A Guide to Linux Graphics Software 01: bitmap vs vector

Graphics software for Linux is superior to most other software for several reasons. Since the Linux system is inherently more efficient than other systems, memory-hungry graphics operations will always run faster, better, and more reliably on a Linux box than on, say, a Windows box, all else being equal. Day to day graphic needs can be met with a wider range of software on Linux than on other systems. Most of the available applications are OpenSource, so not only are they free and easier to install, but no puppies were mutilated during the production of the software.

The purpose of this sequence of posts is to assemble basic information for the Noob Linux User about what software packages you should consider installing. In this first post, we will look only at the graphic files themselves, mainly at the two distinct types of graphics that you must know about before moving on to any other topics: Pixel, or bitmap graphics on one hand vs. vector graphics on the other.

This distinction applies to all operating systems so if you are a Windows user this will still be useful. You don't need to know any of this if you are a Mac users because Mac's just work, and the user only needs to know how to click on things.

But seriously....

There are two kinds of graphics, and thus roughly two kinds of graphics programs (though this is an oversimplification): pixel/bitmap and vector. Pixel or bitmap based graphics are pictures made up of dots, and vector based graphics are pictures made up of commands written in a programming language that tells a rendering machine (software or hardware) about specific shapes and where to put them. This is a fundamental difference that must be understood.

Here, I used a vector based graphics program to draw a thick line segment:

I then exported this graphic to a bitmap format and opened the resulting file with a bitmap graphics program:

Notice that the lines look the same. However, they are not the same. The vector line is made, from the user's point of view, of a small number of points that are connected by curves. So, if I use the correct tool (chosen with the mouse) to click on the line, I can move the points and change the shape of the line. Here, I have done this and you can see the points that define the line highlighted and note that the line shape has changed:

Note that these curves can be made more curvy or less curvy (or the direction of the curve can be changed) by moving the levers with the red dots on them, which are more clearly shown in this closeup:

In the bitmap/pixel graphic, by contract, the line consists of several hundred black dots arranged to appear to be a line on a white background. There are no tools to manipulate this line as a line because it is not really a line. It is merely a representation of a line. Here is what the line looks like when I use some bitmap graphics tools to play around with it:

You can see that I've smudged (moved pixels) the line in a few places. In another location I've cut out a chunk of the line and moved it do a different location on the page. So I can do things with the line, but it is hard to manipulate it as a line. I'm merely doing things to pixels that happen to be laid out in such a way as to represent a line.

If I showed you the inside of the bitmap graphic file, it could look something like this:

700,700 --- indicating the hight and width of the file in pixels

black dot at 100,300 --- indicating the 2D
black dot at 100,301 --- coordinates of the black
black dot at 100,302 --- pixels.

...

black dot at 600,368
black dot at 600,369
black dot at 600,370

... but with several thousand lines each indicating a white or black dot and it's exact two dimensional coordinate.

Now, there is no graphics file format that looks exactly like this, but you get the idea.

If I showed you the inside of the graphics file for the vector line, it would look like this:

xml version="1.0" standalone="no"?>
svg xmlns="http://www.w3.org/2000/svg" version="1.2">
Generator: Xara Xtreme 2.0, SVG Export Plug-In Version: 0.4 -->
Generated 2008-08-06 07:46:46 -->
not very useful exporter for now... -->
entity tag="40" -->
entity tag="1" -->
entity tag="4114" -->
entity tag="4116" -->
entity tag="4124" -->
entity tag="80" -->
entity tag="41" -->
entity tag="1" -->
entity tag="42" -->
entity tag="1" -->
entity tag="45" -->
entity tag="53" -->
entity tag="4031" -->
entity tag="43" -->
entity tag="1" -->
entity tag="48" -->
entity tag="115" -->
entity tag="1" -->
entity tag="111" -->
entity tag="152" -->
entity tag="51" -->
...
/svg>

That is actually a big portion of the file for the line as altered in the example I provide above, with parts that needed to be removed in order to not break the internet. It is much, much shorter than the bitmap file (of which I've only shown you a tiny bit).

