GEOG2021 Environmental Remote Sensing Lecture 2 Image Display and Enhancement

Image Display and Enhancement Purpose visual enhancement to aid interpretation enhancement for improvement of information extraction techniques

Image Display The quality of image display depends on the quality of the display device used and the way it is set up / used … computer screen - RGB colour guns e.g. 24 bit screen (16777216) 8 bits/colour (28) or address differently

Colour Composites ‘Real Colour’ composite Swanley, Landsat TM 1988 red band on red green band on green blue band on blue Swanley, Landsat TM 1988

Colour Composites ‘Real Colour’ composite red band on red

Colour Composites ‘Real Colour’ composite red band on red green band on green

Colour Composites ‘Real Colour’ composite red band on red green band on green blue band on blue approximation to ‘real colour’...

Colour Composites ‘False Colour’ composite NIR band on red red band on green green band on blue

Colour Composites ‘False Colour’ composite NIR band on red red band on green green band on blue

Colour Composites ‘False Colour’ composite many channel data, much not comparable to RGB (visible) e.g. Multi-polarisation SAR HH: Horizontal transmitted polarization and Horizontal received polarization VV: Vertical transmitted polarization and Vertical received polarization HV: Horizontal transmitted polarization and Vertical received polarization

April; August; September Colour Composites ‘False Colour’ composite many channel data, much not comparable to RGB (visible) e.g. Multi-temporal data AVHRR MVC 1995 April August September April; August; September

Colour Composites ‘False Colour’ composite many channel data, much not comparable to RGB (visible) e.g. MISR -Multi-angular data (August 2000) 0o; +45o; -45o RCC Northeast Botswana

Greyscale Display Put same information on R,G,B: August 1995

Density Slicing

Density Slicing

Density Slicing Density slicing: a crude form of classification Don’t always want to use full dynamic range of display Density slicing: a crude form of classification

Density Slicing Or use single cutoff = Thresholding

Density Slicing ‘Semi-Thresholding’ Or use single cutoff with grey level after that point ‘Semi-Thresholding’

Pseudocolour use colour to enhance features in a single band each DN assigned a different 'colour' in the image display

Pseudocolour Or combine with density slicing / thresholding

Image Arithmetic Combine multiple channels of information to enhance features e.g. NDVI (NIR-R)/(NIR+R)

Image Arithmetic Combine multiple channels of information to enhance features e.g. NDVI (NIR-R)/(NIR+R)

Image Arithmetic Common operators: Ratio Landsat TM 1992 Southern Vietnam: green band what is the ‘shading’?

Image Arithmetic Common operators: Ratio topographic effects visible in all bands FCC

Image Arithmetic Common operators: Ratio (cha/chb) apply band ratio = NIR/red what effect has it had?

Image Arithmetic Common operators: Ratio (cha/chb) Reduces topographic effects Enhance/reduce spectral features e.g. ratio vegetation indices (SAVI, NDVI++)

Image Arithmetic Common operators: Subtraction An active burn near the Okavango Delta, Botswana NOAA-11 AVHRR LAC data (1.1km pixels) September 1989. Red indicates the positions of active fires NDVI provides poor burned/unburned discrimination Smoke plumes >500km long examine CHANGE e.g. in land cover

Top left AVHRR Ch3 day 235 Top Right AVHRR Ch3 day 236 Bottom difference pseudocolur scale: black - none blue - low red - high Botswana (approximately 300 * 300km)

Image Arithmetic + = Common operators: Addition Reduce noise (increase SNR) averaging, smoothing ... Normalisation (as in NDVI) + =

Image Arithmetic Common operators: Multiplication rarely used per se: logical operations? land/sea mask

Histogram Manipluation WHAT IS A HISTOGRAM?

Histogram Manipluation WHAT IS A HISTOGRAM?

Histogram Manipluation WHAT IS A HISTOGRAM? Frequency of occurrence (of specific DN)

Histogram Manipluation Analysis of histogram information on the dynamic range and distribution of DN attempts at visual enhancement also useful for analysis, e.g. when a multimodal distibution is observed

Histogram Manipluation Analysis of histogram information on the dynamic range and distribution of DN attempts at visual enhancement also useful for analysis, e.g. when a multimodal distibution is observed

Histogram Manipluation Typical histogram manipulation algorithms: Linear Transformation 255 output 255 input

Histogram Manipluation Typical histogram manipulation algorithms: Linear Transformation 255 output 255 input

Histogram Manipluation Typical histogram manipulation algorithms: Linear Transformation Can automatically scale between upper and lower limits or apply manual limits or apply piecewise operator But automatic not always useful ...

Histogram Manipluation Typical histogram manipulation algorithms: Histogram Equalisation Attempt is made to ‘equalise’ the frequency distribution across the full DN range

Histogram Manipluation Typical histogram manipulation algorithms: Histogram Equalisation Attempt to split the histogram into ‘equal areas’

Histogram Manipluation Typical histogram manipulation algorithms: Histogram Equalisation Resultant histogram uses DN range in proportion to frequency of occurrence

Histogram Manipluation Typical histogram manipulation algorithms: Histogram Equalisation Useful ‘automatic’ operation, attempting to produce ‘flat’ histogram Doesn’t suffer from ‘tail’ problems of linear transformation Like all these transforms, not always successful Histogram Normalisation is similar idea Attempts to produce ‘normal’ distribution in output histogram both useful when a distribution is very skewed or multimodal skewed

Histogram Manipluation Typical histogram manipulation algorithms: Gamma Correction Monitor output not linearly-related to voltage applied Screen brightness, B, a power of voltage, V: B = aV Hence use term ‘gamma correction’ 13 for most screens

Colour Spaces Define ‘colour space’ in terms of RGB Only for visible part of spectrum:

Colour Spaces RGB axes:

Colour Spaces RGB (primaries) as axes

Colour Spaces Alternative: CMYK ‘subtractive primaries’ often used for printing (& some TV)

Colour Spaces Alternative: CMYK ‘subtractive primaries’

Colour Spaces Other important concept: HSI transforms Hue (which shade of color) Saturation (how much color) Intensity also, HSV (value), HSL (lightness)

Colour Spaces Other important concept: HSI transforms

Colour Spaces Fusion application SPOT data fusion 10m PAN 3 ‘colour’ bands (NRG) at 20m 1 ‘panchromatic’ band at 10m Perform RGB-HSI transformation Replace I by higher resolution Perform HSI-RGB 10m PAN 20m XS fused 10+20m

Summary Display Image arithmetic Histogram Manipulation Colour spaces Colour composites, greyscale Display, density slicing, pseudocoluor Image arithmetic +­ Histogram Manipulation properties, transformations Colour spaces transforms, fusion

Summary Followup: web material http://www.geog.ucl.ac.uk/~plewis/geog2021 Mather chapters Follow up material on web and other RS texts Learn to use Science Direct for Journals