IMAGING SYSTEMS
1 Kim Bubello
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Contents 4
Fundamentals
12 Input 20 Processing 26 Output
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SECTION ONE
FUNDAMENTALS
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SPATIAL RESOLUTION Spatial resolution is defined by the pixel count of the image. For example a higher spatial resolution allows for a larger image and image size and more detail in the image. When we resize images we have to make up for the numebrs and values in the image that weren’t there before. To the right is an image that has not been affected by interpolation methods. The image was then resized to 500% of the original using the different interpolation methods that you can find in photoshop. These methods were used to compare artifact generation, speed of the process and color gradients in the image used.
Participants of the New York State fair watch one of the rides as the sun sets at the end of the day in Syracuse, New York on September 2, 2014.
Examples of Resolution techniques that can be applied to images in photoshop. (Image > Image Size)
Nearest Neighbor Took 0.1 seconds to apply and resize image.
Bilinear Took 0.2 seconds to apply and resize image.
Bicubic Took 0.3 seconds to apply and resize image.
Nearest Neighbor creates the values of the photograph by using the nearest pixel. It is simple and fast but can cause jagged edges in your image and changes in color.
The Bilinear method samples data from the four nearest pixels (x and y or 2x2) and then averages the data together to give you the values. This method will give you a more realistic transition on edges and colors.
The Bicubic method takes averages the 8 nearest pixels (4x4). This creates a much smoother image than the other methods.
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Participants of the New York State fair watch one of the rides as the sun sets at the end of the day in Syracuse, New York on September 2, 2014.
TONAL RESOLUTION Tonal resolution a type of resolution that is measured in bit depth. The term bit depth is defined as a unit that determines how many tones can be displayed in a photograph. The more tones or levels pixels can display in that photograph, the more continuous tones and colors you are going to see and that are going to appear. The number of levels in a bit depth is equal to, 2 to the power of the bit depth. The continuous neutral tones for print and the human eye occurs at 7 bits and 5 bits for color. The color image shows bit depth starting from the bottom. (i.e. An 8 bit image is 28 or 256 levels)
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4 16
32 255
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SPECTRAL RESOLUTION Spectral resolution is the color data that the sensor in your camera can capture. In most cameras these channels are known as the Red, Green and Blue channels(RGB) (also known as additive primary colors). Then there are the subtracive colors that are made by mixing the additive colors together. Those would be Cyan, Magenta and Yellow. In the world of printing , mixing cyan, magenta and yellow, with an added channel to achieve black are used.
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RGB
CMY
HISTOGRAMS Histograms display the frequency of pixels at different tones or brightness values. (The values range from 0-255 in an 8-bit image). A properply exposed image would have a historgram that shows a spread and well distributed amount of values throughout the image.
High Contrast
This is a high contrast image, the histogram shows peaks at either end. Although this may seen good the veruse of contrast can lead to loss of detail in dark and light pixels in the photograph.
Underexposed
This is an underexposed image. The histogram slides the values to the left and causes loss of detail in the backgroun of the image as well as in the subjects face and back
Overexposed
This image is underexposed. The histogram shows most of the values towards the right. Lighter colors and highlights start to lose detail in the image. 9
FILE SIZE File size can be calculated using the following formula: the result will be expressed in Megabytes (mb)
Spatial • Tonal • Spectal = X mb Multiply the pixel dimensions of your image for spatial resolution, then to convert the total number of pixels to bits divide by 1 because there are 8 bits per pixel. Next multiply by the bit depth to get file size in bits. Divide this number by 8 because they’re 8 bits per byte. This number is the size of one channel , so multiply by 3 to get your final file size in bytes.
To get your file size in megabytes, divide this number twice by 1024 (1024 bytes in a kilobyte, 1024 kilobytes in a megabyte). Assume you have an 8-bit, 1024 x 1024 image (1 megapixel).
1,048,576 Pixels • 8 bits • 3 1,048,576 Pixels • 8 bits/1 pixel • 3 8,388,608 bits • 1 byte/8 bits • 3 1,048,576 bytes • 3 3,145,728 bytes • 1 byte/1024 kb 3,072 kb • 1 mb/1024 mb 3 mb
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DPI VS. PPI Resolution in prints and displays are measured in two ways: dots per inch and pixels per inch. For example DPI in printing is a mthod that puts small dots of color and disperss them over the page at different screen angles to convince your eye that there are continuous tones in the image. PPI is simply the number of pixels in an inch of a display.
