1–bitmap images e.g. photographs
Bitmaps are made of many tiny dots, or pixels, each a different tone or colour. When a camera or scanner’s sensor is exposed to light it registers the light at each point and gives it a number to describe its location, colour and tone. These numbers can then be translated back into an image by a computer screen or printer.
bitmap resolution. More pixels mean more detail, but uses more memory to store the extra data. Bitmap resolution can be measured in pixels per inch–ppi (or dots per inch–dpi). Another common way to measure image resolution is the area of an image in pixels, e.g. 3,000 pixels wide x 2000 pixels high = 6,000,000 pixels which is commonly shortened to 6 Mega Pixels (6MP).
bit depth. A ‘bit’ is a binary digit; 1 or 0. Bit depth is the number of bits used to record the colour of a pixel. 1 bit can record black or white (recorded as 1 or 0). 24 bit (or 24 binary digits) can record 16,777,216 colours! So more bit depth can show more detail, but again uses more memory to store the extra data.
Compression. For images with lots of detail you need lots of pixels which makes larger files. Storing and displaying large images on the internet is slow so we compress the data. This usually means some data will be lost and we need to balance compression with image quality. JPG is the most common compression.
hi res image printing you need files that are 300dpi at 100%, e.g. to print an image at A4 size it needs to be 21 x 29.7cm at 300dpi. This equates to 2480 x 3508 pixels or 8.7MP. An uncompressed file at this size uses 30MB of data. Compressed as a JPG with quality set at 10 uses 2MB, and at a quality setting of 4 uses 0.5MB. You can see that file size is not a good guide to resolution.
hi res images on screen you need files that are 72dpi at 100%, e.g. to display a landscape image full page on a website it needs to be about 1000 x 650 pixels at 72dpi or 0.65MP. An uncompressed file at this size uses 1.8MB of data. Saved as a JPG with quality set at 10 uses 0.5MB, and at a quality setting of 4 uses 0.2MB. You can see that file size is not a good guide to resolution.
Bitmap file formats To retain the maximum detail you should use the native file format of your software, e.g. Photoshop uses ‘PSD’ files, which is ‘lossless’ so will always keep all your data but makes big files. To store or share files you should use a format that will compress them, like JPG which is ‘lossy’, each time you open and save the file you will lose a bit more data. At a quality setting of ‘10’ JPGs will be much smaller than native files.
2–vector images e.g. logos
made with lines that define a shape containing colour. You can draw a shape by defining corner points and curves using software like Adobe Illustrator. Lots of shapes can be drawn to kane an image. The vector file saves the data for each point and curve and what colour the shape is filled with, so a large area can be recorded using much less data.
vector resolution vector files have no ‘resolution’ as they contain only data describing the location of points relative to each other and what colour fills the shape made. It is irrelevant if the points are 1mm apart or 200m apart the data is the same and a smooth line will always join the points together, resulting in a sharp image at whatever size it’s used.
vector file formats usually depend on the software used to create them, Illustrator saves ‘AI’ files, Corel Draw saves ‘CDR’ files. Another common format is EPS which is compatible with a wider range of software.
RGB or CMYK
All screens (TV/Computer/mobiles) and photographic printers use RGB (Red Green Blue) to recreate images.
Offset printers (magazine/books/ postcards) use CMYK (Cyan Magenta Yellow Black) inks. This applies to both bitmap and vector images.
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