Remote sensing/digital image processing

Color Arithmetic

red+green=yellowgreen+blue=cyanred+blue=magenta

Image Display• Example color combinations

– red(255) + green (255) + blue (255) = white

– red(0) + green (95) + blue (0) = dark green

– red(255) + green (0) + blue (255) = purple

– red(0) + green (255) + blue (255) = cyan

– red(170) + green (170) + blue (170) = gray

– red(0) + green (0) + blue (0) = black

Digital Image Processing: Image Enhancement

Image Enhancement example:

• Contrast Enhancement - “stretching” all or part of input BVs from the image data to the full 0-255 screen output range

Contrast Enhancement example:

• A linear stretch is one of the most common types of contrast enhancement

• Minimum BV is remapped to 0• Maximum BV is remapped to 255

0 255127

0 25560 108 158

Image Display

• Two types of linear stretches

255

255

0

Stretched Output

Emphasizes middle “piece” of input range

255

255

0

Linear Contrast Stretch Piecewise Linear Stretch

BV (input)

BV (output)

BV (input)

BV (output)

True-Color 321 Image

No stretch applied

True-Color 321 Image

Linear Contrast Stretch

Interactions with the Atmosphere

Particles and gases in the atmosphere can affect the incoming light and radiation. These effects are caused by the mechanisms of scattering and absorption.

Scattering

Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path.

sun

Scattering

Scattering is affected by:

• wavelength of the radiation

• the abundance of particles or gases

• the distance the radiation travels through the atmosphere.

There are 3 major types of scattering: 1. Rayleigh Scattering2. Mie Scattering 3. Nonselective Scattering

Rayleigh ScatterRayleigh scatter occurs when radiation interacts with

atmospheric molecules that are much smaller in diameter than the wavelength of the interacting radiation.

This is the major form of scattering that occurs on clear, blue sky days.

Rayleigh scattering occurs mostly in the upper regions of the atmosphere, where most particles are very small.

Rayleigh Scattering

Short wavelengths are scattered by this mechanism more than long wavelengths.

R O Y G B I VShort λLong λ

The sky is blue because of Rayleigh Scattering. Shorter wavelengths (i.e. blue) of the visible spectrum are scattered more than longer visible wavelengths.

So why isn’t the sky indigo or violet?

Mie scattering

Mie scattering exists when atmospheric particle diameters essentially equal the wavelengths of the energy.

Major causes: Water vapor and dust. (other causes - pollen, smoke.)

Influence: longer wavelengths compared to Rayleigh scatter.

Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundant, and dominates with overcast conditions.

Nonselective Scattering Nonselective scattering occurs when the diameters of the particles are much larger than the wavelengths of the energy.

Example: water droplets.

Such particles scatter all visible and Near to Mid-IR wavelengths about equally. This scatter is “nonselective” with respect to wavelength. Hence fog and clouds appear white.

Nonselective scattering

Correct for scattering in remotely sensed images

• Various algorithms are sometimes used to correct for each type of scattering.

• Mie scattering, in particular, is different on different days, depending mostly on the amount of atmospheric haze.– Therefore the correction algorithm should be different

on different days.• Classifications can be made without correction.

Often no correction is used.

AbsorptionAbsorption is the other main mechanism at work when electromagnetic radiation interacts with the atmosphere.

In contrast to scattering, this phenomenon causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone, carbon dioxide, and water vapor are the three main atmospheric constituents which absorb radiation.

Absorption (Cont.)Ozone serves to absorb the harmful (to most living

things) ultraviolet radiation from the sun. Without this protective layer in the atmosphere our skin would burn when exposed to sunlight.

Carbon dioxide is referred to as a greenhouse gas. It tends to absorb radiation strongly in the far infrared portion of the spectrum - the area associated with thermal heating.

Water vapor absorbs much of the incoming long wave infrared and shortwave microwave radiation (between 22mm and 1m).

Atmospheric Windows

Those areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called atmospheric windows.

Image Pre-Processing

• Radiometric Corrections– changing the image data BVs to correct for errors or distortions

• atmospheric effects (scattering and absorption)• sensor errors

• Geometric Corrections– changing the geometric/spatial properties of the image data – also called

• image rectification• rubber sheeting

Geometric Correction

• Four Basic Steps of Rectification1. Collect ground control points (GCPs)2. “Tie” points on the image to GCPs.3. Transform all image pixel coordinates using mathematical functions

that allow “tied” points to stay correctly mapped to GCPs.4. Resample the pixel values (BVs) from the input image to put values

in the newly georeferenced image

Geometric Correction• Three Types of Resampling

– Nearest Neighbor - assign the new BV from the closest input pixel.This method does not change any values.

– Bilinear Interpolation - distance-weighted average of the BVs from the 4 closest input pixels

– Cubic Convolution - fits a polynomial equation to interpolate a “surface” based on the nearest 16 input pixels; new BV taken from surface

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