Min + Max power and power scale -> implicit behaviour?

I came across a thing that I don’t understand. It is related to this question: https://forum.lightburnsoftware.com/t/another-power-scale-question/33570
I also used a file with generated power scale values. I have three objects. The power scale is set to 20, 40 and 60%. Erroneously, min power and max power are both set to 100%. LightBurn gives a warning when you want to preview it, BUT the output seems to be correct! So the output seems to be 20%, 40% and 60%.
Here is my question: if min power and max power are set to 100% and you apply a power scale, is the min power implicitely set to 0%?
Or could the reason be that I use a grbl controller that does ignore the min power anyhow? But this would mean that you can’t create power scales with grbl controllers, since it would make a difference if min power is set to 0 or any other value for calculation…
I hope you understand what I want to say/ask :slight_smile:

Because you are using a GCode controller, Min Power is automatically set to 0 for you because the hardware doesn’t actually support the Min Power setting. Power scale will generate an output power by scaling just the Max Power value, so it does work, and the error message shouldn’t be shown for GCode systems. I’ll fix that.

So are you saying that the Min power setting does not do anything when using a GRBL controller? I am using a JTech 7W laser. When engraving images, should I be changing the power scale setting instead of the Min power setting?

As far as I know, the one time that Min power is relevant for G-code lasers is when using Grayscale mode for images. In that case the dynamic range of the grayscale is bookended between the min and max values.

For non grayscale modes laser power is applied at max power wherever a burn is expected and no power when no burn is expected. It’s binary. So I don’t see any reason to use power scale instead of just using Max power to control power level.

Are you trying to do something specific that you think would call for using power scale?

This is also stated in the documentation: Redirecting... (see the note about grbl systems) But I don’t know about the case with images.

I would agree. For normal usage, just use the max power setting according to the requirements. Just a side note, grbl controllers actually DO use a power gradient, but that one is calculated internally. So, the actual power output is not constant, but depending on speed. But external systems like LightBurn have no influence here.

One question that still is not completely clear to me (@LightBurn): If grbl systems set min power to 0 implicitly, what happens in this case:
Min power: 50%, max power 100%, power scale: 50%
This should result in 75% power. Is this the same with grbl systems? Or will this setting implicitly result in:
Min power 0%, max power 100%, power scale 50% → 50% output power?

I assume you’re referring to variable power mode introduced in 1.1f. There is one mechanism in LightBurn to affect this. You can effectively disable variable power by using Constant Power Mode in cut settings.

It’s not GRBL that’s setting the min power to 0 implicitly. It’s LightBurn. LB does this for all g-code based systems as far as I know. And it does this for anything other than grayscale images.

So in your example it’s the latter option, 50% output power that would be the result for all non-grayscale image power scales.

With grayscale images power would be scaled proportionately as in your first option but with even more nuance since power is based on black level. I did a writeup on effect if min/max for grayscale if you’re interested.

Right. That’s what I meant, I missed the “for” in that scentence :slight_smile: For grbl-systems, LB sets this to 0.

And yes, I was referring to the variable power mode and “Dynamic Laser Power Scaling with Speed” as well as acceleration planning (and cornering).

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Min Power is ignored completely on vector shapes because it’s not supported by the hardware - it’s always zero, so the power scale example will always be between 0 and Max Power on GCode devices, with the sole exception of grayscale images.

With grayscale, Min Power is the value used when burning white, Max Power is used when burning black, and all other values are ramped between them.

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