Computer Graphics: Pixels, Resolution and Aspect Ratio

Essential Concepts in Computer Graphics - Lecture 2

Pixel

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

Resolution

• Display resolution is the number of distinct pixels in each dimension that can be displayed.
• Resolution is calculated by Width x Height, for example 1024 x 768.
• Common resolutions include:
  • 720p (HD, Standard HD): 1280x720 pixels
  • 1080p (Full HD, FHD): 1920x720 pixels
  • 1440p (2K, QHD): 2560x1440 pixels
  • 4K (Ultra HD, UHD): 3840x2160 pixels
  • 8K (UHD): 7680x4320 pixels

Aspect Ratio

• The aspect ratio describes the correlation between width and height.
• Common monitor aspect ratios include 4:3, 16:9, and 21:9.

DPI/PPI

• DPI (dots per inch) indicates the number of dots within a one-inch line of a scanned image or print.
• PPI (pixels per inch) indicates 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 that surrounds something such as a picture, door, or windowpane.
• Frames are rectangular areas for inserting graphics and text, allowing users to place objects wherever they want on the 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, also referred to as 'chroma'.
• The more saturated the image, the more colorful and vibrant it will appear.
• Less color saturation will make an image appear subdued or muted.

Contrast

• Contrast describes tones, specifically the relationship between the darkest and brightest parts of an image.
• High contrast images have very bright highlights and very dark shadows.

Rasterization

• Rasterization is the technique of taking an image described in a vector graphics format and transforming it into a set of pixels for output on a screen.

Aliasing

• Aliasing is the process by 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 intensities of the pixels along the line, or by increasing the resolution of the screen.

Foreshortening

• Foreshortening is a technique that represents an object or human body in a picture so as 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.
• It can occur when the video refresh rate is too low.

Halftone

• Halftone is a reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or spacing.

Dithering

• Dithering is a trick graphic applications use to simulate more colors or grey tones on the screen than are really there
• It achieves this optical illusion by mixing together different colored pixels to trick the eye into thinking that a totally new color exists.

Plotter

• A plotter is a graphics printer using pens to draw images.
• Plotters draw point-to-point lines directly from vector graphics files.
• Plotter user a pen, pencil, marker, or another writing tool to draw multiple, continuous lines onto paper rather than a series of dots.

Display Device

• A display device is used to view video, images, or text, and also referred to as a desktop display or simply a display.
• It is an output device for the presentation of information in visual or tactile form.
• Examples: TV monitor, mobile screen, desktop monitor.

Video Display Devices

• 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 :

• Primary output device for video.
• Standard video monitor design.

Refresh CRT

• A beam of electrons hits a phosphor-coated screen.
• Light is emitted by phosphor.
• An electron beam directly impacts the screen multiple times, maintaining phosphor activation.
• The frequency at which the picture is redrawn is referred to as the "refresh rate".
• The maximum number of points that can be displayed on a CRT is referred to as the "resolution".

CRT Display Principles

• Raster-Scan Displays:
  • Based on TV technology.
  • An electron beam is swept across the screen one row at a time from top to bottom.
  • Each row is referred to as a scan line.
  • Picture elements: a screen point is referred to as a "Pixel."
  • Picture description is stored in the memory area, also called the "refresh buffer" or "frame buffer".
  • The number of bits per pixel in the frame buffer is called depth (or bit planes).
    • Buffer with 1 bit per pixel - Bitmap
    • Buffer with multiple bits per pixel - Pixmap
  • Interlaced refresh procedure, which beams sweeps across every other scan line.

Frame Buffer

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

Specifying Color

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

Refresh Rates and Bandwidth

• Measured in Frames per second (FPS).

Interlaced Scanning

• Scan frame 30 times per second
• To reduce flicker, divide the frame into two fields: one 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.

Raster Displays: Pros and Cons

• Advantages:
  • Enables real images.
  • Many colors can be produced.
  • Allows dark scenes to be pictured/filled with regions/shaded images.
• Disadvantages:
  • It has a discrete representation.
  • Continuous primitives must be scan-converted (i.e., fill in the appropriate scan lines).
  • Aliasing or "jaggies" arise due to sampling error when converting from a continuous to a discrete representation.

