Session 25 – Laser Engraving Photo Replication – Introduction

The Concise RDWorks Learning Lab Series

Welcome to Module 3 of the new Concise RDWorks Learning Lab Series with Russ Sadler. Module 3 will build on the information learned in the previous modules and will be targeted on the differing types of laser engraving methods and the techniques needed to consistently achieve great engraving results. So lets learn how to master Laser Engraving Photo Replication.

In this Session, Russ takes us to the top of the engraving mountain with the topic of photo replication. He explains the principles and  theory behind historical printing technologies and how they transfer onto the laser engraving process. Russ also explains how cheap diode lasers and expensive RF laser systems cannot achieve laser engraving photo replication and the tricks that are used by users of these systems to just get an “OK” engraving result.

Release Date: 31st December 2021

Over the last 6 years, Russ has built up a formidable YouTube following for his RDWorks Learning Lab series which currently has over 200 videos.

The original RDWorks Learning Lab series on his “Sarbar Multimedia” YouTube Channel, follows Russ as he tries to make sense of his new Chinese laser machine and to sort out the truths, half truths and outright misleading information that is available on the web.

Six years later with over 3 million YouTube Views under his belt, Russ has become the go to resource for everything related to the Chinese CO2 laser machine user or wannabe user.

Laser Engraving Photo Replication
Laser Engraving Photo Replication

In this new series, Russ has condensed his knowledge and experience of the last 6 years to provide valuable information and insights into the purchasing, understanding, use, repair and maintenance of the Chinese CO2 laser machines and their key component parts.

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Transcript for Laser Engraving Photo Replication

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The Concise RDWorks Learning Lab with Russ Sadler. Session 25: Photo Replication – Introduction. Over the past few sessions,

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we’ve dealt with all the types of engraving that you can do on this machine.

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Today, we’ve reached the top of the mountain. We’ve got on the screen in front of us.

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An example of what most people would call photo engraving.

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Now, I think that that’s a much maligned term, and I don’t like to use the term photo engraving for what I do. Now I’ve nearly created World

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War Three by saying that some people and some machines just cannot do what I can do.

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What you see on the screen there, is a photo replication. Now, I took the photograph.

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Did nothing to it other than dither it, and then I did that with the laser machine. Many machines cannot do proper photo replication.

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In the last session, we did grayscale engraving of photographs.

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It was pretty rubbish, and the reason why it was pretty rubbish was because I made this machine have an instant response.

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And as we demonstrated when we examined some of the tests, instant response cannot produce single dots.

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It produces sausage shaped dots, basically two dots for one pixel.

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That’s not what this image is. Photo replication is one dot equals one pixel. If you have two dots for one pixel,

Transcript for Laser Engraving Photo Replication (Cont…)

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you have to do a lot of modification to the picture, to make it look reasonable.

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Many of the videos that you’ll see on YouTube are showing you how to do photo engraving and they just do

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not have a clue about the basics of digital graphics behind what they’re trying to demonstrate to you.

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So, I would like you to waste the next minute of your life watching this little video.

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Transcript for Laser Engraving Photo Replication (Cont…)

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We’re going to use this young lady as my demonstration photo today, and I’m going to do all sorts of strange things with it later on.

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But first. I’m going to make no apology for going over some of the stuff of graphics that we’ve already touched on before,

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many years before photography became available. Newspapers could only print text.

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Black ink on white paper.

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This became a problem when they wanted to put pictures into their newspapers, the only way that they could generate pictures

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was like this. They had to get copperplate scribed and filled with ink so they could get their black ink onto white paper.

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Now, that’s an artist’s job, a very skilled job to do that.

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So it was quite a few years before they managed to solve this problem and create ways of breaking up that picture from a continuous analog scale,

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which went from very dark brown, nearly black to almost white.

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It’s a sepia. It hasn’t got a complete black and white colour range, but it’s got a very wide colour range.

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There are just gradual changes of tone across the whole of the picture.

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So somehow they had to break that image down into black and white.

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Now, there had been many early attempts, but eventually they came up with a process called half tone printing.

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Half tone printing allows you to break that picture down into a selection of dots.

