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What is a Raster?

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  • Written By: Malcolm Tatum
  • Edited By: Bronwyn Harris
  • Last Modified Date: 23 March 2014
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    Conjecture Corporation
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Rasters are the sections of cathode ray tubes and liquid crystal display monitors that make it possible to create images for display. Essentially, a raster functions by creating a series of horizontal lines that run across the viewing area. An electron beam scans the lines using a dual scanning pattern that runs the beams left to right and up and down over the horizontal lines. This continuous action results in the ability to view a complete image.

While the basic function of a raster is the same with just about any type of cathode ray tube or CRT display, there are some differences in the process used by different devices. Televisions that may use of the CRT model often will not have quite as sharp an image display as a computer monitor using the same basic approach. The reason for this is that the setup of the CRT on a television set is normally referred to as an interlaced raster pattern, while the computer monitor is likely to use a raster scanning approach that is considered to be non-interlaced.

When a liquid crystal display or LCD resolution is used with a device, the scanning process may also be somewhat different. Unlike a CRT raster function, the LCD process of rendering images involves displaying elements of the image individually. In fact, the raster process for LCD is usually referred to as a grid rather than as a raster.

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In both situations, the scanning activity of the raster is what makes it possible to view moving images. Over the years, technology has refined the efficiency of the scanning even as the basic creation of the raster sequence of horizontal lines has remained more or less the same. The end result is that images are displayed with greater clarity than in times past. This enhanced clarity has been particularly important as desktop computers have become more commonplace in the home. The enhanced raster resolution has also made such devices as laptops and the displays on handheld devices possible.

One outside factor that will impact the resolution of the raster is the quality of the signal received for display. This is the reason why a television set that contained state of the art resolution equipment will not perform as well with a broadcast signal picked up by an antenna. A secure transmission, such as via cable or satellite, provides a stronger signal that is translated with more clarity than a signal picked up by rabbit ears or an old fashioned exterior antenna.

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Discuss this Article

SkyWhisperer
Post 4

@NathanG - I’d like to point out that the raster image format, with all its pluses and minuses, is not the only kid on the block in this regard.

The new thing these days is the vector format. Unlike the raster image, which stores everything as pixels, vectors use mathematical formulas to compute the lines and curves; the upshot is that the final image is completely scalable.

You can even do a raster conversion where you convert from raster to vector and vice-versa.

I did this once and the results were amazing.

I got a GIF animated raster image, and used a vector image program to convert it from raster to vector format.

I was then able to scale my small GIF animation to where it filled the whole screen, with no loss of resolution! Talk about an easy way to do an animation.

miriam98
Post 3

@David09 - You’re not kidding. My friend has a still camera with something on the order of 12 megapixel resolution for his images. He is able to create posters with these images, all with rich color and stunning clarity.

What I like also about the raster format is that, regardless of the resolution, raster data is easy to compress. The reason is that all of the pixels are essentially just numbers. A compression program looks for patterns of repetition among these numbers and squeezes the compressed file down to smaller size.

David09
Post 2

@NathanG - That’s true; the raster grid doesn’t look as good when you expand it. I’ve noticed that myself when I play around with some image editing programs and try to scale up an image. For that reason I just stay with the default resolution of the image.

In the early days of digital camcorders with still camera capability, they took pictures in low resolution. These pictures were okay on a computer monitor, but if you printed them out you could only print out a small picture because they couldn’t scale up to fill a 4 by 6 or 5 by 7 photo print.

Nowadays of course there are still cameras that have high resolution raster graphic capability; you can print at much higher resolution.

NathanG
Post 1

I’ve worked a lot with raster graphics, and like the article says, the images are split up into grids of pixels.

As a matter of fact, if you open up a raster graphic image in a paint program you can zoom in and see the individual pixels. At the zoomed in level, it becomes possible to individually manipulate the image one pixel at a time (if you want) using the paint bucket and other tools.

One disadvantage of raster images however is that they don’t scale very well. For instance, if you take an image that is 320 by 240 resolution and you attempt to scale it so that its dimensions have doubled, you will notice some pixelation.

The image will lose its clarity. You can see this in some of the home movies that are shot in this resolution and which are stretched to fit your TV monitor. There is some loss of resolution.

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