
add cases for vector constants <x,y,z>, vector operators, and swizzles. (Parser
emits calls into fncode helpers.)
mankier
Standard library intrinsics: decide which built‑ins get vector overloads
in the VM (e.g., min, max, clamp, dot, cross, length, normalized, noise,
f_noise3d style). Ensure the overload set matches SDL expression semantics.
(Parser & VM dispatch)
mankier
3) Swizzling semantics
Read‑only swizzles are easy: treat V.xyz as an inline permutation/extract


write‑back masks. Unless there is a strong need, start with r‑values
only.

expression. Tokenizer already recognizes .; you only need to validate the

parser)
povray
+1

4) Type system & diagnostics
Add a minimal static type notion in the function compiler so errors like dot(
scalar , vector ) are caught at compile time rather than at run time.
Improve error messages (line/column from scanner are already tracked; keep
passing them through). (scanner.cpp, parser nodes)
programmersought
5) Backwards compatibility
Keep scalar‑only functions working as today.
Do not change existing precedence or numeric literal rules.
Gate any syntax that could be ambiguous behind #version 3.8+ or similar, just as
POV‑Ray already uses Version/#version to select language behavior.
stackoverflow
Work breakdown & effort estimate



Task Main files Est. effort

Tokenizer & Parser: swizzle selectors parser_tokenizer.cpp,

VM & bytecode: vector value type; opcodes; interpreter changes; emitter helpers

Intrinsics: implement vector overloads for common math ops; promote parser




with the amount of code in the parser/VM and the care needed for versioned
behavior. (The parser core is >8k LOC; even small changes propagate.)
mankier

Risk & complexity hotspots
Mixed scalar/vector expressions: decide on promotion (broadcast scalars) and
forbidden mixes; mirror existing SDL expression behavior to avoid surprises.
(Parser + VM)
Noise/isosurface functions: Some built‑ins accept vectors already at

existing evaluation. (Parser + core math calls)
mankier
Performance: The interpreter is scalar‑centric; vectors will push more
data per instruction. Consider packed ops to reduce opcode dispatch overhead
(e.g., VADD instead of three scalar ADDs). (VM)
mankier
Error recovery: swizzle typos (.xxw) must produce crisp errors and recover to
the next statement; lean on the existing reporting in scanner/parser.
programmersought
+1
Minimal viable path (recommended)
Phase A: Read‑only swizzles and vector returns/args for functions, plus
basic vector ops (+ - * with scalar broadcast, dot, cross, length, normalize).
Phase B: Extend intrinsics and add convenience ops (min/max/clamp/mix),
color‑vector aliases if desired.
Phase C: (Optional) writeable swizzles and 4‑component vectors (RGBA).
Phase D: micro‑opt (pack opcodes), more intrinsics, documentation polish.
Each phase is shippable and can be guarded behind a #version gate.


Example (what users would gain)

#version 3.8;

#declare F_NORMAL = function(u, v) {
  // Return a vector (unit normal on a wavy surface)
  #local p = <u, v, sin(2*pi*u)*cos(2*pi*v)>;
  // swizzle to drop z ripple into x,y direction and normalize
  normalize(<1,0,0>*p.x + <0,1,0>*p.y + <0,0,1>*p.z)
};

#declare F_SHADING = function(n) { // n is a vector
  // swizzle use: emphasize yz
  let k = dot(n.yz, <0.6, 0.8>);
  clamp(k, 0, 1)
};


Why the estimate is plausible

The code base already separates scanning/tokenization from parsing and has a

just extending types/opcodes and codegen. [programmersought.com], [mankier.com]


[stackoverflow.com]
DeepWiki overviews outline the front‑end pipeline
(scanner→tokenizer→parser→symbol table→function VM),
providing a reliable map of where to touch. [math.hws.edu],
[programmersought.com]

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