Pixels and Digital Images Yrd. Doc. Dr. Ahmet Sayar Kocaeli Universitesi Bilgisayar Muhendisligi Ileri Bilgisayar Grafikleri.

Pixels and Digital Images

Yrd. Doc. Dr. Ahmet SayarKocaeli Universitesi Bilgisayar Muhendisligi

Ileri Bilgisayar Grafikleri

Digital Image

• Any image from a scanner, or from a digital camera, or in a computer, is a digital image

• Converts the real world color picture to the numeric computer data consisting of rows and columns of millions of color samples

Icons• Icons are graphic images

with size of 32x32 pixels.

• This image was not photographed nor scanned, instead it was created by hand in a graphics computer program

• If we blow up the icon image about a dozen times, we see the individual pixels of the image

Pixels• A pixel (or picture element) is the smallest

addressable screen element in raster graphics; each pixel has its own address

• It is the smallest unit of picture that can be controlled• The address of a pixel corresponds to its coordinates

• Digital images are composed of pixels, and that digital images are therefore dimensioned in pixels (not inches).

• In your photo editor program, zoom an image to about 500% size on the screen, and you will see the pixels

Pixels (2)

• Pixels are like colored mosaic tiles:– From a reasonable viewing distance, we do not

notice the individual small tiles, our brain just sees the overall picture represented by them

• The size of the image is dimensioned in pixels, X columns wide and Y rows tall

• We must think of that image as pixels, simply because that is what it is, and how things work.

Pixels

• The measures dots per inch (dpi) and pixels per inch (ppi) are sometimes used interchangeably

• What is resolution?– The more pixels used to represent an image, the closer the

result can resemble the original. The number of pixels in an image is sometimes called the resolution

• What does 3mp-camera say to us? – Pixel counts can be expressed as a single number (such as a 3

million pixels)– Or as a pair of numbers, as in a " 2048 x 1536 display", which

has 2048 pixels from side to side and 1536 from top to bottom– A 3 megapixel camera has a grid of 2048x1536 sensors (3 million

of them)

Pixels (3)• #of distinct colors that can be represented by a pixel depends on

the number of bits per pixel (bpp)– 1bpp?

• A digital color image pixel is just a RGB data value (Red, Green, Blue)

• Each pixel's color sample has three numerical RGB components (Red, Green, Blue) to represent the color

• These three RGB components are three 8-bit numbers for each pixel.

• Three 8-bit bytes (one byte for each of RGB) is called 24 bit color• Each 8 bit RGB component can have 256 possible values, ranging

from 0 to 255• 255 is the maximum possible number that can be stored in an 8 bit

byte. Larger numbers require multiple bytes, for example two bytes (16 bits) can hold up to 256x256 = 65536 unique values

Pixels (4)• 24 bit RGB color images use 3 bytes, and can have 256 shades of

red, and 256 shades of green, and 256 shades of blue– 256x256x256 = 16.7 million possible combinations or colors for 24 bit

RGB color images• In the RGB system, we know Red and Green make Yellow. So, (255,

255, 0)• Black is a RGB value of (0, 0, 0) and White is (255, 255, 255). • How about gray

– (220, 220, 220) is a light gray (near white), and (40,40,40) is a dark gray (near black).

• NOTE: Since gray has equal values in RGB, Black & White grayscale images only use one byte of 8 bit data per pixel instead of three. The byte still holds values 0 to 255, to represent 256 shades of gray

An Image File

• The image file contains three color values for every RGB pixel, or location, in the image grid of rows and columns

• If the image size were say 1000x750 pixels (written as width x height by convention), then there would be 1000 columns and 750 rows of data values, or 1000x750 = 750,000 pixels total.

• For 24 bit color, each pixel's data contains three 8-bit RGB byte values, or 750,000 x 3 = 2,250,000 bytes.

Exercises

Raster Images• are composed of lots of small dots. This lends

itself to photographic or artistic effects because color gradations can be made very smooth in a bitmap format. However, bitmap images lose their clarity when they are increased in size because the dots get bigger.

• Formats– Graphic Interchange Format (.GIF)– Joint Photographers Expert Group (.JPEG, .JPG)– Tagged Image File Format (.TIFF, .TIF)

Vector Images

• This kind of artwork is made up of lines and filled areas only, which are mathematically drawn and calculated (hence the term vector) by the software you use. They can be stretched in size without losing any clarity or sharpness. Because of their mathematical origins, vector images are readily scalable without distortion.

• Formats– Encapsulated Postscript (.EPS)– PostScript (.PS)– Windows Meta File (WMF)

Raterization

Vectorization

Little Square Model

• The little square model pretends to represents a pixel (picture element) as a geometric square

• Thus pixel (i, j) is assumed to correspond to the area of the plane bounded by the square– { (x, y) | i-.5 <= x <= i+.5, j-.5 <= y <= j+.5 }

• There is typically an argument about whether the pixel “center” lies on the integers or the half-integers???