The most prevalent type of image files found in the computer graphics and printing industry realm are images composed of pixels saved in a bitmap format or file type. A bitmap image is an image wherein each pixel is defined by the number of computer bits needed to describe its color. In black and white images, only two bits are needed to define the color of a pixel (e.g., 0 = black, 1 = white). In full color images, 24 bits per pixel is standard. The pixels are then arrayed or defined in the digital file as in a map overlayed by a grid: this pixel occupies this specific x, y coordinate (e.g., row 34, position 128). All pixel-based images are bitmap images. However, in general graphics usage, the term bitmap is used to refer to images that are strictly black and white; that is, they contain no gray color tones.
What is a pixel?
A pixel (a word created by combining the terms “picture” and “element”) is the basic unit of color on a computer monitor or in a digital image. A pixel in a digital file is not a physical unit with a set dimension. Rather, the size of the pixel would vary with the amount of data the digital image contains. For example, in a 6” x 4” photograph saved at 300 ppi (pixels per inch), the pixel size would be smaller than in a 6” x 4” photograph saved at 150 ppi.
How many pixels are there in an image?
The number of pixels an image contains is determined by the device used to capture or create the image. Modern digital cameras and smartphones typically capture pixels numbered in the millions. Desktop scanners can also capture images with millions of pixels. The key question is how many pixels are required for an image to print at full resolution? That answer is dependent on at what size the image will be printed. For example, to print an 8.5” x 11” sheet of paper with a full-bleed photo requires a 300 ppi image containing 9,315,000 pixels. Image-editing software, such as Photoshop, can readily re-size an image to contain fewer pixels (called downsampling) and can also add pixels to an image (called upsampling). However, adding pixels to an image invariably degrades the quality of the image except if done in small increments (i.e., increasing the size of an image 5-10% by adding pixels is not problematic; increasing beyond 10% is not advisable).
Why is it important to know about bitmap and pixel images?
In our next blog post we are going to discuss the topic of resolution as it applies printing. The internet has given people access to millions of pixel-based images. However, not all of these images contain enough pixels to print with clarity, particularly if they are enlarged beyond their intended display size.
If you have any questions about bitmap & pixel images, give us a call (330-597-8560). We’re happy to help you get the most out of your printing project.
