Tuesday, December 18, 2007

Raster graphics

While there are alternative ways to display images, the predominant displays (televisions and computer monitors) are what as known as raster devices. This means that the display is broken into a number of horizontal lines. Each horizontal line can then be broken into discrete chunks which are known as picture elements or pixels for short. While we know that the best resolution in HD televisions is 1080P which means there are 1080 horizontal lines with each horizontal line having 1920 pixels. For the B8VGC project, we are going to have to assume that non-HD televisions will be the display device.

Being in Canada, television broadcasts are in the NTSC (National Television Standards Committee) format. This format has 525 scan lines which are refreshed at 30 frames per second. Television broadcasts are interleaved, meaning that each of the 30 frames are broken into two fields. The odd lines are drawn first, then the even lines. This is what is known as interlacing. Not all the scan lines are visible, as some are used for the vertical blanking interval so the real visible resolution is roughly 480 lines. NTSC isn't the only format, there is also PAL (Phase Alternating Line) and SECAM which has 625 lines 576 visible at 25 fps.

The horizontal resolution of the display is a bit different beast. Because television signals are analog in nature, there is no absolute resolution. Instead, the number of pixels on a scan line is determined by the timing of the signal. DVD uses 720 pixel scan lines. These, however are not square pixels. If you want square pixels, then you will want 640 pixel scan lines. Programmers prefer square pixels so we will go with a television resolution of 640x480. That, unfortunately, will not be the resolution that the console will be using due to memory issues.

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