DPI Is used in printing. CMYK (Cyan, Magenta, Yellow and Black) dots are distributed across the page at varying angles to produce color. This method is what we call halftoning. PPI Is used in displays and sensors. Pixels display varying intensities of RGB to make color. PPI and LPI (lines per inch) are also equal values to PPI.
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SECTION TWO
INPUT
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IMAGING PROCESSING PIPELINE INPUT ADC RAW Capture Image Photons enter camera through lens and are converted into electrons by the sensor in the camera. 14
Sensor The camera’s sensor detects the amount of light at each pizel and sends the electrical information to the ADC. (Analog Digital Converter)
ADC The Analog Digital Converter takes the voltage generated at the pixel sites and quantizes it into numerical values.
Container The raw numerical data is pulled into a container which will then be read by a processing software.
PROCESSING NB CFA
ADJUSTMENTS
GAMMA
RAW Raw Processor The data is opened in a raw converter which translates the raw nurmerical data into a visable image.
Neutral Balance CFA Interpolation Gamma Correction
Interpretive Adjustments Required adjustments to an image end here, further adjustments can be made here before output. Examples: exposure, brightness, color, etc.
Raster Image Processor Assuming the final output is print then the file is sent to an RIP, which controls the conversion from vector to raster image. This can be controlled in the printer or software you are using.
Print Image The RIP sends the final raster umage information to the printer which then creates the image.
OUTPUT Convert File Type Depending on the type of output different file types will be used.
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SENSORS An image sensor is a device that converts an optical image into an electrical signal. The photons that enter the lens are read by many photosites on the senor, each photosite translating the light into voltage for indivisual pixels. While all senors serve this function there are many different types of sensor technologies. There are three primary types of digital sensors. These are CCD, CMOS and Foveon sensors which will be explain in the coming pages.
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CMOS Sensors(Complementary Metal Oxide Semiconductor) sensors are larger than CCD’s and are commonly used in DSLR cameras. They use less power and instantaneously collect luminance information through integrated circutry. Early versions had issues with dynamic range. Technologies in recent years have allowed CMOS to match that of CCD.
A Foveon Sensors is a “direct image sensor” that is made up of three layers of pizels that simutaleously capture all of the color information. The technology is possible due to silicon’s inheret ability to absorb different colors at different paths. A foveon sensor uses a standard CMOS semiconductor process. The necessary processing power is reduced by the mothod because there is no interpolaion neccessary. Manufacutueres claim that these sensors result in more vivid colors and sharper images. Foveon sesnor pixels collect information from all three RGB channels.
Foveon Sensors
CMOS & CCD
CCD Sensors (Charge Coupled Device) are smaller sensors tht are often used in compact camera and cell phones. They are more light sensitive than CMOS sensors, which make them less susceptible to noise. In general CCDs utilize a bucket brigade or conveyer belt proces in oerder to capture a charge.
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Red, green and blue filters seperate the visible light so that each photosite in a sensor collects color information for only one fo the RGB channels. There are twice as many green filters as red and blie in ordeer to mimic the color sensitivity of the human eye.
BAYER PATTERN The Bayer Pattern is a version of color filter array that is used in CCD and CMOS sensors. Red, green and blue filters are arranged ina particular pattern to create a photo sensor. In a bayer pattern there is twice the amount of green than red and blue in order to mimic the color sensitivity of a human eye. The raw image is referred to as a bayer pattern image. However, this process results in only a third of the color data and a demosaicing algorithm is used to fill in the missing data.
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SECTION THREE
PROCESSING
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FILE FORMATS
JPEG RAW
A raw file refers to the mosaiced image that come directly out of the camera. However, it must undergo demosaicing, gamme correction, and noise reduction before it is viewed as a photograph on a display. Raw files require raw engines to interpret their data. They generally range in size from about 10 MB to 40 MB of data. Non-destructive changes applied during processing are made in metadata instructions. They allow for more manipulation without image degredation.
Raw formats are created by different camera manufacturers. Examples of these are NEF, CR2, ORF, etc. They are often locked in order to precent non-first party software from writing to them. 22
JPEG stands for Joint Photographic Experts Group, which is the organization whoch created this format. JPEG are generally 8-bits per color and range in file size of 500 KB to 8 MB. There is generational degradation whoch occurs with repeated saves. While shooting, photographers have the option of creating in camera JPEGs. These files take a 12-bit raw file and compress it in camera. This allows for faster write-speed. JPEGs can help speef up the photographer’s workflow, but editing the images quickly results in compression artifacts.
