Computer Graphics: Pixels, Resolution, Aspect Ratio

Essential Concepts in Computer Graphics - Lecture 2

Pixel

• A pixel, or picture element, represents the smallest unit of a digital image or graphic.
• Pixels can be displayed and represented on a digital display device.

Resolution

• Display resolution, or display modes, refers to the number of distinct pixels in each dimension that a digital television, monitor, or display device can show.
• Resolution is calculated by multiplying width by height, for example, 1024 x 768.
• Common resolutions include:
  • 720p (HD, Standard HD): 1280x720
  • 1080p (Full HD, FHD): 1920x720
  • 1440p (2K, QHD): 2560x1440
  • 4K (Ultra HD, UHD): 3840x2160
  • 8K (UHD): 7680x4320

Aspect Ratio

• Aspect ratio refers to the correlation between width and height of a display.
• Monitor aspect ratios such as 4:3, 16:9, and 21:9 are commonly encountered.

DPI/PPI

• DPI (dots per inch) is the number of dots found within a one-inch line of a scan or print.
• PPI (pixels per inch) signifies the number of pixels within a one-inch line of a digital screen.
• Both DPI and PPI define the resolution, or clarity, of an image.

Frames

• A frame is a rigid structure surrounding something like a picture, door, or windowpane.
• Frames are rectangular areas designed for inserting graphics and text, enabling users to place objects anywhere on a page.
• In video and animation, frames are individual pictures in a sequence of images.

Saturation

• In photography, saturation describes the depth or intensity of color present within an image, and is referred to as chroma.
• Images appear more vibrant with increased saturation, and more subdued with less.

Contrast

• Contrast describes tones, specifically, the relationship between the darkest and brightest parts of an image.
• The shadows are very dark and the highlights are very bright in an image with high contrast.

Rasterization

• Rasterization transforms an image described in vector graphics format into a set of pixels for output on a screen.

Aliasing

• Aliasing is the process in which smooth curves and other lines become jagged because the resolution of the graphics device or file is not high enough to represent a smooth curve.
• The aliasing effect can be reduced by adjusting the intensities of the pixels along the line, and by increasing the resolution of screen

Foreshortening

• Foreshortening is a technique of representing an object or human body in a picture to produce an illusion of projection or extension in space.

Flickering

• Flicker is the appearance of flashing or unsteadiness in an image on a display screen.
• Flickering can occur when the video refresh rate is too low

Halftone

• Halftone is a reprographic technique simulating continuous-tone imagery through the use of dots, varied in size or spacing.

Dithering

• Dithering is a trick graphic applications use to fool your eye into seeing more colors (or grey tones) on the screen than are really there.
• Computers achieve this optical illusion by mixing different colored pixels to trick the eye into thinking that a totally new color exists.

Plotter

• A plotter is a graphics printer, used for printing vector graphics that draws images with ink pens.
• Instead of toner, plotters use a pen, pencil, marker, or another writing tool to draw multiple, continuous lines onto paper.

Display Device

• A display device a term used to describe the device used to view video, images, or text; also referred to as desktop display.
• A display device is an output device for the presentation of information in visual or tactile form, such as a TV monitor, mobile screen, or desktop monitor.

Video Display Devices

• Video display devices include:
  • Cathode-ray tube (CRT) Monitor
    • Raster-Scan Displays
    • Random-Scan Displays
  • Color CRT Monitors
  • Flat-Panel Displays
  • Liquid crystal display (LCD)
  • Light Emitting Diode (LED)
  • Direct View Storage Tubes (DVST)
  • Plasma Display
  • 3D Display

Cathode-Ray Tube (CRT) Monitors

• CRT monitors are primary output devices; the standard design of video monitors.
• Refresh CRT
  • A beam of electrons hit a phosphor-coated screen, with light emitted by the phosphor.
  • The electron beam is directed to the same screen repeatedly, keeping the phosphor activated.
  • The frequency at which a picture is redrawn is the refresh rate.
  • The maximum number of points is the "resolution".

CRT Display Principles

• Raster-Scan Displays are based on TV technology.
  • An electron beam sweeps across the screen, one row at a time, from top to bottom.
  • Each row is a scan line.
  • Screen point is referred to as a pixel.
  • Picture description is stored in the Refresh buffer, or Frame Buffer.
  • Description is stored in the memory area called as Refresh buffer, or Frame Buffer.
  • The number of bits per pixel in the frame buffer, called depth or bit planes. Includes:
    • Buffer with 1 bit per pixel - Bitmap
    • Buffer with multiple bits per pixel - Pixmap

Frame Buffer

• A frame buffer is characterized by size, x, y, and pixel depth.
• Resolution of a frame buffer is the number of pixels in the display (e.g., 1024x1024).
• Bit Planes or Bit Depth is the number of bits corresponding to each pixel, determining the color resolution of the buffer.
  • Bilevel or monochrome displays have 1 bit/pixel.
  • 8bits/pixel -> 256 simultaneous colors
  • 24bits/pixel -> 16 million simultaneous colors

Specifying Color

• Direct color: each pixel directly specifies a color value.
  • e.g., 24bit: 8bits(R) + 8bits(G) + 8 bits(B)
• Palette-based color: indirect specification, uses palette (CLUT)
  • eg., 8 bits pixel can represent 256 colors.

Refresh Rates and Bandwidth

• Frames per second (FPS).