Random-Scan Display Principles

• Calligraphic Displays are also called vector, stroke, or line drawing graphics.
• The electron beam is directed only to the points of the picture to be displayed.
• Vector displays use electron beams to trace out lines to generate pictures.
• After compilation of picture drawing, the system cycles back to the first line and creates all the lines of the picture 30 to 60 times per second.
• The picture is stored as a set of line-drawing commands.
  • Storage is referred to as a display list, refresh display file, vector file, or display program.

Comparing Raster and Vector

• Vector advantages: - Very fine detail of line drawings (sometimes curves), whereas raster suffers from a jagged edge problem due to pixels (aliasing, quantization errors). - Handles geometry objects (lines) whereas raster only handles pixels. - e.g., 1000 line plot: vector display computes 2000 endpoints. - Raster display computes all pixels on each line.
• Raster advantages: - Cheaper. - Color, textures, realism - Unlimited complexity of picture: whatever you put in refresh buffer, whereas vector complexity limited by refresh rate.

Color CRT Monitors

• Uses a combination of phosphors that emit different-colored light.
  • Beam-penetration:
    • Used in random-scan monitors.
    • Uses red and green phosphors layers.
    • Color depends on the penetrated length of electrons.
  • Shadow mask:
    • Used in raster-scan systems.
    • Produces a wide range of colors with the RGB color model.
• Color CRTs are more complicated, requires manufacturing very precise geometry,
• Uses a pattern of color phosphors on the screen:

Color CRT Monitors: Pros and Cons

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

Flat-Panel Displays

• A class of video devices with reduced volume, weight, and power requirement, compared with CRTs.
• Two main categories:
  • Emissive Displays:
    • Converts electrical energy to light energy.
    • e.g., Plasma panels.
  • Non-emissive Displays:
    • Uses optical effects to convert light from other sources into graphics patterns.
    • e.g., LCD monitors.

Plasma Panel Display

• Plasma panels (gas-discharge display).
• Contracted by filling the region between two glass plates with a mixture of gases.
• It uses a refresh buffer to store the 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, flow like a liquid.
  • Active-matrix LCD.
  • Passive-matrix LCD.
    • The voltage is applied to intersecting conductors to control light twisting.
  • Uses thin-film transistor technology, placing a transistor at each pixel location.

3D Display

• The same as stereoscope display technology.
• Technology capable of bringing depth perception to the viewer.
• Used for 3D gaming and 3D TVs
  • E.g., Fog Display, Holographic Display, Retina Display.
• Advantages: Impressive Picture Quality. Disadvantage:
  • Expensive
  • Binocular Fusion

Video Output Devices

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

Traditional Input Device

• Commonly used.
• Mouse-like devices.
  • Mouse
  • Wheel mouse
  • Trackball
• Keyboards
• Pen-based devices -Pressure sensitive -Absolute positioning -Tablet computers -IPAQ, WinCE machines -Microsoft eTablet -palm-top devices -Handspring Visor, PalmOS
• Joysticks.
  • Game pads.
  • Flight sticks.
• Touchscreens
• Microphones -Wireless vs. wired.
• Headset
• Digital still and video cameras, scanners
• MIDI devices -Input from electronic musical instruments More convenient than entering scores with just a mouse/keyboard

3D Input Device (1/2)

• Electromagnetic trackers.
• Can be attached to any head, hands, joints, objects.
  • Polhemus FASTRAKT (used in Brown's Cave).

3D Input Device (2/2)

• Gloves
  • Attach electromagnetic tracker to the hand
• Pinch gloves
  • Contact between digits is a “pinch” gesture
• In 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. -This digitization process is called Scan Conversion.

Graphics Software

• Refers to a program or collection of programs that enable a person to manipulate images or models visually on a computer.
• Classifications:
  • Special purpose package:
    • Designed for non-programmers.
    • No graphics procedures involved.
    • Communicate with a set of menus .
  • General programming package:
    • Designed for programmers.
    • Provide a library of graphics functions used in programming language.

Graphic Standards

• Agreed upon specifications that define the common interfaces between computer systems or subsystems.