Fix M166 Gradient Mix on delta printers

[Kadah]

Description

The updates for M166 gradient mix on delta printers use the z tower stepper position instead of the toolhead cartesian z position. This leads to current gradient mix being constantly changing during a single layer as seen here and results in the gradient being rotated and offset.

Example:

I am not sure if the fix I have made is the correct way to do it. I have been running with this patch and it does fix the issue. This likely creates additional cycles with having to call for a forward kinematics update more often, though a delta+mixing setup should already preclude 8-bit controllers.

Benefits

Allows M166 to work as intended on delta printers.

Configurations

RGB_Delta.zip

Related Issues

#18917

[thinkyhead]

That looks much better, thanks!

[AnHardt]

There is always a way to speed things up.

There the problem is the expensive back calculation of the Cartesian Z.

Currently the mix for a block is set and calculated at the end of _populate_block(). The catch could be to only copy in the mix there, but calculating it as long the Cartesian Z is still available. It's just a matter of finding the right place for that calculation. I'd search near the kinematic conversion.

[Kadah]

Wouldn't this duplicative since mixer.update_mix_from_gradient() calls it?

[AnHardt]

Isn't this the critical path? (from 2.0.x)

Marlin/Marlin/src/module/planner.cpp

Lines 2593 to 2599 in 9ade72a

	TERN_(HAS_POSITION_FLOAT, position_float = target_float);
	TERN_(GRADIENT_MIX, mixer.gradient_control(target_float.z));
	TERN_(POWER_LOSS_RECOVERY, block->sdpos = recovery.command_sdpos());
	return true; // Movement was accepted
	} // _populate_block()

Marlin/Marlin/src/feature/mixing.h

Lines 186 to 193 in 92076c6

	static inline void gradient_control(const float z) {
	if (gradient.enabled) {
	if (z >= gradient.end_z)
	T(gradient.end_vtool);
	else
	update_gradient_for_z(z);
	}
	}

Marlin/Marlin/src/feature/mixing.cpp

Lines 165 to 180 in 92076c6

	void Mixer::update_gradient_for_z(const float z) {
	if (z == prev_z) return;
	prev_z = z;
	const float slice = gradient.end_z - gradient.start_z;
	float pct = (z - gradient.start_z) / slice;
	NOLESS(pct, 0.0f); NOMORE(pct, 1.0f);
	MIXER_STEPPER_LOOP(i) {
	const mixer_perc_t sm = gradient.start_mix[i];
	mix[i] = sm + (gradient.end_mix[i] - sm) * pct;
	}
	copy_mix_to_color(gradient.color);
	}