What is Rasterization

This is an article which talks about rasterization which is a technology used by all modern computers to display graphical user interface. This article tries to easily explain how a computer draws a line or a 3D image on your screen.

Rasterization is a word full of jargon. A Google search on the internet will show you a thousand or more results; and when you open them, well they can be mystical. You can see the term in many of the image editing software. Sure you do not hear it in the Microsoft Paint, but as everyone knows, MS Paint is not a full image editing software. The software like GIMP, Adobe Photoshop and Paint.NET give you a lot more control over the editing procedure of images. It is in those software that you will see the terms of rasterization. Also, if you are a Gamer and love to play games and have tried to go even a bit deeper in what happens in a game and how do you see all the trees cars and gunfire on screen, you probably have heard of the term 'rasterization'. In this article, I will try to demystify "Rasterization" in as simple terms as possible.

Rasterization is basically the process by which the computer graphics systems come to know about "how to display a particular graphics". While a lot can be said about rasterization, we will keep it simple and try to understand what it means to the computers of today and how is it done. So let us see it in step by step, by taking an example of how the computer displays a "Window" on screen.

1. The computer first gets the command to draw a Window. It then understands the command and checks the settings.
2. Next step is to see what ratio of the screen space will that window cover.
3. Then the computer will calculate the dimensions of that Window.
4. Computer will calculate the lines to be drawn for making the outline of the window.
5. After calculating the length of the lines and their end points, the computer needs to calculate the number of pixels to fill in the line and from where to where in the original screen.
6. The computer repeats the process for all the borders and then again for the elements inside the window and then starts to draw them on screen. The drawing usually takes step by step and the elements are drawn one after another. If your computer has every slowed down too much, you would have noticed that when windows appear on screen, they slowly get filled with one element appearing after another.
7. This is one of the simplest examples of rasterization.

Now let us take another example of creation of a 3D object, such as in a Game:

1. The computer first loads the data that is needed to display the image. This data is usually made of the point coordinates of the lines which make an object.
2. The data is calculated upon and real line segments are created. This is still in virtual space. By virtual space, we mean that, while the screen size may be just in a few inches, the calculation takes place for meters of view in that game.
3. The computer then calculates the area of scene that is to be displayed. This is done by scaling the actual image with that of the actual resolution.
4. The lines are then converted into pixels by using the calculations which the computer got during the scaling step.
5. Colors are filled in according to the graphics data supplied and the image is finally displayed.

A few points about rasterization:

1. It is very heavy or resource intensive computation.
2. It would usually need a graphics card to do rasterization for 3D objects.
3. Windows AERO transparency effect uses quite a fair amount of this technology. This is one of the biggest reasons why Windows Vista and Windows 7 take more resources and need a good graphics controller to enable AERO.

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