Interlaced Scanning

• Scan frame 30 times per second.
• To reduce flicker, divide frame frame into two fields, consisting of the even scan lines and the other of the odd scan lines.
• Even and odd fields are scanned out alternately to produce an interlaced image.

CRT Displays: Pros and Cons

• Advantages:
  • Real image.
  • Many colors can be produced.
  • Dark scenes can be pictured/filled regions/shaded images.
• Disadvantages to raster displays:
  • The discrete of representation, continuous primitives must be "scan-converted".
  • Aliasing or "jaggies" Arises due to sampling error when converting from a continuous to a discrete representation.

Random Scan Displays

• Calligraphic Displays is also called vector, stroke, or line drawing graphics.
  • An electron beam is directed only to the points of the picture to be displayed.
  • Images store as a set of line-drawing commands, referred to as: display list, refresh display file, vector file, or display program.

Raster vs Vector Displays

• Advantages of vector:
  • Very fine detail of line drawings (sometimes curves), whereas raster suffers from jagged edge problem due to pixels (aliasing, quantization errors).
  • Geometry objects (lines) whereas raster only handles pixels.
  • eg. 1000 line plot: vector display computes 2000 endpoints.
  • Raster display computes all pixels on each line
• Advantages of raster:
  • Cheaper.
  • Colours, textures, realism.
  • Unlimited complexity of picture: whatever is put in refresh buffer, whereas vector complexity is limited by refresh rate.

Color CRT Monitors

• Color CRT Monitors use a combination of phosphors that emit different-colored light.
• Beam-penetration monitors:
  • Used in random-scan monitors.
  • Use red and green phosphors layers.
  • The color depends on the penetrated length of electrons.
• Shadow mask monitors:
  • Used in raster-scan systems.
  • Produce wide range of color with RGB color model.
• Color CRTs are much more complicated, require precise geometry, and use a pattern of color phosphors on the screen: -Delta electron gun arrangement -In-line electron gun arrangement

CRT Color Monitors: Pros and Cons

• Advantages:
  • A wider range picture.
  • Display realistic images.
  • In-line arrangement of RGB color.
• Disadvantages:
  • Difficult to cover all three beams on the same hole.
  • Poor Resolution.

Flat-Panel Displays

• Flat panel displays reduce volume, weight, and power requirement compared with CRT and are divided into two categories:
  • Emissive Displays: Convert electrical energy to light energy (e.g., Plasma panels).
  • Non-emissive Displays: Use optical effects to convert light from other sources into graphics patterns (e.g., LCD monitors).

Plasma Panel Display

• Plasma panels (gas-discharge display)
  • The region between two glass plates when filled with a mixture of gases
  • A refresh buffer is used to store picture description
  • Firing voltages applied to refresh the pixel positions

Liquid-Crystal Displays

• Liquid-crystal displays (LCD) are commonly used in small systems.
  • Liquid crystal compounds have a crystalline arrangement of molecules, flowing like a liquid.
  • Passive-matrix LCD To control light twisting, the voltage applied to intersecting conductors to align the molecules.
  • Active-matrix LCD Using thin-film transistor technology, place a transistor at each pixel location.

3D Display Advantages

• It is also called stereoscope display technology is capable of bringing depth perception to the viewer, and used for 3D gaming and 3D TVs.
• The Fog Display, Holographic Display, and Retina Display are examples of 3D Displays.
• Advantages:
  • Impressive Picture Quality
• Disadvantage:
  • Expensive
  • Binocular Fusion

Video Output Devices

• Includes:
  • Desktop
  • Vector display
  • CRT
  • LCD flatpanel
  • Plasma
  • Workstation displays (Sun Lab)
  • PC and Mac laptops
  • Tablet computers
  • Wacom's display tablet
  • Digital Micromirror Devices (projectors
  • Field Emission Devices (FEDs)
  • Organic Light-Emitting Diode (OLED)

Immersive Video Output Devices

• Include:
  • Head-mounted displays (HMD)
  • Stereo shutter glasses
  • Virtual Retinal Display (VRD)
  • CAVE™

Traditional Input Devices

• Include:
  • Standard mouse
  • Tracker ball mouse
  • Keyboards
  • Pen-based devices
    • Touch screen
    • Tablet computers
  • Palm-top devices
  • Joysticks
    • game pads
    • flightsticks
  • Touchscreens
• Microphones
  • Wireless vs. wired
  • Headset
• Digital still and video cameras & scanners
• MIDI devices
  • input from electronic musical instruments for entering scores

3D Input Devices

• include:
  • Electromagnetic trackers, which can be attached to any head, hands, joints, objects.
    • Polhemus FASTRAKT™ (used in Brown's Cave)
  • Gloves:
    • Attach electromagnetic tracker to the hand
    • Pinch gloves contact between digits is a "pinch" gesture.
      • CAVE, extended Fakespace PINCH™ gloves with extra contacts

Display Processor

• The display processor digitizes picture definitions given in an application program into a set of pixel intensity values for storage in the frame buffer (Scan conversion).

Graphics Software

• Graphics software refers to a program or collection of programs that enable a person to manipulate images or models visually on a computer.

Categories of Graphics Software

• Special purpose: is designed for non-programmers and communicates with a set of menus.
• General programming: which is for programmers, and provides a library of graphics functions used in programming language.

Graphic Standards

• Graphic Standards refer to agreed specifications which define the common interfaces between computer systems or subsystems.