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Raster vs. Vector

Digital Graphics Basics

Improve Your Graphics

I mentioned in my blog that my 17-year old niece, Alex, explained the difference between raster and vector images to me – something I had been struggling to understand.

One of our readers, Beth, posted a comment, “Can you please explain raster vs. vector image for me? I don't understand it either!”

I would be glad to. I’m happy to hear I’m not the only one who didn’t get it.

A Bunch of Pixels = a Raster Image

Anyone who uses a scanner or digital camera or paint program like Photoshop is familiar with pixels.

“Pixel” is short for “picture element”. A pixel is a dot of color. To create a raster image, pixels of varying color values are laid out in a grid. The human eye perceives the grid as a smoothly blended picture UNLESS …

If, for example, you download a photo from an e-mail you most likely have a 72 psi (pixel per square inch) file and its size might be 2 by 2 inches. That's 144 pixels wide and 144 pixels high for a total of 20,736 pixels in your downloaded file. That’s fine for the Internet and viewing on screen.

However if you want to print the photo, you probably want it at least 4 inches by 4 inches. When you try to stretch the image, you are spreading 20,736 pixels over 16 square inches and your file resolution will be only 36 psi! (As a point of comparison, the tiny icons on your computer desktop are 32 psi.)

Your picture will be very pixelated, meaning the eye sees the individual pixels rather than a blended image.

The number of pixels is fixed or, said another way; a raster image is resolution-dependent. I’ll talk about the implications of that after explaining what a vector image is.

A Math Equation = a Vector Image

The statement that raster images are resolution-dependent is the perfect segue into a discussion of vector images because vector images are not.

You can stretch a vector image to the size of a billboard and it will look precisely the same. No loss in image quality. No pixelation, BECAUSE there are no pixels in a vector image.

What then comprises a vector image? Being an artist, you may not like the answer.

Vector images are made using math. Any clearly-defined shape can be expressed as a mathematical equation. The good news is that computers are very good at math, so you don’t have to be.

If you want to enlarge a vector image, you just stretch it out or type in a new image size. The computer recalculates the equation to fill the larger space. Vector images are ideal for elements that frequently need to be resized like fonts and digital scrapbooking embellishments.

Use the Right Tool or Suffer the Consequences

Some file formats that use vector images are .doc, .txt, .xls and .png.

Examples of raster image file formats are .jpg, .gif, .tiff and .bmp. Choose the right tool for the job and you too can achieve professional graphic design results.

Maybe I can best illustrate this with an example of what not to do. Here is a good one – or perhaps I should call it a bad one.

Here is a chart I made in Excel before I knew anything about raster vs. vector images.

Using Excel was a good start because Excel is a vector-based program which provides a clear image no matter how you size it. Why then does this chart appear blurry? Because, in order to load it up to the website, I copied it into Photoshop and saved it as a .jpg file.

So now it is a raster file with color gradations and shading rather than crisp clear black and white. There is no going back. I could try to clean up all the stray pixels that cause the blur and re-save it as a .png file, but it would be less work to start over in Excel.

Raster Image Tips

As I explained above, a raster image has a fixed number of pixels. There are ways around that but, if you plan your design ahead, you won’t have to fool around with often less-than-satisfactory fixes.

Think about how you will use a project before you begin. If you plan to make a 12-inch by 12-inch scrapbook page to print for an album and then want to post it in the online gallery at Scrap Girls, make a 12 by 12-inch document using 300 psi.

After creating and saving your layout at 300 psi, reduce the resolution to 72 psi and save it as a .jpg for the web.

In Photoshop or Photoshop Elements, select Image>Image Size and type 72 into the resolution box. Close the file without re-saving the .psd file. You already saved it at 300 psi and don’t want to overwrite that with the smaller version.

In summary, 72 psi is good for e-mails and the web, 300 psi is good for most printing. There may be times when you want even higher quality for printing. The tradeoff is file size.

Because we’re measuring in square inches, the growth in file size is exponential. A 1-inch 72 psi file contains 5,184 pixels, while a 1-inch 300 psi file has 90,000. The color versions contain three times as many (R, G and B).

I said that a raster image has a fixed number of pixels. Now you know one way to change the number of pixels: reduce the resolution. What happens when you reduce the resolution is that pixels are thrown away.

Once you’ve done that and saved the file, there’s no getting them back. You threw them in the garbage and the garbage man already came and hauled them away.

Photoshop does provide a way to add more pixels when you absolutely must increase the resolution of a low rez file. If you reduced the resolution yourself, they won’t be the same pixels you threw away but rather interpolations of the remaining pixels.

So plan ahead. Start with the highest rez and largest size your project requires. Save that to one name. Then make your smaller versions, if any, and save them to different names.

If you are forced to use a low rez raster image in a larger file using Photoshop, select Image>Image Size. Type in the higher resolution, make sure the Resample Image box is checked and choose Bicubic Smoother from the drop down menu.

That tells Photoshop to interpolate new pixels between the existing ones moving from each pixel’s color to the next in as smooth a fashion as possible.

P.S. When you use the Image Size box to reduce resolution, you can sometimes get better results by selecting Bicubic Sharper from the Resample Image menu. Try both ways and see which you like.

Remember you can undo work in Photoshop simply by going to the History palette and backing up a few steps. If you don’t see the History palette, select Window and put a check mark next to History.

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