Thanks Tweakie,
I am close to a release version of PicDither, but as you know some other personal things slowed me up for a while, but I'm getting back up to speed now.  As always, your work is first-class and your pioneering work is much appreciated.  Please keep the examples coming.  Maybe someday my little laser diode will grow up into a full blown CO2 laser  

Dan, I can not use Art's plug-in as it does not work well with my lower powered, slower firing, 1 watt laser diode.  But if I understand correctly, Art's plugin varies the on/off cycle time of a laser based upon the gray value of an image pixel.  This means as a pixel goes from black towards white, the laser will fire a series of shorter bursts for each pixel to simulate a gray value.  Converting (dithering) an image changes all pixels of an image to either white (laser off) to black (laser on) according to a matrix formula, of which there are several.  This allows the laser to fire full power for each pixel that is black in a dithered image.  This is similar to the process that newsprint photos are printed (look closely at a newspaper or magazine with a magnifying glass to see the "dots" that make up an image).

My program will apply any of several different dithering matrix algorithms to an image to produce a 2-bit image suitable for impact or laser engraving.  I have not tested impact yet, but hope to do so soon.  Tweakiie has posted some great examples of of his impact work here on site.

Regards All,
John Champlain
www.picengrave.com
picengrave@verizon.net  

On it's own, the plugin will not always smooth a gradient of tone without producing 'steps' as can be seen in the background of this image which is a smooth transition from light to dark.

Tweakie, just out of curiosity I would try engraving the same image rotated by 90°.

John, I think the plugin outputs equal length pulses for each pixel. As far as I understand it's the spacing which creates the toning.

Dan

EDIT: Found original Art's description:

http://groups.yahoo.com/group/mach1mach2cnc/message/74912

As examples, here are three pictures - original, grayscale, and Floyd-Steinberg dithered.  I hope this helps to clarify the process a little.

John Champlain
www.picengrave.com
picengrave@verizon.net

Something ate the other two pics - trying again.

OK, think now the original image will post  - got my fingers crossed 

Dan, I probably do misunderstand the way Art's plug-in works, but tried to base my explanation my personal trial and this:

"Shades are achieved within the Engraving plugin by varying the Pulse Repetition Frequency (not PWM) on the digital trigger output. A PRF of 100% would be equivalent to maximum power and a PRF of 0% would be equivalent to minimum power. In practice the Engraving plugin never gets anywhere near to a 100% PRF (see my pdf document for values related to feedrate http://www.machsupport.com/forum/index.php?action=dlattach;topic=12444.0;attach=26734 ) but manual control of the maximum tube current can, to a large extent, compensate for this.
The manual control of tube current can be adjusted at any time during operation to achieve the appropriate laser power for the job in hand. A multiturn potentiometer with calibrated dial is good because the adjustment is quite fine and the dial enables the correct settings to be recorded so that they can easily be used again at a future date for a similar job.
The Engraving plugin, as mentioned earlier, controls the laser’s output power by adjusting the PRF and this, in turn, is directionally proportional to federate and pixel value (high federate = high laser output power : low federate = low laser output power : no feed at all = laser off). It’s really just as simple as that.

I hope I have explained all your questions.

Tweakie."


At any rate, it did not work for me, hence my search for another way.

Regards,
John Champlain
www.picengrave.com
picengrave@verizon.net

Well, I think the plugin doesn't control the laser power whatsoever. All it does is trigger it on and off.

Dan

Well, I think the plugin doesn't control the laser power whatsoever. All it does is trigger it on and off.

Dan

Hi Dan,

In a way you are right – the plugin does indeed just switch the laser on and off but it does in a cunning way called modulation. As laser ‘burn’ is related to both laser power and exposure time when the plugin switches the laser on it does it in pulses or dots per pixel – a few pulses on top of one another produces a light mark and many pulses on top of one another produces a dark mark. You have seen from Art’s description that these pulses or dots are pretty quick and providing the step-over has been set correctly they will blend and this in turn produces the shades of grey. I have tried to describe the process in my PDF http://www.cooperman.talktalk.net/Engraving.pdf which I am sure you have probably read already.

Overall, the function is not dissimilar to a PWM spindle speed controller – it only turns the spindle on and off at full power but by varying the duty cycle it controls the speed. There are a few more refinements in a speed controller such as measuring the back EMF and adjusting the pulse width so that a constant speed can be maintained under varying tool loading but basically all it does is turn the spindle on and off.

Tweakie.

Hi Tweakie,

I agree with this. It's just that I think that using the word "power" to describe this process is misleading as the laser is being turned on to the same (predefined by some other means) power each time.

And to be more correct, I think that the laser never really does several pulses on top of one another (well if it's focused fine enough that is), as the axis is in a constant move. I think the correct way of thinking about it is the dots intensity. And the other thing to consider while at it is that the exposure time is determined by the axis feed rate.

Yes, I had read through your PDF and I must say you've done great job there!! Thanks for that!

Dan

And to be more correct, I think that the laser never really does several pulses on top of one another (well if it's focused fine enough that is), as the axis is in a constant move. I think the correct way of thinking about it is the dots intensity. And the other thing to consider while at it is that the exposure time is determined by the axis feed rate.

Hi Dan,

You can do the maths – when the laser is pulsing at twenty thousand dots per second with a line scan speed of just four hundred pixels per second, there can indeed be many dots on top of one another. If it was not for this fact, the plugin would not, I think, be able to produce any shades of grey. 

Tweakie.

Hi Tweakie,

I agree again. I was just trying to say it was not theoretically correct a way of description. There will always be the slightest offset between the two dots. It might be small, but it's there. And without it you wouldn't again be able to produce shades of grey

Dan