Definition: Raster Image (bitmap image)

Raster images | Glossary entry

Raster image (also called a bitmap image)

A picture created in a camera comes from an array of tiny sensors (photosites) on the digital image sensor. When the sensor data from all the photosites are translated into an image on your screen, each photosite becomes an individual pixel in the image.

Each of the tiny pixels in a raster image are too small to see individually. Together the pixels merge before your eyes into one coherent whole. The full raster image is the composite of all the pixels put together as the image showing on your screen.

Raster image formats for storage

A raster image, also known as a bitmap, can be stored in a variety of formats. In most cases the images are picture formats like photographs. They may be found as *.jpg, *.gif or *.PNG; or any of a wide range of other formats. These other formats include RAW formats.

The *.jpg, *.gif and *.PNG; formats are created by a process that compresses the amount of data found in the file. This is to enable a faster transfer of the data for use over computer networks and the World Wide Web (WWW). This compression leaves only the data that describes the actual colour of each pixel. This is in contrast to the RAW images where all the data is in the file. While the latter are easier to edit the files are too large to use on the WWW.

Compression reduces the file size and makes file transmission faster. It also affects the quality. The more a file is compressed the more the quality deteriorates. See also: Important File formats – JPG.

Measurement of a Raster Image

Typically a raster image is measured by the number of pixels wide by the number of pixels high. However, every pixel also has a colour. An accompanying number indicates the the Colour Depth. Colours in a raster image are themselves represented in a number of ways. In photography colours are represented by a colour model. Common colour models include RGB, CMYK and some proprietary ones branded by companies. These models describe each of the colours, how they graduate between colours, and how many colours are included in the model.

Raster image resolution

The raster image cannot change its scale. Attempts to change the size of the image will cause apparent damage to the visible quality of the image.

Some image editors have some good tricks available to mimic increase in size. However, these rely on putting new near-colour pixels around existing ones. As the image gets bigger this does not fool the eye. A computer cannot add new data to make an image look real. So as the size increases the image begins to blotch and form artifacts in the image that are not seen in the real world.

A raster image can be reduced in size with less apparent damage. The eyes are better at seeing things that are smaller and recognizing them even with reduced detail. Reductions in size of a raster image involve dumping of the pixels between colours in a way that tends to preserve the colour gradient. With a little sharpening to preserve edges, the image can be reduced by large percentages before the quality is badly affected. To what extent the damage affects the viability of the raster image depends on the image itself and the degree of size reduction. You have to experiment with different images to see how they work in reduction.

The resolution of a raster image is also dependent on the hardware it is displayed on or the printer that prints the image. Modern high resolution screens can display raster images in very high quality because the individual pixels are smaller and closer together. This makes the images look much sharper and smooth edged. Likewise, modern printers can print at high resolution too. The quality of some printers allows printing resolutions into the thousands of Dots per Inch.

It should be remembered that in printing, the resolution of an individual dot is not the same as an individual pixel. To get the colour right in a colour dot on a page, three (or more) ink colours are used. Together these represent the colour of one pixel. Modern printers have much higher resolutions to match the individual pixels in the raster image. A printer matching 200 pixels per inch (PPI) screen resolution my have to have a resolution exceeding 1000 dots per inch (DPI) to represent it on the printed page.

Editing raster images

Image editors can make changes to raster images by changing the character of individual pixels. A white window on a screen is actually an array of white pixels. To paint a line the paint brush tool sweeps the new colour across the pixels it passes over. The substitution from white to the new colour (on the brush) viewed as a brush stroke of the chosen colour. The tools used in the editor all have a similar effect on the pixels. However, the individual tool has its own ways for that tool to create the shape and colour of the expression of the artists stroke.

Raster images vs. Vector images

Raster images are composed of an array of pixels. This contrasts with vector images. The latter use mathematical equations to define the way pixels are coloured and where they are placed on the screen. Each individual brush stoke, or drawn line or shape has a mathematical model to describe its characteristics. These individual components can all be redefined mathematically. In this way components can be resized, shaped or otherwise transformed without damage or loss of quality. Vector graphics are not produced in a camera.

Raster images and vector images in the same editor

The different file formats are normally separated. However, like word processors and spreadsheets, they can be partially mixed. Sophisticated image editors are able to create “raster” layers for raster images and likewise for vector images. It is possible to do this by creating ‘layers’ which do not mix the two types of graphical object. To mix or merge them they will first have to be converted to the same type. Thus, in an image editor that is of raster image type, the vector image objects will have to be “rasterized” to create the final file.

Comparison – raster image format and vector image format

For a comparison between the two types of file view this table.
More detailed information on Vector images.

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Damon Guy - Netkonnexion

Damon Guy (Netkonnexion)

Damon is a writer-photographer and editor of this site. He has also run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photogs.
See also: Editors ‘Bio’.
By Damon Guy see his profile on Google+.

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