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This whole principle of half tone printing relies on your brain being fooled and filling in the gaps between the spaces.

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It does basically a color mixing of black and white into various shades of gray within your brain.

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So depending on the size of the dots that you’ve got in the black, the black gradually changes to a sort of grey and then the grey,

Transcript for Laser Engraving Photo Replication (Cont…)

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the mid-grey’s here change to white as the black gets less and less.

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That is a very crude representation of what the half tone printing principle is.

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The problem with that, when we try and apply it to our laser system, we’ve got small black dots and we’ve got big black dots.

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Now, the only thing that we can do with the laser machine is make dark spots or we can burn something into the surface of a material.

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We can only create one colour generally.

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Now, a couple of the large industrial type laser machines base their photo engraving software on this principle, but it has a major weakness.

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You can’t produce different sized dots on a laser machine.

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Sounds pretty obvious, doesn’t it? But before we get onto the details of that. This picture here,

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although it’s an analog picture, has been reproduced in modern times to go onto a computer.

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So therefore, it is no longer an analog picture. And if we zoom in on this, you can clearly see that it’s not completely analog.

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It’s digital. It’s made up of little square blocks called pixels.

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And this is the modern way in which photographs and images are stored and used.

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There isn’t that amazingly detailed. What I’m going to do is we’re going to analyze this picture to try and illustrate several points.

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First of all, I’m in Photoshop here, and it gives me the opportunity to find out exactly how this picture is constructed.

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If we take a look at the top here, we’ll find that the picture contains a total of twenty one point seven million pixels.

Transcript for Laser Engraving Photo Replication (Cont…)

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Of those little squares that we just saw. Now, you can’t see any squares on there, can you?

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If I zoom in and there we go, look, you can see the pixels there. At the moment,

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the resolution of this picture, it looks a stunning quality picture is 300 pixels per inch.

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I wonder what happens if I change that resolution to 10 pixels per inch.

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Do you think the picture is going to get better or worse?

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There we go. Trust me, I’ve changed it.

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I’ve cancelled it and we’ll come in and we’ll look at the image again, image size.

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Ten pixels per inch. Twenty one point seven million pixels, still the same number of pixels that we started off with.

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The only difference is, if we look here at the width of the picture.

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It’s now mm, eight thousand seven hundred ninety millimeters, eight meters wide and five and a half meters tall.

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Let’s just change that back to 300 where it was. Ah, where has the eight meters gone?

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It’s now 293 millimeters. The very important point I want to make here is that pixels are not fixed elements.

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A pixel can be as big as your imagination. I can have a pixel that is a meter square.

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You would see that picture from 10 miles off, probably.

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It was still have twenty one point seven million pixels,

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but the pixels would be huge. If you got up close to it, you would not be able to see what the image was.

Transcript for Laser Engraving Photo Replication (Cont…)

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The bigger the pixels, the further away you have to be to see the image.

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So by that understanding, if you’re only going to look at it at arm’s length, you need a high resolution image.

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But if you’re looking at it as a billboard, for example, you could find that

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the pixels on a billboard might be one millimetre square if you walked up close to it.

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So that’s important feature number one. Pixels are as big as you want them to be.

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OK, we will come back to that one shortly, because that’s an important feature when it comes to deciding what we’re going to do with our images. Here

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we’ve got a coloured image. Now, we can’t work with coloured images on our laser machine because we’ve already decided we’ve only got two opportunities.

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We either burn or we don’t burn. We’re effectively doing the same as an ink image, black or white.

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White is nothing and black is ink on a background. So the first thing we’ve got to do with this image, if we want to use it for laser printing,

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is we’ve got to take away the color and turn it into something called grayscale.

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OK, discard the colour information. And now we’ve got a grayscale image.

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Now, grayscale is an interesting point because, unlike sepia images, they run from black to white and there are no steps in between.

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It’s all completely gradual. With the grayscale system,

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black is defined as number zero and white is defined as 255.

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Well, there’s 254 shades of grey between black and white, so there’s a total of 256 colours in the range.

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Every one of these is a different colour, a different shade of grey.

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Each one of these little squares has got a different number attached to its colour.

