Raster Graphics

Graphics fall into to two main categories:

Vector graphics
Bitmap graphics
The difference between these two types is what they're made up of. Vector graphics are made up of lines and curves. Bitmap graphics are made up of little squares called pixels.

Notice that even blown up this large, it's still fairly smooth. You only see a little bit of jagginess because your monitor can only display this graphic using pixels. If you were to print it out, you'd see that it's as smooth as the same graphic at 100%.

Here's the same logo — except it's a bitmap graphic — blown up roughly the same amount. Doesn't look the same does it? You can see obvious jagginess or "stairstepping", and the text is unreadable.

That's because this is a bitmap graphic. It's composed of little squares called pixels. When you enlarge the graphic, the software must guess where to put the extra pixels necessary to make it larger. Let's zoom in even closer on these graphics.

Here's a closeup of the bar on the k in the bitmap graphic. Now you can see the pixels, close up and personal. The reason they're different colors is because of antialiasing.

Here's a closeup of the bar on the k in the vector graphic. Looks nice and smooth, doesn't it?

Here's a closeup of the bar on the k in the bitmap graphic, after it's been converted to bitmap mode. It's no longer antialiased, so you can really see the jaggies — the individual little squares, or pixels. Ugh!

Finally, let's take a look at what happens to this same graphic — in both vector and bitmap format — when it's reduced in size. This comparison is a little unfair, because we're comparing apples and oranges, so to speak. We've got to zoom in on the reduced graphic in order to see anything: this changes nothing in a vector graphic, but as we've seen before, zooming does interesting things to bitmap graphics.

Above is the vector version of the INK logo. It's been reduced 50%, and you can see that basically it's the same. The reason the lines are thicker is because of the settings in CorelDRAW!, nothing to do with the vector graphic format.


This is the bitmap version of the logo reduced 50%, with a zoomed in view. You can see from both that reducing a bitmap graphic doesn't create a pretty picture.

Basically, when you enlarge a bitmap graphic you'll notice pixelation (jaggies or stairstepping shown above). That's because the software has to guess where to put the extra pixels.

When you reduce the graphic, the software has to decide which pixels to throw out. Both the addition and discarding of pixels in software is what's know as interpolation. You'll usually get less pixelation if you reduce or enlarge by even amounts, like 25%, 50%, and 75%.

Graphics File Formats
There's a bewildering amount of graphics file formats out there. When the pixels settle, the most common file formats in the graphic design world can be counted on two hands.

Vector Encapsulated Postscript (EPS)
Adobe Illustrator (AI)
Windows Metafile (WMF)
Bitmap

TIFF
JPG
GIF
A word about "native formats": almost every software program has a file format that is native to that program, and that program alone. Photoshop's is PSD; CorelDRAW! is CDR; and so on. Always save your graphic in your software's native format before saving it in the file format you'll be using in the end. That makes it really easy to make changes to the original graphic.

So which format do you use?
Hopefully by now you understand the difference between bitmap and vector graphics. You may even already suspect when you should use which. But I won't leave you wondering.

At the moment, the only format that can be easily viewed on the Web is bitmap graphics, GIF and JPG. There are a few vector formats that can be viewed on the Web, but as of this writing, they all require plugins. Your viewers shouldn't be left in the cold if they don't happen to have a plugin installed.

Given the limitations of the bitmap format, you may be wondering why you'd ever use it outside of the Web. If you scan a photograph, you'll be forced to save it as a bitmap; the same is true of digital pictures.

Vector graphics are great because of their easy scaleability. Be careful, though: EPS graphics require a PostScript printer to print correctly. If you try to print an EPS graphic to a non-PostScript printer, the only thing that will print is the low resolution header.

On the Windows platform, WMF is a common vector format. But if you'll be going to a service bureau, chances are they won't know what to make of your WMFs. WMF is fine if you'll be use your laser printer output as camera ready art, but if you'll be getting film run stick with EPS or AI

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