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Forensic Applications of Video and Digital Photography. What is a Digital Camera?. Film Cameras. Scanners. CD/Floppy. Cell Phones. Retailers. email. Devices. Computer. Home Printing. Internet. Digital Cameras. Digital Imaging Evolution. How Digital Cameras Work.
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Film Cameras Scanners CD/Floppy Cell Phones Retailers email Devices Computer Home Printing Internet Digital Cameras Digital Imaging Evolution
How Digital Cameras Work • Digital Cameras have a sensor that converts light into electrical charges. • The image sensor employed by most digital cameras is a charge coupled device (CCD). • Some low-end cameras use complementary metal oxide semiconductor (CMOS) technology. • While CMOS sensors will almost certainly improve and become more popular in the future, they probably won't replace CCD sensors in higher-end digital cameras.
How Digital Cameras Work • The CCD is a collection of tiny light-sensitive diodes, which convert photons (light) into electrons (electrical charge). • These diodes are called photosites. In a nutshell, each photosite is sensitive to light -- the brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site.
Several Types of Digital Cameras • Floppy Disk • 3 inch CD-R • Memory Stick • Compact Flash Card • SmartMedia Cards • Built-in Memory
Image Acquisition • Solid-state arrays are composed of discrete silicon imaging elements (photosites) providing voltage output • Line scan sensors (resolution: 256 to 4096) • Area sensors (32 x 32, 256 x 256, 640 x 480, 1280 x 1024) with CCD chip: 2048 x 2048
Pixel A digital image is made up of hundreds of thousands of tiny dots Resolution Number of horizontal pixels multiplied by the number of vertical pixels More Pixels the sharper the image Cropping photos allows one to make high resolution photos small enough to print and keep a lot of detail 640 X 480 web, email 1024 X 768 = 3” X 5” 1280 X 960 = 5” X 7 “ 1600 X 1200 = 8” X 10” Or extra-sharp 5” X 7” 1920 X 1600 Very High Resolution Resolution
How to Compare Digital Cameras • The higher the amount of megapixels the better the resolution • For long range photography you will want a digital zoom • Small vs. Big- The Sony Mavica FD97s shoot a great picture but are very bulky and heavy • What machine will you be downloading to? Windows 95 machines without an adaptor will not accept memory cards. • Convenience: memory cards, speed of downloads
How to Compare Digital Cameras • Glass lens vs.plastic lens • If you will only output pictures to a computer monitor (for viewing, Web page use or email) an inexpensive digital camera with a 640X480 pixel resolution will provide satisfactory results • If you plan to print photographs on a good (at least 720 dots per inch) color printer, look for a high resolution camera
Optical Zoom Magnifies the image using a real multifocal-length lens 2X, 3X, ….14X Means that if the camera’s minimum focal length is 50mm, then it has the ability to take pictures up to 100mm Or 2X as far as the image in a fixed lens. Digital Zoom Magnifies the image using the pixels in the image. Does not telescope into a subject It Interpolates the picture magnifying the existing pixels Best used in high resolution pictures Leads to fuzzier, less defined image Choosing a Camera
Exposure Compensation Allows for minor adjustments to the automatic exposure settings, allowing one to lighten or darken scenes with difficult lighting situations, such as backlit or high contrast scenes White Balance Allows one to set the “white point” for the picture. Setting a manual “white point” will allow one to adjust one’s picture’s colors to get what the actual color is really. Digital Camera Features
Photo Compensation If you wish to make the result whitish, operate the exposure compensation button (or dial) to + (plus) side (at +2 EV, the entire frame becomes almost pure white).On the other hand, if you wish to make it blackish, adjust it to - (minus) side (at -2 EV, the entire frame becomes almost pure black).
White Balance Example of a shot taken under the light of a tungsten bulb with "Fine (="SUNNY")" white balance setting. Example of a shot under the lighting of a fluorescent lamp with "AUTO" white balance setting Example of a shot taken under the sun with "Incandescent" white balance setting.