TIFF
TIFF stands for Tagged Image File Format. Images saved in this way can be 8-bit or 16- bit and tilize eaither lossy or lossless compression. Lossy compression discards information that can becomes visible while lossless compression reduces the file size without losing image quality. This is possible due to statistical reducndancy such as inter-pixel or psycho-visual redundancy.
High Brightness Levels
High Contrast
Low Brightness Levels
Low Contrast
EXPOSURE CORRECTION Histograms A histogram is a graphical representation of the tonal distribution of an image. It shows the frequency of the pixels at each light level in the image. There are 256 levels in a normal 8 bit images. The x axis on the histogram will represent the tone, with the blacks at 0 to the far left and the whites all the way to 256 on the right. Then, the y-axis represents the number of pixels at each value.
Birghtness The brightness of an image refers to the overall concerntration of tones along the histogram. To adjust brightness a mathermatical function is used to either add or subtract a given numnber to all the the pixels in an image. If you are adding to these numbers in the pixels the image will becoming and appear brighter but if you are subtracting from these numbers the image will then appear darker.
Contrast The contrast of an image refers to the differance between the brightest collection and the darkest collection of pixels. The greater difference the greater amount of contrast that will appear in the image. To adjust this contrast a mathematical funtion is utilized to adjust all of the pixel values of an image based on a given number. When you multiply you get greater contrast in the images but when you divide you then will see less contrast.
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LAB COLOR LAB Color or Lab color space is a 3D model arranged on three axises. Lab is designed to mimic human vision , unlike RGB or CMYK. This color space has a lightness component, also to match human perception. Lab color is based off of the Munsell color system.
L in LAB represents lightness or density, 0 being black and 100 being white or lightness.
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In a and b channels values range from -128 to 127. Positive a values yeild red or magenta, positive b values yeild yellow. Negitive a values yielf green and negitive b values yield blue.
Original Image
Sharpened Image
Over-Sharpened Image
SHARPENING Image Sharpening is a tool used to exaggerate texture and clarify focus. There are many applications that can be used to sharpen an image but if you aren’t careful, sharpening can have degrading affects, like sharpened noise and sharpening halos.
The most commonly used method of sharpening, even if you don’t know it is being used, is the unsharp mask which was put to use when develping in the days of film.
Blurred portions of an image have a low frequency and short aspects have a high frequency. When sharpening an image you’ve effectively decreased the amount of low frequencies.
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SECTION FOUR
OUTPUT
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CALIBRATION Calibration: Color is an important characteristic of an image and it is ideal that the same color profile carries all the way through from capture to print. The eaisest way to stay consitent is by placing a color checker into the scene. Color checkers come equipped with colored patches that can be used when color balancing an image in post processing while in Lightroom or Photoshop.
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Color balancing your monitor is a key part of keeping color conistent throughout the entire image process. Every display monitor and printer are characterized differently, so profiles vary with each device.
Tools like the Xrite Color-Munki aid in calibration to acheive a consistent color profile between a monitor and printing device.
PRINTING Laser printing is a digital printing process that passes a laser beam over a charged drum, which is then heated to keep the image in place. Laser printing is fast, cheap, and is best for high quality text or graphics.
Inkjet printers are the most commonly used printing methods. They can range from the small desktop printers that many people use at home to other large profssional printing machines. Continuious inkjet printers will use pressure to propel droplets of ink onto the paper. Drop-on demand will use heat to bubble ink and have it drop onto the paper.
Dye Sublimation printers use heat to transfer dye onto paper. These are not to be confused with inkjet printers. Dye-sub printers create true continuious tone images whereas inkjet printers produce individual droplets of ink that can be seen under magnification.
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HALFTONING Halftoning is used in non continuous devices and must be used when printing. It is used to to simulate continuous tone by using individual dots and varying them in size, shape and frequency in order to produce an image. Halftoning uses put inks in the colors of: black, cyan, magnta, and yellow and are used to create the illusion of a continuous tone in the images that is printed. To create a halftone image, a continuous tone image is projected through a screen of dots. The areas of the image with larger dots or more dots appear
darker than other sections of the image. Areas with less dots, along with the combination of black dots and whitr paper will then create different shades of grey. In the case of halftoning spatial resolution is sacrifed in order to gain tonal resolution.
Right: Original Image
Left: Halftoned Image Above: Close up of dots in halftoned image
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Kim Bubello
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