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Saturday
Oct272007

Bitmapped Images

Digital pictures are made up of a series of pixels (or picture elements). The resolution of an image is the number of pixels in the x and y direction. With a digital camera, the quality of the lens is more important than the number of pixels available. A 2 megapixel camera with a really good lens can take better pictures than a 7 megapixel camera with a less good lens.

Bitmapped images are also known as raster images. Apparently raster is a grid square. A second definition for raster is the parallel lines that form the scan pattern on display screens or tvs. They are good at representing real-world images, where there is a comples variation in colours, shades and shapes. (As opposed to vector graphics.)

The size that the image will be displayed at affects the resolution that should be used to store it. So a small image or thumbnail can be stored at a really low resolution and still get across the data. If that same image was displayed at a larger size it would look all rubbish and pixelated. Thsi is the problem with lots of 'show large image' links - where clicking just increases the size to show a nasty grainy image and gives no extra information. Getting this right is a major concern in multimedia presentations, because you want to avoid things looking crap.

Resolution is often specified in terms of Dots Per Inch (DPI). Printers are normally very high resolution at 600-1200 dpi, scanners from 300-3600 dpi, and monitors are 70-204dpi. Interesting that the iPod Nano has the highest definition screen ever, because they need to squeeze more detail and information out of a very small display area. Smaller pixels allows finer detail, and provides a more readable (and vivid) display. One side effect of the difference in resolution between a monitor and a printer is that an image that fills the screen on a 72dpi monitor will look tiny on a 600dpi printer. To avoid this being an issue, most image formats allow you to specify the resolution in pixels per inch.

A 4:3 aspect ratio is normal for PAL TV format. That's the same for a lot of screen resolutions, and standard sizes like 320x240, 640x480, 800x600... 1280x1024 is an exception at 5:4, so circles drawn on a screen at that resolution will not be circles when displayed on a 4:3 screen. Digital TV and widescreen stuff has introduced a whole new range of aspect ratios.

One thing to remember is that a graphics package such as Photoshop is better at resizing than a browser, so it's better to resize photos to the right size and save them that way, rather than using the width and height tags to do it on the fly.

The simplest data model used in images is the true colour image data model. Each pixel has a 24 bit value, containing a RGB value (or the HSV, or YUV - basically three pieces of information).

Alternatively a palette colour model can be used. The palette colour has an 8 bit pixel index, which refers to a colour in a palette. The palette stores the full RGB (or HSV or YUV) value for that colour. Gifs use this system.

True colour vs. palette: True colour gives a very high quality image, but takes 3 times the memory to store. Palette limits the colour range used, but is cheaper storage-wise.  

 

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