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And every one of those pixels is mapped into something in machine memory, in your computer memory. Although we’ve got individual blocks,

Transcript for Laser Engraving Photo Replication (Cont…)

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pixels there on that screen, we can’t work with them because we can only produce black and no black, i.e. white.

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There is a different sort of half tone printing. It’s something called stochastic half tone, which is basically random half tone.

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This is patterned half tone printing where you’ve got a regular pattern and different sized dots.

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Now, stochastic half tone printing is exactly the opposite to that.

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And let me just show you, I’m going to, first of all,

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take this image and I’m going to turn it away from being a grayscale image into being something called a bitmap image.

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Now, the resolution, I’m going to leave it at 300 at the moment, pixels per inch, and we’re going to use something here called a diffusion dither.

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We’ll just say OK, I’ve just destroyed the image. Well, no, I haven’t, not really.

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What you see there is something called an aliasing using pattern.

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It’s the mixing of the pixels that are in the background of the image with the pixels that

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are on the screen that you’re seeing. As I change the magnification, look goes back to normal.

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So if I zoom in now. We’ll see what’s happened.

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Every one of these pixels that we can see here, is either black or white.

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We can now work with this on our laser. If we just consider one of those lines across there.

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Look, we can scan across there and here we are.

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We can turn the computer on and we can burn a black line. We can turn the computer off,

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nothing, on black line, off nothing, on off on off.

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Just a series of on off signals that correspond with the Morse code that you see on the screen.

Transcript for Laser Engraving Photo Replication (Cont…)

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So it’s really very simple. The problem is the size of that pixel.

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What is it? Well, remember what I said, that pixel can be as big or small as you want, if I made that pixel, for instance, 100 millimetres square.

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Could I print it with a single dot?

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No, obviously, so in essence, what we’re saying here is the size of the dot that we can produce with the machine, controls

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the resolution of our image. If the smallest dot that I can produce is naught point one millimetres,

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there will be ten of those dots in a millimeter and there will be 254 of those dots in an inch.

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So therefore, what I’ve now got to do, I’ve got to come back to this image.

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And I’ve got to set the image size to two hundred and fifty four. Now,

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it won’t change anything on this screen, but what it does, it changes the physical size of the pixels.

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And if we look again, you’ll see that, first of all, it’s changed the number of pixels and the resolution is 254.

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But the size of the image now is 219 by 346.

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If we don’t want an image that size, then we’ll have to change it.

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So, now, I could if I wanted, tick these boxes to constrain the image. Let’s just say I wanted the image

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it’s 100 millimetres wide or 120 millimetres wide, 120.

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So I’ve got 120 white, 75 millimeters tall, and it’s still got 254 pixels per inch.

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I have not changed the resolution of the picture. And you’ll notice the number of pixels has now dropped even more.

Transcript for Laser Engraving Photo Replication (Cont…)

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And that’s an image which we can print. Because, A: we can produce dots that are naught point one millimetres diameter.

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And B: it’s a size we can get onto the machine. OK, so what you’re looking at here is a very crude representation of my little DOT test pattern.

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Now, it’s been very carefully designed, this test pattern, for several good reasons.

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First of all, I want to be able to visibly, quickly check the size of a DOT, estimate the size of the dot.

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So I made my pixels here naught point one millimetres wide.

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The gap between the pixels is also naught point one millimetres wide.

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So that’s effectively a black pixel. A white pixel, black pixel, white pixel like that, so all these are black,

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these images that I’ve got on here. The way your laser interprets these is very simple.

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It comes along here and it gets to this point just here and it switches the laser beam on.

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And it draws a burn line until it gets to here and then it switches off. Because when there’s white,

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there’s no laser beam. On, off, on, off.

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The same applies to all of these, they’re all on for black and off for white.

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But there’s a fundamental problem here. We can’t, first of all, manufacture square pixels with our laser.

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We can only produce round burn dots.

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And there’s another problem when the laser comes up to this point here, it says switch the beam on which it does instantly.

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But of course, we’ve got a round dot. It gets to this point and it says switch off.

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But again, we’ve got a round dot. Here, here.

Transcript for Laser Engraving Photo Replication (Cont…)

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On, off. On, off. Now, it might not seem too much of a problem to you, along there because they still dashes.

