02-digital Image(raster And Vector Image)

Digital Image (2D Computer Graphics) -Multimedia Design Technologies2nd Lecture

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Contents 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

What is a digital Image? Type of digital images Bitmap image Bitmap and Resolution Bitmap and Image size Resolution for Publishing Photos Type of bitmap images Image representations Color Depth Data volumes 2

What is a Digital Image? A digital image is a representation of a twodimensional image as a finite set of digital values, called picture elements or pixels

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What is a Digital Image? (cont…) Pixel values typically represent gray levels, colors, opacities (transparent levels), etc Remember digitization implies that a digital image is an approximation of a real scene 1 pixel

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Type of digital images z Raster/Bitmap Image • pixels in a grid • resolution dependent • resizing reduces quality • easily converted • restricted to rectangle • minimal support for transparency Original Object

A Bitmap Image

z Vector Image { Vector images describe the image in the term of coordinates and mathematical transformations.

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Bitmap/Raster Image z Common bitmap-based formats are JPEG/JPG, GIF, TIFF, PNG, PICT (Macintosh), PSD (Adobe Photoshop) ,and BMP. z Popular bitmap image editing programs are 1 pixel { Microsoft Paint { Microsoft Photo Editor { Adobe Photoshop { Corel Photo-Paint { Corel Paint Shop Pro , etc z Bitmap-based images are comprised of pixels in a grid. Each pixel or bit in the image contains information about the color to be displayed. z Bitmap images have a fixed resolution and cannot be resized without losing image quality. z Most bitmap images can be converted to other bitmap-based formats very easily. z Bitmap images tend to have much large file sizes than vector graphics and they are often compressed to reduce their size. 6

Bitmap Image and Resolution z Bitmap images are resolution dependent. z Resolution is a measurement of the output quality of an image, usually in terms of samples, pixels, dots, or lines per inch. z The terminology varies according to the intended output device. PPI (pixels per inch) refers to screen resolution, DPI (dots per inch) refers to print resolution, and SPI (samples per inch) refers to scanning resolution. z Resolution

pixels PPI = inches

Exercise: Calculate resolution (PPI) of your monitor! (in the case of IPS room’s monitors and its default setting, size 12x9 inches (monitor type is 15 inches), 1024x768 pixels)

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Bitmap Image and Resolution (cont…) z Bitmap images are referred to as high resolution (hi-res) or low resolution (low-res). z High resolution would be an image intended for print, generally having 300 ppi or more. z Low resolution refers to images only intended for screen display, generally having 100 ppi or less. z Bitmap images are displayed on your computer screen at screen resolution: approximately 100 ppi. z When printing bitmaps, your printer needs much more image data than a monitor. In order to render a bitmap image accurately, the typical desktop printer needs 150-300 ppi. 8

Bitmap Image and Image Size z When you reduce the size of a bitmap image through your software's resize command, you must throw away pixels. z When you increase the size of a bitmap image through your software's resize command, the software has to create new pixels. z When creating pixels, the software must estimate the color values of the new pixels based on the surrounding pixels. This process is called interpolation.

increase the size

reduce the size

100x75 pixels

Original image 200x150 pixels

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800x600 pixels

How to Calculate Resolution for Publishing Photos z A real problem: I want to print a photo. And, I need it to be a high resolution photo (300 PPI or more) in 5x8 inches. The photo I have is a 702KB, 1538 x 2048 jpeg. Can I print the photos in the high resolution? z The photo is 1538 x 2048 and I need a print size of 5x8 inches… The math you need is: { pixels/inch=PPI { 1538/5=307 PPI { 2048/8=256 PPI

z That means that 256 is the maximum PPI you can get from this image to print the longest side at 8 inches without letting your software add new pixels. z Please, Calculate minimum pixel sizes that I need to print 5x8 inches a high resolution photo! z The math you need is: inchxPPI=pixels z 5x300 = 1500 z 8x300 = 2400 ∴The minimum pixels that I need is 1500x2400 Exercise: How many number of pixels do you need to print a high resolution photo on a postcard(148x200 mm)

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Type of Bitmap Image Common image formats include: {1 sample per point (B&W or Grayscale) {3 samples per point (Red, Green, and Blue) {4 samples per point (Red, Green, Blue, and “Alpha”)

