Digilabs Technologies Blog

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

Posted on: June 24, 2009

In the previous post we spoke about  image resolution, scaling, and graphics. So, how is that related to the various types of PDF?

A PDF can be built of two types of graphics objects. Vector graphics and raster images. Vector graphics in PDF, as in PostScript, are constructed with paths. Vector graphics are what I called in the last post line art. A line art is an image that is better defined as a combination of lines, curves and colors. Raster images, on the other side, are defined by a grid of pixels. This is what I called a continuous tone or a bitmap.

A vector PDF is built out those two graphic object types. Each image is kept separated to itself and is represented by a raster, while other art, like text, lines or clip art is represented by vector graphics objects. (The “Each image is kept separated to itself…” thing is important. I’ll get to it latter in post III)

A raster PDF is a PDF where all objects on the page were converted first into a raster graphic, such as a jpeg/tiff image, and than added to the PDF as a complete page.

In the previous post I showed why scaling the vector as a vector produces better results than scaling the raster (or bitmap). That’s obvious. However, is this really the case with a PDF? After all, we can raster the image to the resolution we want it to be printed anyway so we will not need to scale later.

Well, if just life was so simple. What happens is that not all resolutions are created equal. Or maybe its more correct to say that not all graphics are created equal when it comes to resolution?

When we look at an image, out eyes will view it as a continues tone. As such, our eyes can not see the difference in resolution (or more pixel data) at one point. Print at 300 dpi and above, it all looks the same. Between 200-300, its pretty good but the trained eye will notice. Print at than 200 dpi or less and you will likely notice. Print at less than a 100 and, well, just don’t go there.

When we look at line art or text or clip art, our eyes are less forgiving. Look at text for example. While an image is built of pixels that change all the time, a character has a solid color and ends at once on an edge. You have a curved line, and than you don’t! Our eye sees that. This why in the traditional repress industry, text is eventually converted into a raster at anywhere between 1200-2400 dpi. If you don’t believe me, go print some black text on your favorite laser printer and look at it really close. Look at a X or a B, look at thin text, and you’ll see. Same goes for clip arts and lines.

So now, you have a problem. What dpi should I raster my page is I go with a raster PDF? 300 dpi is good for the images, but only reasonably okay for my text and clip art. Or, I can raster to 1200 dpi. That will work,with a cost. When I save my images at 1200 dpi they are not 4 times bigger than a 300 dpi but 16 times bigger! (yes, its not 4 but 4 times 4). So if a page in my calendar is 8.5×11, at a 300 dpi raster it will be, roughly, 2 MB, depending on the compression level I am willing to take. The same page, at 1200 dpi, will be come 32 MB. Given a calendar might have 26 pages, my calendar just grew from around 50 MB to around 850 MB. This by itself is enough to kill this idea, which is does. Almost anyone will do it to 300 dpi or less. Some will settle on 200 as well. The temptation (and the files) are just to big. There are even more problems intreduced into the page tis way, but lets ignore them. They are small compared to this one anyway.

To sum it up. Your print will not look as good as it can, especialy text and graphics.

To be honest, this might not be a big deal. Many customers, at least at this stage of the game will not notice anyway. If you do not have allot of text or clip arts this is not even a problem. On the other hand, why be less than you can be?

This however, is the smaller problem of raster PDFs. The next post will describe the main problem. If you want a hint, rememebr the sentence: “Each image is kept separated to itself…”? The next post will discuss why keeping each graphics ellemnt separate is a big deal, even if we evntualy print them on the same page.


1 Response to "All PDF’s are not created equal. Part II (out of III)"

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