Digilabs Technologies Blog

All PDF’s are not created equal. Part I (out of III)

Posted on: June 18, 2009

One of the many different things we prepare our output is the usage of a Vector PDF. But what’s the big deal? Doesn’t PDF stand for a Portable Document Format? Isn’t a PDF just a PDF?

This turned out to be a long post. Longer than I thought it will be when i started. So, I broke it into three parts. The first part will layout the basics. Some of you might find it doesn’t tell them anything they don’t know already. Feel free to skip it if this is the case. The second post will tie it all together with emphasis on printing and output.

Well, a PDF is indeed a portable document, and it will look “the same” on different devices. It just does not mean that all PDF’s are created equal. In other words, looking the same is the not the same as looking the best it can. So the way the PDF is built will effect the results you get, even if you don’t know it yet.

Let me explain. Many of you might know it already, but some who are new to the world of digital printing might not yet be aware of all those details. I’ll try to put some sense into it all. I will avoid many technical details, but we’ll have to go into some of it from time to time.

We live in an analog world. In this world things are continues. I draw a line. I take my pencil, start form one point and go to another point and I have a line the width of my pencil. I’ll take a magnifying glass and look at it, it’s there! It’s looks wider, but is still the same line I drew. That is the analog world.

A digital world, buy contrast, uses discontinuous values. The combination of a series of discontinuous values may fool us to think it is continues when in fact it is not. In real life we move around. This is analog. In the movies, the actors picture on the screen change at 30 frames per second, fooling our brain to think they are moving when in fact its a combination of discontinuous frames. This is digital.

Computers are digital. The screen is a digital device. It is built out of many small points that can be turned on or off, and if you put a lot of them inside a tiny place it might fool the eye, but this is still a visual effect, not reality.

Enough with this mambo jumbo! lets add some context. Remember the line I drew in the first paragraph? Lets draw it on a computer screen. I have a starting point and an end point. I turn pixels on the screen on and off to get the illusion of the line. But since the screen is digital, pixels are either on or off, and since my screen is a matrix of discontinuous pixels, they don’t always fall where the line should be. On the left, you can see my line on paper. On the right, how the same line might be represented on a screen.

line

Okay, I admit. I did simplify things somewhat. Many smart people make a good living doing a better job in representing this line. They will use all kind of smoothing techniques, such as anti-aliasing or others to get better results, but you get the point.

When we represent things on screen or paper, we can divide our inputs into two camps. The continuous tone camp and the line art camp.

For simplicity, lets say digital images are better represented by a continuous tone representation. In the real world it means a big array of pixels, each one with its own color. When enough pixels are crammed together is a small place, they fool the eye to look like a continuous tone image. We will call the number of pixels we put together in a given space the “display resolution”. There are more than one usage for the term resolution, but here we actually mean: “how many dots per inch do we have”. So when we say the image is displayed at 72 dots per inch (dpi), we actually say that when we display it on the screen, every square inch of the image contains an array of 72 by 72 pixels with color information in each. We can refer to this graphical representation as a bitmap (or a pixel map) image.

The image on the right is a small image on a scree. On the left, you can see what it really looks like on the screen if we look through a magnifier.

pixels

On the other end of the spectrum, we have the line arts. A line art is an image that is better defined as a combination of lines, curves and colors. Think about a clip-art or a cartoon. It is easier defines as a series of strokes, lines, geometrical shapes and colors than by a bitmap. Since those images are defined by geometrical shapes, they do not have an inherited resolution. I can take the geometrical shapes and scale them mathematically to any size I want before I render them to the screen (which we know by now is a digital device). So, when I use a line art image, I can get the best possible result for the specific output device. But more on that later.

I know. By this time you all nod your head in understanding, but back in your mind you think, “who cares?”. After all, we mostly print and and deal with images, right? Wrong! The most common form of a “line art” we use are fonts. Okay, fonts are more complex than a typical line art and someone out there is annoyed by me treating them the same. So let me say just that. Fonts are more complex in their technology and definition and usage than I might suggest here, but for our little discussion lets define “line art” as anything that can be defined with lines and curves, including fonts.

How do you make a font look good at all sizes? Fonts, or at least modern fonts (like truetype or postcript), are defined as mathematical definitions of lines and curves to shape the look of a character. When displayed or printed, they are first mathematically scaled to the desired size and than are rendered to the output device. Did I say no inherited resolution? I guess the term resolution independent is a better term to use.

Look at those two example.

When we represent what should be a line art as a bitmap image, we impose a resolution on the item first. From now one, if we scale it, we always start with its resolution, same as the example of the image above. On the left we see the letter D. On the right is the top part of the same letter at a larger size. Since we started form a bitmap of the letter D and scaled it up, look how it looks like. This is called pixelation.

font2

Below we have the same text, but this time, we started with the line art description of the font and kept it this way until the last minute before displaying it on screen. Note how the lines look compared to the same text above.

font

This is true not only for fonts, but for anything which is better described as a line art. Take a look at the clip art below. Left is the original, center when scaled from a bitmap and right when scaled from a vector file.

clip

So what all of that has to do with PDF quality? This will be the subject of the next post.

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