Digital Photogrammetry 7
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Transcript of Digital Photogrammetry 7
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Digital Image processingMethods used in photogrammetry
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Fundamentals
Photogrammetric image procesing are developed and applied in the fields of image acquisition, pre-processing and segmentation.
Methods: Image measuring Line following Image matching Object recognition
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Handling image data
Image pyramids Gaussian shmooted Dimension +30%
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Compression
Lossless compression
Lossy compression
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Image pre-processing Histogram
Provides the frequency distribution of the pixel values in the image
The most important are:
Relative frequency
Min,Max
contrast
Mean, variance
entropy, simmetry
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Contrast enhancement
Manipulating brightness and contrast of an image results in a change of the pixel value distribution, for example along an image edge.
Linear contrast stretching Histogram equalization
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Other simple operations
Thresholding
Image combination (aritmetic, Logical, bitwise)
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Filter operations
Smoothing filters (low-pass filter) are mainly used for pixel noise suppression. Gaussian filter possess optimal smoothing properties
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Smoothing filter
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Smoothing filter
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Morphological operators
Application of a non-linear filter for the enhancement or suppression of black an white regions with (known) properties.
Two fundamental functions based on boolean operator are defined:
Erosion, which leads to the shrinking of regions. The value 1 is set up if all pixels in the filter region (e.g. 3x3 elements) correspond to the structure elements. Otherwise 0.
Dilatationwhich yields to the extension of connected regions. 1 is setted if at least one matching pixel is present.
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Morphological operators
Sequential application of dilatation and erosion can be used. Opening erososion followed by dilatation Remove
small objects closing dilatation followed by erosion close gaps
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Edge extraction
Edge are primary images structures used by human visual system for object recognition.
Charachteristics a significant change in adjacent pixel values
perpendicular to the edge direction. edges have direction and magnitude formed by small images structures High frequencies into frequency domain.
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Edge extraction
First order differential filter
Sobel operator
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Edge extraction
Second order differential filter
Laplacian filter
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Edge extraction
Laplacian of Gaussian filter
Laplacian filter is senitive to noise a better results would be expected if the image is smooted in advance with a gaussian filter.
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Hough Transform
The hough transform is based on the condition that all point on an nalytical curve can be defined by one common set of parameters.
Based on that fact, the straight line y = mx + b can be represented as a point (r, φ) in the parameter space
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Sample
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Enanched Edge operator
Simple methods for edge detector often do not deliver satisfactory results. An edge filter should have following chaacteristics: Robustness simple parametrization (with out interactive input) High subpixel accuracy minimum computational effort.
Canny and Deriche operator
edge extraction in image pyramids (kÖthe 1997)
Least square edge operators (el hakim 1996)
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Geometric Image transformation
The term rectification denotes a general modification of pixel coordinates e.g. for:
Translation and rotation Change of scale or size Correction of distrosion effect Projective rectification orthophoto production Texture mapping
Generally is performed into two stages Transformation Of Pixel Coordinates Calculation of (output) pixel values
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Geometric Image transformation
Stereo image rectification
This process is useful for stereo vision, because the 2-D stereo correspondence problem is reduced to a 1-D problem.
Steps: Find homologous points
Remove outliners
Compute fundamental matrix
Rectify images
Fusiello, Andrea (2000-03-17). "Epipolar Rectification”
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Geometric Image transformation
Geometric Rectification
Raw remotely sensed data gathered by satellite or aircraft are representations of the irregular surface of the Earth. Remotely sensed images are distorted by both the curvatures of the Earth and the sensor being used. The process of shifting pixel locations to remove distortion is known as rectification or georectification.
Spatial interpolation
Intensity interpolation
The geometric relationship between the input pixel coordinates (column and row; referred to as x’, y’) and the associated map coordinates of this same point (X, Y) must be identified.
Polynomial equations are used to convert source file coordinates into the referencing map coordinates
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Geometric Image transformation
Intensity interpolation involves the extraction of a brightness value from an x′, y′ location in the original (distorted) input image and its relocation to the appropriate x, y coordinate location in the rectified output image.
There are several methods of brightness value (BV) intensity interpolation that can be applied, including:
nearest neighbor,
bilinear interpolation, and
cubic convolution.
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NEAREST NEIGHBOUR
Nearest neighbour resampling uses the digital value from the pixel in the original image which is nearest to the new pixel location in the corrected image. This is the simplest method and does not alter the original values, but may result in some pixel values being duplicated while others are lost. This method also tends to result in a disjointed or blocky image appearance
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BILINEAR INTERPOLATION
Bilinear interpolation resampling takes a weighted average of 4 pixels in the original image nearest to the new pixel location. The averaging process alters the original pixel values and creates entirely new digital values in the output image. This may be undesirable if further processing and analysis, such as classification based on spectral response, is to be done. If this is the case, resampling may best be done after the classification process.
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CUBIC CONVOLUTION
Cubic convolution resampling calculates a distance weighted average of a block of sixteen pixels from the original image which surround the new output pixel location. As with bilinear interpolation, this method results in completely new pixel values. However, these two methods both produce images which have a much sharper appearance and avoid the blocky appearance of the nearest neighbour method