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optimization: Build the G-code preview in C++ - #4481

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alex-pres wants to merge 11 commits into
LinuxCNC:masterfrom
alex-pres:pr-render-preview-cpp
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optimization: Build the G-code preview in C++#4481
alex-pres wants to merge 11 commits into
LinuxCNC:masterfrom
alex-pres:pr-render-preview-cpp

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@alex-pres

@alex-pres alex-pres commented Aug 27, 2026

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Follows PR##4293

Adds C++ GCodeRenderer, which builds the whole preview - transform, arcs, taps, suppression, vertices, extents, lengths and event records - during gcode.parse and hands it over once, replacing the Python rendering engine. C++ handles Vec[9] -> X,Y,Z transform completely, following all interpretator details, and Python renders it into OpenGL using shaders.

Parsing is ~2.9x faster on move-shaped programs and peaks at less memory; and specially for arcs, the last per-move Python call kept in code, go from 1.58 s to 0.075 s at 1.28M segments.

Even without speed benefit it is anyway better because resulting code is much more clean and easy to follow. Previous code was hard to read, understand and test. Right now we can test GCodeRenderer engine directly, it is independent of presentation.

Benchmark results, compare to current master

Headless, fractal-1M.ngc (1,000,149 moves):

master this branch
parse+build 2.705 s 0.732 s (3.7x)
total load 2.712 s 0.745 s
peak RSS 166.8 MB 163.5 MB
VBO bytes 20,003,000 20,003,000
CPU geometry 28,004,200 28,004,200
extents identical identical

Byte-identical geometry out, ~3.7x less time to produce it.

Results could be much better if ngc file use arcs (current master calculates arcs in Python which was bad idea, after moving to C++ it is 100x faster).

@alex-pres
alex-pres marked this pull request as draft August 27, 2026 15:50
@alex-pres
alex-pres force-pushed the pr-render-preview-cpp branch from bd037e7 to c0bf905 Compare August 27, 2026 16:01
@alex-pres
alex-pres marked this pull request as ready for review August 27, 2026 16:06
@BsAtHome

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What are all those json files doing in there? It seems you want to do some "bake" test, but I did not see a test{.hal,.sh,} in that directory. Did I miss it?
You also need to disable any test(s) that cannot run in the headless CI run. All GUI/Graphics related dependencies are not installed in the headless CI run.

Another point, you are "redecorating" the gcodemodule. Would it be an idea if we move this to pybind11 at the same time? Doing this work twice may be more work and may also be error prone.

@alex-pres
alex-pres force-pushed the pr-render-preview-cpp branch from c0bf905 to 0b8fa5f Compare August 27, 2026 16:20
@alex-pres

alex-pres commented Aug 27, 2026

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What are all those json files doing in there? It seems you want to do some "bake" test, but I did not see a test{.hal,.sh,} in that directory. Did I miss it? You also need to disable any test(s) that cannot run in the headless CI run. All GUI/Graphics related dependencies are not installed in the headless CI run.

My fault, forgot to add test.sh - done now. JSON files are reference results for related .ngc files from another folder.

No problem with headless - these tests are for C++ routines that do math only - no GUI, no graphics.

Another point, you are "redecorating" the gcodemodule. Would it be an idea if we move this to pybind11 at the same time? Doing this work twice may be more work and may also be error prone.

Any WIP on this or samples?

Yes, I can work on pybind11 conversion, but any existing refrerences to such change in LinuxCNC will be helpful. If any

@alex-pres

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I agree test files do not look good. I will rework it to easily see what each case is testing.

In time being, I'd appreciate feedback on the rest of PR.

@alex-pres

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Another point, you are "redecorating" the gcodemodule. Would it be an idea if we move this to pybind11 at the same time? > Doing this work twice may be more work and may also be error prone.

OK, I have found halquery is using pybind now. I will try to rework gcodemodule to use it too in this branch.

@alex-pres
alex-pres marked this pull request as draft August 27, 2026 19:49
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alex-pres force-pushed the pr-render-preview-cpp branch from 0b8fa5f to 7597f74 Compare August 28, 2026 12:44
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alex-pres marked this pull request as ready for review August 28, 2026 12:45
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alex-pres commented Aug 28, 2026

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Reworked with pybind11 , it is a pleasure to work with. Resulting code is much easier to read and follow! Added some modern C++ which gcodemodule.cc deserve )

Tests reworked too.

Comment thread src/emc/rs274ngc/gcodemodule.cc
Comment thread src/emc/rs274ngc/Submakefile
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Last commit is breaking compatibility - returns tuples instead of arrays is not good idea, breaks code on Python side. I will revert this at Monday.

