Graphics

F.4 CIT – Graphics

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Introduction The origination of computer graphics can be in digital or analogue form. We can use computer software to draw a picture in the computer. This picture is already in digital format. But very often, the images come from the real world. In such case, the brilliant pictures that we see are composed of different colours which are actually light waves of different frequencies. Light wave is analogue, so we have to digitize it so as to be processed by the computer.

Figure 1. Visible light spectrum

As the digital image is formed, it is very often shown on the screen in the form of an 2D array of pixels. Pixel is the smallest logical unit of a single colour on a computer screen of graphical image. Digitization of Light 1. Sampling Usually, we uses digital camera to catch a moment and scanner to scan a picture or hand-written document or even a 3D object! In many digital cameras and most of the scanners, there are CCD (Charge-Coupled Device) arrays inside. CCD converts light signal to electrical signal for digitization. The density of the CCDs determines the quality of the output image signal, and is highly related to the device resolution. Device resolution of scanner is the measure of the number of dots (i.e. pixels on screen) can be shown in a unit length and is measured in dpi (dots per inch). Since the electrical signal from the CCD resembles the sample of the original image, the device resolution can be used as a measure of the sampling rate. More about: Resolution However, in monitor, the resolution is meant by the number of pixels that can be shown in the horizontal and vertical axes of the monitor. For example, a screen resolution of 800×600 implies that there are 800 pixels on each horizontal line and 600 pixels on each vertical line. The screen resolution can be adjusted manually and there is a direct proportion relationship between the device resolution and the screen resolution. For example, a 17 inch monitor whose actual resolution of 72dpi will result in a screen resolution of about 832×624.

F.4 CIT – Graphics

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Quantization The sample size decides the number of colours that can be recognized. In computer graphics, the sample size is the colour depth (or bit depth). To be more precise and general, color depth is the number of bits used to store the color of one pixel.

Representation of Colours The number of colours that can be used to store an image depends on the colour depth and the colour model used. Common Colour Model: Black and White: It uses 0 and 1 to represent white and black dots. Brightness of colour is represented by density of black and white dots.

Original Image

B&W Figure 2.

B & W using pattern

Black and White Image

Greyscale: It normally uses 8 bits to store the shades of grey from white (255) to black (0).

Original Image

Greyscale Image

Figure 3. Greyscale Image

RGB: It usually uses 24 bits to store the colour of one dot: 8 bits for the red level, 8 bits for the green level and 8 bit for the blue level. The colours are usually stored in three 2D arrays. Each 2D array stores the matrix of 1 primitive colour and is called a channel.

F.4 CIT – Graphics

Original Image

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Red Channel

Blue Channel

Green Channel

Figure 4. The three channels of a colourful image

Sometimes, we use 32 bits for RGB model instead of 24 bits. The extra 8 bits are used for a channel which stores the degree of transparency which is called the alpha channel. CMYK: Colour Depth 1 bit 8 bit 8 bit 16 bit 16 bit 24 bit 32 bit

Colour Model Black and White Greyscale Indexed Colours Greyscale RGB High Colour RGB True Colour RGB True Colour

Common Image File Types

No. of Colours

Remark