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Raster image

Photoshop is used to edit and create digital images. It is the standard in digital image editing in the Graphics Industry. Advertisement agencies, digital artist and almost all industries who work with digital images us the tools to enhance, alter or combine digital images.



Photoshop is used to edit Raster images. These are images who consist of a rectangular grid of squares, called pixels, where every pixel contains one color. The pixels are organized in an image in a rectangular grid or Raster, hence the Raster Image.

It is important to familiarize yourself with some important terms that are involved in working with Photoshop.

Pixels vs. vectors

Images can be divided in two main categories: raster images and vector images. Raster images are also referred to as bitmap or pixel images. This pages discusses both types as well as their differences.


Example of a bitmap raster image

Pixel or raster images are the most commonly used type of images. Digital photos are raster images, for example. Generally speaking, they consist of pixels (squares) with a certain color value.

Often pixel images are referred to as raster images because an image consists of a raster or grid of pixels. Because raster images are based on pixels they have certain disadvantages. The main disadvantages becomes apparent when scaling or zooming-in to a part of the image. The raster becomes apparent and the image will no longer have sufficient detail for smooth shapes. This makes it impossible to scale a raster image without loss of quality.

Raster images can be edited with software such as Photoshop, GIMP, Paint and Paint Shop Pro.

Photoshop primarily works with raster images and has limited support for vector graphics.

Most used extension for bitmap images are:

  • JPG or JPEG
  • PNG
  • GIF
  • BMP
  • TIF or TIFF
  • TGA or TARGA
  • PSD (standard Photoshop extension)

Applications for creating and editing bitmap or raster images are:

  • Adobe Photoshop
  • GIMP
  • Micosoft Paint
  • Jasc Paint Shop Pro

There are several methods available for saving files, all using different ways of compression in order to limit the file size. These methods can be divided in two categories: lossy and non-lossy. Each file format has it's own way to compress. JPEG only uses lossy-compression whereas TIFF rarely or never uses lossy-compression.


Example of a vector drawing

Vector images consist of lines and shapes that are mathematically describes. Therefore vector graphics are always sharp, no matter how far you zoom in; the precise shape is calculated.

Vector graphics don't have a 'resolution', which is used as an indicator for quality in raster images, which means they can be scaled to any size and printed at any resolution without losing detail or clarity.

Vector graphics are well-suited for line drawings, schemes, graphs and such. Since vector objects can only depict a single color or simple gradients, they are not suited for depicting photographs or other images with subtle, irregular and multi directional color differences.

The example at the right shows a typical drawing made in Illustrator, where the sharpness and precision of the program result in better control over the lines and drawing as a whole.

The comparison

Comparison between raster and vector images

This example at the right shows a raster (pixel) version of a filled circle (left) and a vector version (right), which illustrates the difference between raster and vector images. A part of the edge is magnified. When magnifying, the saw-tooth pattern is clearly visible. Hence there evidently is a detail limit for bitmaps. When zooming in on a vector object, the shape can still be accurately calculated due to the mathematical equations.

Because of the elemental difference between pixel and vector images, the one is better suitable for certain types of images then the other. This becomes clear when comparing a pixel image photo to a traced (vector) photo. It is evident that the vector photo has a lack of detail and shade gradients. Therefore vectors are more suitable for line-art images, whilst pixels are more suitable for images that require subtle details in colors (photo's, hand-drawings a.o.).

Converting vector to bitmap

The process is which a vector file is converted to a bitmap is called rasterizing. One could see it as super imposing a grid on a vector object and then determining the color value. Especially sloped and curved lines tend to show a sawtooth pattern. A method to counter this effect is called anti-aliasing. Examples are importing a pdf file in Photoshop and also before printing vectors are rasterized.

Colour mode – RGB or CMYK

Each pixel can contain one color. The color range is depending on the Mode of the image. The Mode defines the base colors which are used to generate all the colors in the color range by mixing them together. There are two main Modes used for defining colored images. This is the RGB and the CMYK image.

Red Green Blue – Screen displays are based on mixing red, green and blue light for generating colors. Hence, the RGB mode is standard for digital photographs. The images using this combination of colors are called RGB images.
Cyan Magenta Yellow Key – This standard is used for digital images that will be printed as printers use cyan, magenta, yellow and black (key) to mix colors while printing. The additional black makes it possible to print real black instead of mixing it. These images are called CMYK images.

The Mode of the image can have an effect on the available tools that can be used to edit the image within Photoshop. If some tool does not work while it should, check the Mode of the image.

Interface cd.jpg


A raster image is a grid of pixels, each containing one color, describing the image. It consists of o rectangular row of pixels in a horizontal and vertical direction. The resolution of an image is defined in the amount of pixels used to define the image. For example a 640 by 480 pixel image will have 640 pixels horizontally and 480 pixels vertically. A higher resolution than the 640-480 image will mean that there are more pixels horizontally and vertically.

If there are more pixels in an image they are more accurately describing the content of the image. So the higher the resolution and thereby the amount of pixels used the more accurately the image is defined. Although this is obvious it is often forgotten when images are printed or used as textures in Maya. A printout of a low resolution will become blurred as there is not enough information in the image.

Interface ca.jpg

The resolution of an image is often defined by the amount op pixels horizontally and vertically like 640x480

However there is also another definition of resolution – DPI or Dots Per Inch . The difference is that this definition is related to print size. This definition describes the amount of pixels used per inch of printed image. Keep in mind this is an indirect description of resolution because the total amount of pixels depends on a COMBINATION of Dots Per Inch and the size of the print itself. So DPI alone won’t tell you anything about the total size in pixels of the image.

DPI is used when the images are used for printing in combination of the actual print size in inches. It's used because it will give you an idea if the resolution is high enough and thereby the quality of the print for printing independent of the print size. For print work a resolution of 300 DPI is used for example. They use 300 DPI because the eye won’t be able to distinguish the separate pixels in such an high resolution.

For large prints like an A1, 150 DPI will suffice because the viewer will look at the image from a larger distance. Large billboards can even have a resolution of only 25 DPI or less depending on the viewing distance. The whole idea behind the DPI description is to define a print resolution where the pixels can’t be distinguished as separate pixels by the viewer. Making a 300 DPI print on an A1 format is a waste of effort because the viewer will observe the print from a distance. A 300 DPI image in book makes more sense as the image is viewed close up. The DPI description is always an indirect description of the actual total resolution of an image.

In programs like Maya we will always work with the traditional resolution description of for example 640x480. If an image has to be rendered for an A1 on a resolution of 150 DPI we calculate the total resolution of the image in horizontal en vertical pixels. That resolution will be rendered in Maya.


Adobe Troubleshooting covers common problems or pitfalls you may encounter in Adbobe Photoshop and Illustrator.

Further Reading

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