Tag Archives: bitmap

Photos and Your Computer: The Basics

Besides surfing the web and sending email, one of the most common reasons for people to get a computer is pictures. Whether it’s scanning old photos to share, uploading digital pictures to an online photo album, or sending your camera phone pictures to Facebook, just about everyone has some use for digital pictures. There are a lot of good photo editing software packages out there that make common adjustments to photos easy to do. But with that software often comes a bewildering choice — what format to save the end result in?

First we need to know a little bit about digital photos. Digital photos are made up of a grid of little dots called pixels. If you’ve seen a photo say that it’s 400 x 300, that means it’s 400 dots across and 300 dots high. When the dots are small enough, our eyes blend it all into one smooth picture. Each of those dots has a number associated with it that also tells the computer what color the dot is and how bright that color is. There are different ways to code the color and brightness information, but it still comes down to storing a number for that pixel.

So why do we have different formats? It all comes down to size.

Let’s say you have a 5 megapixel camera. Not very large as cameras today go. 5 megapixels is 5 million pixels. Each of those pixels has a number associated with it. My digital camera stores the color information in about 2 bytes of data per pixel, which means each picture is about 10 megabytes in its raw format. If I want to email a dozen pictures to a friend, especially a friend with a slow internet connection, they aren’t going to be very happy about the size of those photos.

What can be done? Well, a lot of pictures have relatively large patches of the same color. Your computer might notice that there are 100 pixels in a row of blue sky. So instead of storing the number for sky blue 100 times, it make a notation of the color and that it will be repeated 100 times — storing just two pieces of information instead of a hundred. This is called compression and it makes the file smaller, making everyone is happier.

Naturally, it’s not quite that easy in real life. We might think the sky is just blue, but each pixel might be just one shade of blue different from the next pixel. Now instead of saying we have 100 pixels of sky blue, your computer starts storing twice as much information as the original, because it is storing a color, saying it repeats for a single pixel, storing another color, saying that repeats for another pixel, etc.

What to do? This is where the different formats come in. Each format has strengths a weaknesses — one format may make the photo’s file size smaller, but at the cost of quality. Another format may have a special feature or two that are really useful in some situations and not others. In the next few posts we’ll go over some of the common formats, where they excel, and where they fall short. Then when you are faced with a choice of formats, you’ll be confident you’re choosing the best one for your needs.