Understanding Images on the Computer

1. Understanding Images on the Computer How do images work and why?

2. All Images on the Computer work in one of two ways… • Vector based images • Use math to draw their shapes • Raster based images • Use pixels to make images

3. Vector Based Images • Tend to be graphic in nature (not photographs) • More likely to be made of simple shapes • Can be scaled to any size and not get “pixilated” • Generally do not create large file sizes

4. Raster Images • Are made up of hundreds or thousands or even millions of little pixels. • Used for images hard to describe with math (such as photos) • When enlarged they become “pixilated” • Have something called “resolution” to define their quality.

5. Original raster image Blown up so you can see pixels

6. About Resolution • Measured in DPI (dots per inch) • Measured in LPI (lines per inch) • Measured in PPI (pixels per inch) • Computer monitors generally display at 72 dpi • Ink jet printers print at about 150 dpi • Laser Printers print at 300 – 600 dpi

7. Resolution Continued • Think about DPI as density of pixels. The more dense they are, the finer the quality of the image. • The more pixels there are in an image, the more space that image will take up on your hard disk.

8. Compression - Lossy • JPEG – loses information out of the image to make it smaller. • The higher the compression with jpeg, the worse the quality. You will see “artifacts” • Files end in .jpg or .jpeg

9. Compression - Lossless • GIF – makes a file smaller by using a kind of short hand to keep track of pixels. • Good for graphics without a lot of different colored pixels (not good for photographs) • PNG – Like GIF but includes an alpha channel for transparency.

10. Recommendations • Pick the right kind of compression for the job • Pay attention to size / quality needs of the situation • Don’t let projects get bloated with unnecessarily huge images.