skyview.geometry.sampler

Class Clip

java.lang.Object

skyview.geometry.Sampler

skyview.geometry.sampler.Clip

All Implemented Interfaces:

java.io.Serializable, java.lang.Cloneable, Component

public class Clip
extends Sampler

The class implements a fast flux conserving resampling based on the Sutherland-Hodgman clipping algorithm.

Consider an original image of data in some projection and a new map of an overlapping region with some projection, coordinate system, etc. Assume that the flux within a given pixel in the original image is constant over the area of the pixel. The flux within each pixel in the new map should be the integral of the flux in the region occupied by each pixel in the map.

• Find all of the corners of the pixels in resampled map and project the positions of these corners into the projection of the original image. These should be scaled such that the coordinates run from 0 to inWidth and inHeight where inWidth and inHeight are the dimensions of the original image. I.e., in these coordinate each pixel in the original image is a unit square. This step is done prior to call the primary Clip sampler methods in this class. Note that the corners of the pixels are required rather than the pixel centers.
• For each pixel in the resampled map, find a bounding box in the original image's coordinates. This is used to find a rectangle of candidate pixels in the original image that may contribute flux to this pixel in the resampled map.
• For each candidate image pixel clip the resampled pixel to the image pixels boundaries.
• Calculate the area of the clipped region. This is easy since the clipped region is a convex polygon and we have the vertices. Triangulating the polygon allows us to calculate its area.
• Add a flux to the resampled pixel equal to the area of the clipped resampled pixel times the flux in the original map pixel
• Repeat for all candidate original pixels
• Go to the next resampling pixel.

The instance methods of this class are not thread-safe, however it is possible to generate a separate Clip sampler object for each thread to resample the same input image.

Developed by Tom McGlynn, NASA/GSFC October 3, 2002

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
protected double	drizzArea Drizzle Area
protected double	drizzOffset Drizzle offset
protected int	recurse

Fields inherited from class skyview.geometry.Sampler

bounds, inDepth, inHeight, inImage, inWidth, outDepth, outHeight, outImage, outWidth, trans

Constructor Summary

Constructors

Constructor and Description
Clip()

Method Summary

Modifier and Type	Method and Description
double	clipRectPoly(double xmin, double ymin, double xmax, double ymax, double[] polyx, double[] polyy) This method provides an interface to the simplified Sutherland-Hodges clipper for use outside the sampling context.
static double	convexArea(int n, double[] x, double[] y) Calculate the area of a convex polygon.
java.lang.String	getDescription() Get the description of this component.
java.lang.String	getName() Get the name of this component.
double[]	lastX()
double[]	lastY()
protected static void	printVert(int n, double[] x, double[] y, java.lang.String label) Debugging routine that prints a list of vertices.
int	rectClip(int n, double[] x, double[] y, double[] nx, double[] ny, double minX, double minY, double maxX, double maxY) Clip a polygon by a non-rotated rectangle.
void	sample(int pix) Implement the sample function of the Sampler class.
void	samplePixel(int pix, double[] x, double[] y) Sample a single map pixel.
void	setDrizzle(double drizzle) Set the drizzle factor for sampling
void	setInput(Image in) Set the input image for the sampling
void	setIntensive(boolean intensive)
void	setOutput(Image outImage) Set the output image for the sampling
double	weight() Return the weight associated with the last sampling.

Methods inherited from class skyview.geometry.Sampler

clone, factory, setBounds, setOrder, setTransform

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

recurse

protected int recurse

drizzOffset

protected double drizzOffset

Drizzle offset

drizzArea

protected double drizzArea

Drizzle Area

Constructor Detail

Clip

public Clip()

Method Detail

setOutput

public void setOutput(Image outImage)

Description copied from class: Sampler

Set the output image for the sampling

Overrides:

setOutput in class Sampler

getName

public java.lang.String getName()

Description copied from interface: Component

Get the name of this component.

getDescription

public java.lang.String getDescription()

Description copied from interface: Component

Get the description of this component.

setIntensive

public void setIntensive(boolean intensive)

setInput

public void setInput(Image in)

Description copied from class: Sampler

Set the input image for the sampling

Overrides:

setInput in class Sampler

setDrizzle

public void setDrizzle(double drizzle)

Set the drizzle factor for sampling

Parameters:

drizzle - The drizzle factor should range from 0 to 1 and indicates the length of the side of the pixel inside the original image pixel in which the flux is assumed to be contained.

convexArea

public static double convexArea(int n,
                                double[] x,
                                double[] y)

Calculate the area of a convex polygon. This function calculates the area of a convex polygon by deconvolving the polygon into triangles and summing the areas of the constituents. The user provides the coordinates of the vertices of the polygon in sequence along the circumference (in either direction and starting at any point). Only distinct vertices should be given, i.e., the first vertex should not be repeated at the end of the list. *

Parameters:

n - The number of vertices in the polygon.

x - The x coordinates of the vertices

y - The y coordinates of teh vertices

Returns:

The area of the polygon.

rectClip

public int rectClip(int n,
                    double[] x,
                    double[] y,
                    double[] nx,
                    double[] ny,
                    double minX,
                    double minY,
                    double maxX,
                    double maxY)

Clip a polygon by a non-rotated rectangle. This uses a simplified version of the Sutherland-Hodgeman polygon clipping method. We assume that the region to be clipped is convex. This implies that we will not need to worry about the clipping breaking the input region into multiple disconnected areas. [Proof: Suppose the resulting region is not convex. Then there is a line between two points in the region that crosses the boundary of the clipped region. However the clipped boundaries are all lines from one of the two figures we are intersecting. This would imply that this line crosses one of the boundaries in the original image. Hence either the original polygon or the clipping region would need to be non-convex.] Private arrays are used for intermediate results to minimize allocation costs.

Parameters:

n - Number of vertices in the polygon.

x - X values of vertices

y - Y values of vertices

nx - X values of clipped polygon

ny - Y values of clipped polygon

minX - Minimum X-value

minY - Minimum Y-value

maxX - MAximum X-value

maxY - Maximum Y-value

Returns:

Number of vertices in clipped polygon.

printVert

protected static void printVert(int n,
                                double[] x,
                                double[] y,
                                java.lang.String label)

Debugging routine that prints a list of vertices.

Parameters:

n - The number of vertices in the polygon

x - X coordinates

y - Y coordinates

sample

public void sample(int pix)

Implement the sample function of the Sampler class. This function samples a single pixel.

Specified by:

sample in class Sampler

Parameters:

pix - The index of the pixel.

weight

public double weight()

Return the weight associated with the last sampling. For the Clip sampler, the weight can be associated with the 'area' of the pixel.

samplePixel

public void samplePixel(int pix,
                        double[] x,
                        double[] y)

Sample a single map pixel.

Parameters:

x - The x values of the corners of the pixel [4]

y - The y values of the corners of the pixel [4]

clipRectPoly

public double clipRectPoly(double xmin,
                           double ymin,
                           double xmax,
                           double ymax,
                           double[] polyx,
                           double[] polyy)

This method provides an interface to the simplified Sutherland-Hodges clipper for use outside the sampling context.

Parameters:

polyx - The vertices of an (assumed convex) polygon.

polyy - The vertices of an (assumed convex) polygon.

Returns:

The area of the overlap.

lastX

public double[] lastX()

lastY

public double[] lastY()