One way to think about this is as follows: The vector graphic is a real line, and the pixel graphic is a picture of a line. In meatland (real life) you can take a picture of your roommate, or you can have your actual roommate standing there. If your roommate is standing there, you can say "OK, raise your arm up in the air and swing the dead chicken around over your head." But to make the picture appear to be a person swinging a dead chicken around over their head would be difficult. Better to get the real roommate to do this foolish thing and take a new picture showing this behavior. Then post it on the web, of course.

Here is a concrete example of the difference. Below you can see the same words written out twice. The first time, they are written in a text processing program, displayed on a computer screen, then 'photographed' with a screen shot utility, and saved as a graphic. Go ahead and try to select individual words in the text. Like the text itself says, you can't do it because it is not there! This text, even though it does not exist, is telling you the truth!

The next set of text is simply pasted from that original source into this blog post -- as text. Go ahead and try to select some of the words with your cursor. You can do it! This text, which does exist, lies!

Try to select this text using your mouse.
You can't do it because this is not text. This
is a picture of some text. You can download
this picture as a graphic, but you can't manipulate
the letters as text because they don't exist. Even
though you can see them.

(If you look closely, you can see that whoever wrote this text goofed before they fixed the text, but could not change the picture of the text without a lot of trouble. )

This distinction between vector and pixel also applies to PDF files, and this is a very important fact that many people do not appreciate. A PDF file of a document could consist of a number of instructions indicating where to put which letters/numbers, rule-lines, figures or graphics, etc. etc. In such a document most of what is displayed may be text shown in a particular font, formated in various ways. One could take this PDF file and run it through a conversion program to make the PDF into text, and you'd pretty much have the same document in terms of what is says, though you will loose the formatting.

But a different PDF document may consist of a series of scans (pictures, photographs) ... bitmap graphics ... of the pages of a previously printed document. In this case, the things you see as words on each page are bitmap representations of the words, not rendered text. So you can't select a word, or copy a word using text manipulating tools. If you run such a document through a PDF to text converter you'd get nothing readable because even though you 'see' text, there is no actual text there. Just a picture of the text.

This is an important distinction for several reasons. For instance, if you have a bunch of PDF files in a directory on your computer and try to search through them for a certain string of text, you won't find that text in the scanned-bitmap type of files even if it is "there."

Bitmap/pixel graphics can be much larger than vector graphics because it is often possible to represent a fairly complex shape with very few vector codes. On the other hand, rendering (showing) a bitmap graphic is routine and easy for software, while rendering a vector graphic is not quite as easy.

Bitmap graphics are often compressed so that they take up less disk space and also less internet bandwidth. Imagine a graphic that shows nothing but whiteness (the picture of the line above but without the line, for instance). Since every pixel is identical, such a graphic can be represented by saying "put a white dot in each spot where a dot can be on this graphic of a certain size." That is not going to take up a lot of room in a file!

Compression can involve a loss of data. For instance, let's say that a bitmap graphic has dozens of colors in a complex pattern including lots of greens along the bottom of the graphic and lots of blues along the top. (A landscape showing the Irish country side. With sheep.) One way to compress this might be to average out the blues so instead of having, say, 11 shades of blue, you have only 3, and then specifying approximately which areas of the sky have each color. This would take a lot less data (and thus make for a smaller file) but the graphic would not look as clear.

Methods that compress files but do not reduce the information contained in the graphic are called "lossless." Other than these lossless means of compression, all compression will destroy some of the data. Avoid compressing your original graphics .... compress only copies.

Vector based graphics (and PDF files) can include bitmap graphics as elements, thus adding even more confusion.

OK, now that we have this distinction clearly in hand, we can examine bitmap graphics software and vector graphics software separately. Stay tuned.