Bitmap Images – a primer for authors

Thanks for stopping by. Over the next couple of blog posts, I’m taking a closer look at images and what you need to know about them when working on your own cover or with a designer.

Basic types

There are 2 basic types of digital images…bitmap and vector.

Bitmap images

Bitmap images are made up of pixels within a grid structure. Photographs are good examples of bitmap images.

Early morning in Millview

Taken in Millview, PEI on August 18, 2013.
Copyright (c) 2013 Valerie Bellamy.

Sometimes you will hear these referred to as pixmap or raster images. There are technical differences between these terms, but for the purposes of this discussion we will use the term ‘bitmap’.

Pixels are little square blocks of color where each pixel can only contain one color. The grid structure defines the location of an individual pixel in the image. The number of pixels in an image (expressed in pixels per inch, or ppi) determines the amount of detail in the image…the more pixels you have, the higher the detail possible. Because these files are made up of a finite amount of pixels, scaling the image (making it larger) can be difficult.

Vector images

Vector images are made up of curves, points, lines and other geometric elements. Logos are typically built as vector images.

vector artwork floral background

Vector artwork background from

The curves, points and other elements that make up vector images are based on mathematical equations. Because the files are based on mathematical equations, the equations can be recalculated so the vector image can be scaled (made larger) with no loss in quality. The image is the same quality whether it’s small (like a one inch square) or it’s been blown up very large (like on a billboard).

Bitmap images in detail

Let’s use this image of mine, because I have Spring on the brain (now that it’s finally here in Nova Scotia!), to demonstrate pixels in a bitmap image.

Budding tree branch

Halifax, NS, May 14, 2013.
Copyright (c) 2013 Valerie Bellamy.

Let’s look at those pixels a little closer. Bitmap images are computer code that describes the image pixel by pixel based on a grid structure. That’s why a pixel can be only one color.


A close up screen shot of a leaf demonstrating individual pixels.
Click on the image to enlarge.
Use the back button to return to this post.

See the squares? Those are pixels. To see the pixels in your image, zoom in as close as you can using your image editing software. The larger the image, the closer in you will have to zoom to see each pixel.

Bitmap images are resolution dependent, meaning that they cannot be enlarged without loss in quality. This is really important to keep in mind when choosing an image for your cover. The larger you want to make the image or the closer you want to crop (zoom in) to an element of your image, the larger the image will need to be.

Changing the size of a bitmap image is basic math. I know some people and math don’t get along, so I’m going to show this visually:

In this 5 x 5 pixel checkerboard image, each pixel is either gray or white.

5x5 pixel grid

a 5×5 pixel grid

Now look at what happens to the pixels when we resize that image to 10 x 10 pixels:

10x10 pixel grid

the same pixel grid, enlarged to 10 x 10

Now each gray or white square takes up four pixels of space. It’s been doubled in size both horizontally and vertically, so each square is now four times as big. With this kind of exponential enlargement, the quality of the image will degrade quickly.

Here’s an image of a mallard duck I took a couple of months ago. The original image is 4608 x 3456 pixels in size. I created a smaller version of the same image that’s 1050 x 788 pixels in size. Let’s compare both files at 400% magnification, so we can see the pixels in both sizes.

Click on each image to make them bigger and use the back button to return to this post.

400% close up of image 4608x3456 pixels

400% zoom of the large image.
Click on the image to enlarge.
Use the back button to return to this post.

400% close up of image 105x788 pixels

400% zoom of the small image.
Click on the image to enlarge.
Use the back button to return to this post.

Can you see the difference in the size of the pixels? Can you see the level of detail is different too? This difference may seem subtle on screen and you may be ok with the quality of the second image for your e-book cover, but if you are planning a print version of your book, read on.

Resolution differences between computer screens and printed versions

Are you planning a printed version? If you are using bitmap images, you will need to start with larger files.

Standard screen resolution is 72 pixels per inch. In screen resolution, a 6 inch x 9 inch image used at 100% size would measure 432 pixels x 648 pixels.

Standard printing resolution is much larger…300 pixels per inch. So the same image, to print beautifully at 100% size, should measure 1800 x 2700 pixels (not including any extra you need for bleed).

Tip: If you are planning to use your bitmap images in printed promotional materials, keep in mind the largest size piece you are planning and build your bitmap artwork to that size for best results.

Image enlarging

Now, Photoshop and other image editing programs try to combat this with a process called image interpolation or enlarging, where the software uses algorithms to “guess” at what color the new adjacent pixels should be. The results can vary widely depending on the software and version. Newer versions of Photoshop do a decent job of enlarging images to a certain point.

Tip: I always recommend starting with a larger image when possible. Removing extra pixels is easy. Adding extra pixels is hard.

Files Types

You will see bitmap images in many file types, like JPG, TIF, PSD (Photoshop file), PNG, GIF, and others. EPS and PDF can also support bitmap images and well as vector images. So, file format alone is not always an accurate indication of whether an image is bitmap or vector. Bitmap files can even be exported to vector file format with typically disappointing results (lots of jagged edges).

Tip: Always check the file to be safe. Zoom in as far as you can and if you see pixels, it’s a bitmap.

I hope this sheds some light on bitmap images. Next time we’ll have a look at vector images.

Is there a topic you have been meaning to research and just haven’t got the time? Comment below or send me an email…let me know what you want to know. I’ll do the research for you and write about it in a future blog post.



  1. Vector images - a primer for authors | Dog-ear Book Design - [...] or with a designer. If you missed the blog post all about bitmap images (i.e. photos), you can read…

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