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But hang about, that dot is half a pixel hanging off the end.

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And we’ve got one pixel gap here. So if I’ve got half a pixel there, half a pixel there, half a pixel here and half a pixel here.

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Like that, with my dots. Then how much gap have I got between the pixels?

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Hmm, that’s a problem because remember how the brain is working, it’s looking at the overall colour of the background, which is white,

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and it’s looking at the overall colour of the foreground, which is black, and it’s mixing those two together in your brain.

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So the more white in the image, the lighter the gray you will see and the more black in the image, the darker the image will be.

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Do you see the problem? We’ve lost our white between the pixels, so we’re automatically when we’re doing photo engraving,

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where we’ve got lots of these pixels sitting very close together. We’re automatically going to create a dark image.

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And that is why you will find with every single piece of photo engraving software,

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there is a fairly significant chunk of lightning or brightness control involved to try and enhance the image and reduce this effect.

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I have to admit to being slightly, very slightly involved with some of the early development work in another piece of software called Lightburn.

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Very clever guy designed the software and he’s done a fantastic job of replacing virtually every part

Transcript for Laser Engraving Photo Replication (Cont…)

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of RDWorks and making it five times better than it ever was with all sorts of extra features.

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He identified this problem and decided that the right thing to do would be to delay the start of the beam.

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So he shifted the start of the beam along and finished the beam half a pixel early and half a pixel late.

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So that now you get the start and the finish of your pixel at the right place.

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The problem is when you subtract half a pixel off of there and half a pixel off of there, you’ve got no pixel.

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So you get no dot! And that was the fundamental fault with his logic, everything that was a single pixel disappeared.

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And it changed the quality of the image completely the opposite way to what was intended.

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You will always have this issue and you will always have to lighten your picture.

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Now, this nought point,

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one dimension for this pattern has been very carefully chosen, based on what we could possibly achieve on a CO2 laser with the lens system there.

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You can get less than a 0.1 dot under certain circumstances with certain materials.

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But in general, the smallest dot you’ll be able to burn will be naught

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point one. And therefore, the best resolution that you can create your image at is 254 PPI, which is a naught point one resolution. Pixels per inch

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equals 25.4 millimetres.

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So if we’ve got naught point one, it means we’ve got ten of these pixels per millimetre.

Transcript for Laser Engraving Photo Replication (Cont…)

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So if we multiply 10 by twenty five point four, we get 254.

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So that’s how we arrive at these numbers. Now, good quality photographs are typically 300 pixels per inch.

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The essence of what we have to do to copy that photograph is to make sure that we have dots that are naught point one,

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so we can get 254 dots per inch. So that we can immediately copy every pixel with a dot.

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That’s a very key part of photo engraving.

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You can’t make the dots bigger than the pixels because if you do, they will overlap like this and they will occlude the background,

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which is the thing that’s giving you the impression of light and dark.

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You change the density and the colour of your image.

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I did say that it was very simple and I did say that it was very complex.

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This is the complex part. This is the simple part, very simple.

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One dot equals one pixel.

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So look at the other way around, decide the size of your dot, and that will tell you the resolution of the image that you can reproduce.

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Pixels do not overlap on the screen, therefore you cannot have dots overlapping on your material.

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Well, I’d like you to take a few minutes and just go to YouTube and put in the heading “laser

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Engraving”.

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And you’ll see all these videos, hundreds of them. Now, I will emphasize one of the last points I just made to you, pixels on the screen do not overlap.

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Therefore, the dots on your engraving must not overlap. If they overlap,

Transcript for Laser Engraving Photo Replication (Cont…)

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you will be double and multiple burning of the same spot and it will become a 3D engraving and not a photograph. We’ll stop just here, look

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we’ve got a couple of diode lasers. Now, a diode laser has got an instant switch on, and I’ve

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demonstrated to you what instant switch on does for you. It produces a two dot burn for one pixel. So you can never do a true photo replication.

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And that is why you need to distort your photographs to make sure that they don’t over burn.

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Now you’ll find that there are several commercial bits of software available to you, things like one step photograv.

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Now here in the middle of the screen, it says using imag-r software for engraving with your laser.