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alex-pres marked this pull request as draft August 28, 2026 18:58
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alex-pres marked this pull request as ready for review August 31, 2026 08:25
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Last commit is breaking compatibility - returns tuples instead of arrays is not good idea, breaks code on Python side. I will revert this at Monday.

It was false alarm, problem not caused by this code. Everything seems to be OK and ready for review and testing.

Current live testing results: loading of 418Mb .ngc file (real work file sent by customer, with G2 arcs): 25 seconds to first render.

@grandixximo

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418Mb .ngc file (real work file sent by customer, with G2 arcs): 25 seconds

🤤

@alex-pres alex-pres changed the title Optimization: Build the G-code preview in C++ optimization: Build the G-code preview in C++ Aug 31, 2026
@alex-pres

alex-pres commented Aug 31, 2026

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418Mb .ngc file (real work file sent by customer, with G2 arcs): 25 seconds

🤤

Together with PR #4491 : 12 seconds!

P.S. As said before AXIS [specially AXIS] is using not optimised file editor code that now eats more memory and CPU than graphical preview. So for benchmarking I cut it off. Worth to be optimised too, may be later.

@alex-pres

alex-pres commented Sep 1, 2026

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And with both #4491 and #4492 - 5.7 seconds to first frame on same file.
Peak RSS after file load: +1.04GB.

@grandixximo

grandixximo commented Sep 2, 2026

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Could you please fetch master, then merge or rebase and push, just to let the CI run again, and we shall get the shots of all the UIs to make sure nothing regressed.

Adds C++ GCodeRenderer, which builds the whole preview - transform, arcs, taps,
suppression, vertices, extents, lengths and event records - during gcode.parse
and hands it over once, replacing the Python rendering engine. C++ handles Vec[9] -> X,Y,Z transform completely, and Python renders it into OpenGL using shaders.

2 protocols to interact with python: CallbackCanon (compat) and GCodeRenderer (new). Selected when "parse" method called based on canon properties.
@alex-pres
alex-pres force-pushed the pr-render-preview-cpp branch from 2564163 to 075e6b8 Compare September 2, 2026 14:27
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Could you please fetch master, then merge or rebase and push, just to let the CI run again, and we shall get the shots of all the UIs to make sure nothing regressed.

Done. And squashed to one commit.

@BsAtHome

BsAtHome commented Sep 2, 2026

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And the screenshots seem to be similar to what we see elsewhere.

Comment thread src/rtapi/rtapi_string.h Outdated
Comment thread lib/python/rs274/glcanon.py Outdated
Comment thread lib/python/rs274/glcanon.py Outdated
Comment thread lib/python/rs274/glcanon.py Outdated
Comment thread src/emc/rs274ngc/gcode_renderer.cc Outdated
Comment thread src/emc/rs274ngc/gcode_renderer.cc
Comment thread src/emc/rs274ngc/gcode_renderer.cc Outdated
}
write_vertex(pt, line_number, sub == 0 ? KIND_NOOP : cat, nullptr);
}
data_->cur9 = p2;

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After a failed reserve() this still claims the move completed, so the next one records no jump. Return after the first failed write_vertex instead?

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You're right that it sets cur9 either way. It's dead state though — write_vertex bumps interp_error, and append() (gcode_renderer.hh:394) and arc_feed both bail on that, so no further move reaches fill(). Nothing reads it again.

Does it worth adding bool return v and check for each vertex if it is handled anyway? If yes, I will do that.

return int_array(l->mcodes, ACTIVE_M_CODES);

static void linecode_register(py::module_ &m) {
py::class_<LineCode> c(m, "linecode");

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gcode.linecode() was constructible before and is not now. Deliberate?

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yes, it was broken. restored and added a test
09730f1

This change belongs to the separate pr-perf-rtapi-strlcpy branch and was
merged into this one by mistake. It is submitted as its own pull request,
so drop it here to keep this branch limited to the C++ preview work.
Replaces use_gcode_renderer, a flag a canon with a catch-all __getattr__ could not opt out of.
The renderer transforms in C++ and forwards no offsets, so nothing read them.
set_plane, set_feed_rate, set_spindle_rate and select_plane, with the plane and feedrate they wrote to.
progress_ is a bound method, so it held the canon and its whole program until the next parse.
Drops lo/first_move/xo..wo from the canon contract; the current position reaches a preview as the caller's G53 G0 initcode.
gcode.linecode() constructs again, snapshotting the interpreter the way a
delivery does rather than handing back zeros no later next_line could fill
in. Outside a parse it raises. snapshot_line() is now the one place a
LineCode is filled in, so the two paths cannot drift.
@alex-pres

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Finished with changes, hope all mentioned is fixed now

@grandixximo

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You might want to look at CI the clang is reporting failures, we have it fail on warnings, gotta clean those up.

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3 participants