What’s the difference between a vector file and a bitmap?
The two main types of image that we use are vectors and bitmaps. These can be created by many different programmes, and have many different file extensions. They have specific strengths and weaknesses and are used for different purposes.
Bitmap files are made up out of tiny squares of colour in a grid – pixels – which generally are not visible as individual cells. Each pixel can be a different colour and the format is therefore ideal for photographs or images with complex colour changes and shading. More pixels means greater detail – so long as the pixels exist in the original image.
If you enlarge a bitmap image, more pixels are needed to fill the new, larger size. To do this the computer has add more pixels, and it has to guess at what colour these new fill-in pixels should be. It “interpolates” by adding colours which are related to the adjacent original pixel colours. The result is that the image can look soft or blurry and, as a rule of thumb, it is not generally recommended to enlarge a bitmap image more than 130% for print work. This is assuming that the image is print quality in the first place.
You can generally reduce bitmap images without losing quality until they get so small that there is not enough space for the pixels needed to show a sharp image.
All scanned images and images from digital cameras are bitmaps.
Examples of programmes which create bitmap images: Adobe Photoshop, Corel PhotoPaint, PaintShop Pro
Examples of file extensions used on bitmap files: .jpg, .gif, .png, .tiff, .eps (though this can also be used for vector files) and .bmp, PICT, PCX, PSD (Photoshop)
Advantages of bitmap files: Can handle all the subtleties of colour in photographs, images with complex shading
Disadvantages of bitmap files:
• Can’t enlarge much without drastically reducing quality
• Quality restricted by original creator (camera, settings on paint programme etc)
• Jagged edges (pixilating) if over-enlarged
Mainly used for: Photographs, complex illustrations
Vector files are mathematical instructions of how the end product should be created. These instructions remain the same whatever the final size of the graphic, so vector files can be enlarged with no loss of quality, and no jagged edges to any size.
Vector images have to be relatively simpler than bitmap images – and although very complex vector documents can be created, they take enormous amounts of processing power when outputting for print (and are unlikely to print from a regular office printer).
Their most common use is for logos which can be scaled up and down with no loss of quality. Logos rarely work well as bitmap files, and a professionally designed logo will almost invariably be in vector format.
This is an example of a large and small logo in a vector format.
This is an example of a large and small logo in a bitmap format.
Examples of programmes which create vector images: Adobe Illustrator, CorelDRAW!, Macromedia Freehand, and Macromedia Flash
Examples of file extensions used on vector files: .eps (though .eps can also be a bitmap file – eg Photoshop eps) .wmf and .svg as well as programme-specific extensions .ai (Adobe Illustrator), .cdr (CorelDRAW), and .fla/.swf (Macromedia Flash)
Advantages of vector files: Scalable to any size without loss of quality
Disadvantages of vector files: Can’t get the subtleties of colour and shading needed to show photographic imeages
Mainly used for: Logos, maps, flat colour illustration
Converting between formats: You can change a vector file to a bitmap (rasterise) but of course you are then restricted for resizing just as with any other bitmap image. Having done this, you can’t revert, so designers generally keep the original vector file if they are converting to a bitmap for the final job.
You can import a bitmap image into a vector file as an embedded object but it remains a bitmap – to transform into vector you have to trace over it and remake it as a new vector image. This can be time consuming and sometimes simply not possible!
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