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Now, this is one of those pieces of free software that you can plug your photograph online and get a result back. Well on the left is

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what you put in and on the right is the distorted cartoon like image that you get back for engraving. Tricks and tips,

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well, yeah, they’re tricks and tips on how to put numbers into your software, not what the processes behind are behind photo engraving. Now in the centre of the screen.

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Tonal curve training. Photo engrave on wood. Now, you would think that from somebody like Trotec, this would be an expert masterclass in engraving?

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What on earth has they done to that guy? I mean, that’s not a photograph, that’s not even a cartoon.

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That’s almost like a death mask, you might like to take a look at this one. It’s a good example of what I mean about multi burning pixels.

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The background is very, very deep, maybe one or even two millimetres deep, over burning of pixel after pixel after pixel.

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Sadly, as you will see, many people in these videos are just putting numbers into their software.

Transcript for Laser Engraving Photo Replication (Cont…)

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Now, I’m going to leave you to go down here and waste your time looking at the way that some people approach photo engraving.

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In most instances, it shows a total lack of understanding of the principles associated with digital images.

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And then it goes on to compound that with not understanding what their machines are actually capable of doing.

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Mix the two together. And you get barbecued wood. Now in the case of an R.F. machine and a diode laser machine, it is impossible to get a single pixel.

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You will always get a double pixel burn.

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So in those cases, you have to distort the image to make it lighter, to compensate for the fact that you’re going to produce a double burn.

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Now it’s through lack of knowledge on a CO2 laser machine that people still over burn.

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It’s not necessary to over burn, if you understand what the technology is all about, as we’ve already stated, you can’t deal with a coloured picture.

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So the first thing we have to do is to take the colour out. Now, let’s just go and have a look, zoom right in.

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Now, this is a very high resolution picture with 300 pixels per inch. You can just about see the pixels now that I’ve zoomed in on her eye.

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The first thing that I would do to this picture is nothing. I come down to mode.

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It’s grayscale, and I’ll turn into a bitmap. I would dither it, diffusion dither.

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So those are the, that’s the Morse code that’s basically sent down to the laser to burn.

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So that’s what we have to replicate. Let’s just zoom out and there it is.

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It does look any different than the original image, even though it’s now a simple black and white image. Now,

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it’s a very important point to remember. Your eye cannot resolve those very small dots, what it sees is an average of those dots.

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And that’s how you get the impression that there’s still grayscale there, even though there’s not.

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And now I’ll do a modification: image / adjustments.

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We’ll just use a very crude control, brightness control. OK, now, here we go.

Transcript for Laser Engraving Photo Replication (Cont…)

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We’re going to change the brightness. Now, why on earth have I done that?

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Well, let’s go back and look at the image again as a bitmap.

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Do you notice any difference now? Where have all the pixels gone? Yeah there’s still lots of single pixels there, but look at the gaps between the pixels.

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There are huge white gaps between those black pixels.

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So if we had a pixel, which was double the length, two dots, for example. It would help to fill in the white background.

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So by lightning the image, you can compensate for the two dot burn that you’re going to get per pixel.

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So let’s back step to grayscale again.

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And we’ll do two things to it now. Before I put the brightness into it, I’m going to put something else in which is done,

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but I’m going to do it separately so you can see the effect; sharpen and sharp mask.

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I’ve taken some of the background out and I’ve sharpened all the edges up.

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I’m halfway to a cartoon. So let’s add brightness into this image now.

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So brightness and sharpness. So now let’s convert that into a bitmap for engraving.

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So with just two simple filters,

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I’ve managed to turn that beautiful young lady into somebody who’s got straw hair, staring eyes and pathetically weak lips.

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And that’s all in an attempt to remove pixels and put bigger spaces between the pixels. So you

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can over burn each pixel without it dramatically affecting the colour tone of your picture.

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But of course, you remove the detail, the shading, some of the character in the picture.

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So it’s now an interpretation of a photograph, not a replication of a photograph.

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Because in the next session I’m going to show you how to set the machine so that

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we can manage to achieve what’s on the right rather than what’s on the left.

Transcript for Laser Engraving Photo Replication

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Last updated August 26, 2021

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