Black &White

Grayscale

color (RGB)

11 Color (RGBA)

Black and White Image （Zoom In）

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Grayscale Image（Zoom In）

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Color Image（Zoom In）

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Bitmap Image & Color Depth A Pixel Æ Smallest unit of the digital image elements A Color Depth is number of a color intensity A levels that can be displayed on the screen

pixel's data B&W Image ：White or Black （0 or 1） Grayscale

：Black～White （0～[ N ]）

Color Image;

（Red ：0～[ N]） （Green：0～[ N]） （Blue ：0～[ N]）

a pixel's intensity level data (Color

Depth)

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Color Depth (Grayscale Image) a pixel in a grayscale image has a color information indicate a level of grayscale color from white to black

：White--Black （0 ～ [ N ]）

Grayscale

0

1

2

3

4

5

6 ….N

a pixel's color intensity level data (Color Depth) 16

Color Depth (RGB Color Image) a pixel in a RGB color image has three differences color information; Color intensity level of Red, Green and Blue. Red : white ….Red

1 pixel

Green : white ….Green

Blue : white ….Blue

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Color Depth (RGB Color Image) Color Information is represented by the levels of Red, Green and Blue.

緑

青

赤 18

Bitmap Image Representation (cont…) Digital images are represented by function f(i,j)

i j J

I

The value of a pixel is represented by a set values of function f(I,J) f(i,j)=0,1

B&W Image

f(i,j)=0,1, … , 255

Grayscale

f(i,j)=(r,g,b)

Color Image

r,g,b=0,1,…,255 19

Smallest Unit of memory Recordable data OFF=0 Or ON= 1 a bit

8 bits = a byte Recordable data in the 8bits= a byte of memory 00000000 00000001 00000010 00000011 00000100 00000101 00000110 00000111

00001000 00001001 00001010 00001011 00001100 00001101 00001110 00001111

………

11111000 11111001 11111010 11111011 11111100 11111101 11111110 11111111

= 256 difference data 20

Image Representation (Cont…) f(i,j)=0,1

A pixel’s color information on a memory buffer (8bits) B&W Image 0, 1

f(i,j)=0,1, … , 255

Grayscale

00000000 00000001 00000010 11111110 11111111

f(i,j)=(r,g,b) r,g,b=0,1,…,255

Color Image

00000000 00000001 00000010

00000000 00000000 00000001 00000001 00000010 00000010

11111110 11111111

11111110 11111110 11111111 11111111 21

Number of bits and Color Depth z Color depth is defined by the total number of bits per pixel that can be displayed on the computer screen. Color Depth = 2 ^ Number of bits

z Data is stored in bits. Each bit represents two colors because it has a value of 0 or 1. z The more bits per pixel, the more color that can be displayed. z z z z

For example: Each pixel in a 4-bit image can display one of the possible 16 colors, Each pixel in a 8-bit image can display one of the possible 256 colors, Each pixel in a 16-bit image can display one of the possible 65,535 colors, z Each Pixel in a 24-bit image can display one of 16,777,216 color (over 16-million colors). 22

Data volumes A Data volume is mainly effected by number of pixels, type of image, color dept and others file information. z z z z z

Examples: Black & White Image size: 512x512 pixels/image Results in 512x512=262,144bit/Image≒262kbit/image = 32,768 byte≒32.8kByte

z z z z z

Grayscale Image size: 512x512 pixels/image Color Depth: 256 colors= 8bits/pixel Results in 512x512x8=2,097,152bit/Image≒2.1Mbit/image =262,144Byte≒262.1kByte

z z z z z

RGB Color Image size: 512x512 pixels/image Color Depth: 256 colors = 8bits/color/pixel Results in 512x512x8x3=6,291,456bit/Image≒6,3Mbit/image =786,432 Byte≒786.4kByte 23

Exercise: Please, calculate data volumes of the following Bitmap Images! 1) Black & White Image size: 400x300 pixels/image 2) Grayscale Image size: 400x300 pixels/image Color Depth: 16bits/pixel =65536 colors 3) RGB Color Image size: 400x300pixels/image Color Depth: 24bits/color/pixel =16777216 (difference level/color) 4) How many number of pixels do you need to print a high resolution photo/poster on A3 paper size (297x420 mm) Note: 1 inch=2.54 cm

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