Tips on Taking Photos • Avoid the dreaded blur • Stand Still • Hold the camera steady and firmly with both hands • Use the viewfinder instead of the LCD monitor
Tips Continued • Eye level with subject • Flash • Stay within the effective range of the camera’s flash • 3 to 10 feet • Move to close and get overexposure • To far away and underexposure
Outdoor Flash Makes photos more vibrant with more ‘punch’, separating the subject from the background Yielding sharper photo Especially in low light Be Prepared Carry extra batteries Memory Cards or Disk TIFF and JPEG TIFF longer to record but much higher quality Tips Continued
Storage • Short-term for use during processing: • computer memory • frame buffers (up to 32 Mbytes of storage) • On-line for fast recall • Magnetic discs, optical discs • Archival storage (infrequent access) • Magnetic tapes (short shelf life) • WORM optical discs
Image Storage Image is converted to data and stored on cameras memory card.
Two Factors That Affect Image Quality File Compression & Image Resolution
File Compression • Compressing an image reduces the amount of memory that is taken up by the image but…
File Compression • …compressing an image also reduces the quality of the image.
Pixel 12 x 12 = 144 • A pixel is the smallest piece of information that a digital image is made from. Resolution = 144
Pixel • The greater the number of pixels, the higher the resolution of the image.
Original Image360 x 28050 KBytes Resolution reduced to 150 high x 120wide 18 KBytes
Image enlarged from 150 wide x 120 high back to the original size of 360 wide x 280 high.
Image types .jpg .gif .bmp Graphics Interchange Format Use when graphic has only a few colours Drawings; clipart; maps (or jpeg) Joint Photographics Experts Group Use when image has lots of colours e.g. photograph Bitmaps Made up of dots. Value of dots are stored in bits of data
Digital Camera Formats • Still images are recorded in JPEG formatJPEG (pronounced "jay-peg") is a standardized image compression mechanism. • JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard. • JPEG is designed for compressing either full-color or gray-scale images of natural, real-world scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or line drawings. JPEG handles only still images, but there is a related standard called MPEG for motion pictures. • If one makes changes each time to a JPEG the resolution is compromised over time
Digital Camera Formats • Clip Motion images are stores in GIF Format • GIF stands for "Graphic Image Format", and the format was developed by Compuserve to provide a means of passing an image from one dial-up customer to another, even across different computer hardware platforms. • It is a relatively old format, and was designed to handle a palette of 256 colors (8 bit color), and a single image. When developed, this was near state of the art for most personal computers.
Digital Camera Formats • Movies are shot in MPG M-PEG: • 1. Abbreviation for Motion Picture Experts Group. 2. In digital television signal recording or transmission, the designator that identifies a certain coding algorithm having several levels and profiles, the most sophisticated of which are used for HDTV (high-definition television) applications. • 3.Pertaining to files (with file-name extensions ".mpg" or ".mpeg") that are compressed using the MPEG standard. • Also written MPEG.
Application areas • Image enhancement • Image measurement • Biometric identification
Image Enhancement Video surveillance images are typically of poor quality camera video recorder monitor The quality of the display on the monitor can be misleading. The video recorder runs 24 hours/day. The tape gets worn and the recording heads get dirty. The images that are recorded can be very poor.
Image Enhancement • Common problems in images : • Poor resolution, especially in video images. • Poor contrast due to under or over-exposure. • Corruption with noise. • Motion blur or poor focus. • Misalignment of rows from line jitter and interlacing of motion in video images.
Poor resolution: Zooming in does not help!
Create a `higher resolution’ image by mathematically interpolating pixel values
Super-resolution from multiple low resolution images Images must be aligned accurately. Object should be planar. Irani and Peleg 1991 Capel and Zisserman 1998
Super-resolution from multiple low resolution images: Results from synthetic data can be impressive
Super-resolution from multiple low resolution images: Number plate reconstruction
Denoising Current method of choice is wavelet shrinkage denoising. Donoho and Johnson 1995 original image
Denoising Grey scale enhanced image
original deinterlaced wavelet denoised and contrast enhanced
original deinterlaced wavelet denoised and contrast enhanced
Image Metrology Calibration targets allow views of flat surfaces to be rectified. Rectified views allow measurements to be taken.
Image Metrology Rectified views of the fence and ground. Criminisi, Reid and Zisserman 1999