Bitmap, Vector and Why it Matters

You’ve done your research, stuffed all your data into a spreadsheet or database program, over-analyzed it in SPSS, R, SAS or the like and your results are extraordinary, revolutionary and surely worthy of a Nobel prize. Besides writing your manuscript, there’s just one little detail left to do. The figures. As the old saying goes, “a picture is worth a thousand words.” For your figures to be worth anything and to tell the story, they need to be clear, non-pixelated and readable.

In building a figure, there’s a choice, often determined by the software used, between two formatting options; bitmap or vector. What’s the difference? The difference is in how the image is defined by the program that builds it. The following description is an oversimplification of the actual coding but adequate for our purpose.

The Bitmap
Starting with a 300 x 300 matrix (ie, 300 x 300 pixels), say we want to draw a diagonal line from the upper left corner of the matrix to the lower right corner. The bitmap coding would say, “Go to position (1,1) and print (or display) a dot, then go to position (2, 2) and print a dot, then go to position (3, 3),” and so on until we reached position (300, 300).

The Vector
Starting again with our 300 x 300 matrix and our diagonal line, the vector coding would say, “Start at position (1,1) and draw a straight line, 1 unit wide, to position (300, 300).”

Due to this difference in how the two file types are coded, the ability to enlarge a bitmap image is very limited without losing the sharpness (resolution) of the image, whereas the vector image can be enlarged to virtually any size without losing resolution. To see the difference open this pdf. At 100% size, the two images are fairly similar although the bitmap image is a little fuzzy. Zoom to 200% (a visual equivalent of doubling the image size) and the bitmap gets distinctively fuzzier (or pixelated, as you can begin to see the individual pixels). At 300%, it’s obvious the vector image is far superior.

Why Does it Matter?
If the image is going to be published exclusively online and there will never be any need to zoom in to see details, or the publisher will never have a reason to enlarge the image from the size you created, then it probably doesn’t matter. If, however, the image is ever going to be printed or if the publisher wants to change the size of the image from what you’ve created, then it definitely will matter as the resolution will suffer in two ways. First, if a bitmap image is resized (even reduced in size), it will affect the clarity of the image due to inaccurate interpolation. More importantly, because of the way images are processed for print, a bitmap image that might look alright online or on screen has to be increased in resolution by at least 300% (depending on image content) to print properly (ie, not look fuzzy).

The Solution
Always use a vector image. And how to tell if you’ve created a vector image? The most conclusive method is to save your image file in whatever format your software allows, open the file in a program that can read vector images (see below for possible programs) and zoom in, and zoom in some more, and still more, until you’re sure the image doesn’t get fuzzy or pixelated.

If you have a choice of file format, some of the more common vector formats are: encapsulated Postscript (.eps), Portable Document Format (.pdf), Adobe Illustrator (.ai), CorelDRAW (.cdr), Windows Metafile (.wmf), Enhanced Metafile (.emf), or OpenDocument Graphics (.odg). One of these formats should assure a vector image, however, since many vector files can store bitmap images, you won’t know for sure unless you test the output. Once you’ve created your vector image file, do NOT embed your figure files in your word processing program as it’s possible it might convert it to a bitmap image.

Can you tell by the file type if it’s a vector image? Generally no as most vector image formats can also embed bitmap images. Can you tell, by the file type if it’s a bitmap image? Generally yes. Bitmap images ordinarily would have one of these relatively common file extensions: .gif, jpg, .jpeg, .png, .tif, .tiff, .bmp, .exif, or .raw. There are, however, hundreds of proprietary bitmap formats (and file extensions).

If you absolutely, positively have to use a bitmap image, the only solution is to increase the resolution. Some software express resolution as pixels per inch (PPI) and some as dots per inch (DPI). The two terms are related but not identical. Assuming your image is at what you expect will be the final size as a printed image, the resolution should be 300 for a photo, 600 for drawing with text and 1200 for text only. Keep in mind that a doubling in resolution approximately quadruples the file size.

So, What Good are Bitmap Images?
Photos. Bitmaps are perfect for photos.

 – Some of the (PC) programs that can read vector files —
Some of these can only read certain file formats. Also some files cannot be read directly but have to be imported or inserted.
Adobe Acrobat Pro (or X Pro, or other permutations)
Adobe Illustrator
Adobe Reader (free)
Corel DRAW
LibreOffice Draw – (free, and probably the most flexible solution)
Microsoft Word Draw – (free)

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