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Posted 23 Jul 2007

# Mapping Images on Spherical Surfaces Using C#

, 15 Jul 2016 GPL3
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Mapping images on spherical surfaces using C#

## Introduction

This article describes how to map a flat 2D image (JPG, BMP, or GIF) on a sphere by using basic algebra.

The process is very simple where the x axis of the image will be mapped on sphere longitudes and the y axis of the image will be mapped on sphere latitudes.

The process of mapping is similar to proportion equations x-x0/y-y0 = px-x0/py-y0

```public static double MapCoordinate(double i1, double i2, double w1,
double w2, double p)
{
return ((p - i1) / (i2 - i1)) * (w2 - w1) + w1;
}```

Original image

Resulting image

## Background

### A Sphere Can Be Represented by Spherical Coordinates in R3

• phi (latitude angle)
• theta (longitude angle)

#### Image 2

• Where radius is a constant, phi=[-PI/2,PI/2], and theta=[0,2*PI]

### To Find the Cartesian Coordinates from Spherical Coordinates

• x = radius * sin(phi) * cos(theta)
• y = radius * sin(phi) * sin(theta)
• z = radius * cos(theta)
```double phi0 = 0.0;
double phi1 = Math.PI;
double theta0 = 0.0;
double theta1 = 2.0*Math.PI;```

## The Code

```System.Drawing.Image image1 = new Bitmap(Server.MapPath(
"./images/worldmap4.gif"));
Bitmap imgBitmap = new Bitmap(image1);```

Now we make a loop through the 2 dimensions of the image, map phi and theta angles from image coordinates, get the cartesian 3D coordinates from phi and theta, provide some rotation to the obtained 3D points and plot them with respective image color:

```for (int i = 0; i < imgBitmap.Width; i++)
{
for (int j = 0; j < imgBitmap.Height; j++)
{
// map the angles from image coordinates
double theta = Algebra.MapCoordinate(0.0, imgBitmap.Width - 1,
theta1, theta0, i);
double phi = Algebra.MapCoordinate( 0.0, imgBitmap.Height - 1,phi0,
phi1, j);
// find the cartesian coordinates
double x = radius * Math.Sin(phi) * Math.Cos(theta);
double y = radius * Math.Sin(phi) * Math.Sin(theta);
double z = radius * Math.Cos(phi);
// apply rotation around X and Y axis to reposition the sphere
RotX(1.5, ref y, ref z);
RotY(-2.5, ref x, ref z);
// plot only positive points
if (z > 0)
{
Color color = imgBitmap.GetPixel(i, j);
Brush brs = new SolidBrush(color);
int ix = (int)x + 100;
int iy = (int)y + 100;
graphics.FillRectangle(brs, ix, iy, 1, 1);
brs.Dispose();
}
}
}```

## The Rotation Functions [almost forgot]

Actually I made a 3D Math class, but here you will need only these functions

```public static void RotX(double angle, ref double y, ref double z)
{
double y1 = y * System.Math.Cos(angle) - z * System.Math.Sin(angle);
double z1 = y * System.Math.Sin(angle) + z * System.Math.Cos(angle);
y = y1;
z = z1;
}
public static void RotY(double angle, ref double x, ref double z)
{
double x1 = x * System.Math.Cos(angle) - z * System.Math.Sin(angle);
double z1 = x * System.Math.Sin(angle) + z * System.Math.Cos(angle);
x = x1;
z = z1;
}
public static void RotZ(double angle, ref double x, ref double y)
{
double x1 = x * System.Math.Cos(angle) - y * System.Math.Sin(angle);
double y1 = x * System.Math.Sin(angle) + y * System.Math.Cos(angle);
x = x1;
y = y1;
}```

See sample

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 How do you avoid clashes with remapped pixels? JWhattam11-Nov-15 12:47 JWhattam 11-Nov-15 12:47
 Can not find the complete ource code mans.098724-Jan-13 5:02 mans.0987 24-Jan-13 5:02
 My vote of 5 manoj kumar choubey26-Feb-12 21:44 manoj kumar choubey 26-Feb-12 21:44
 Any ideas to make it faster? Tiramisung1-Apr-10 16:16 Tiramisung 1-Apr-10 16:16
 Re: Any ideas to make it faster? andalmeida2-Apr-10 3:55 andalmeida 2-Apr-10 3:55
 Re: Any ideas to make it faster? Tiramisung4-Apr-10 22:07 Tiramisung 4-Apr-10 22:07
 Re: Any ideas to make it faster? simonxy17-Aug-15 23:55 simonxy 17-Aug-15 23:55
 I really upset! Mr. Cencious12-Nov-07 23:28 Mr. Cencious 12-Nov-07 23:28
 Dear friend, I really upset with the program because it used a Library name Algebra which is not exists in my computer. Can you give me the link to download?? Eg: Algebra.RotX(Math.PI / 2, ref rty, ref rtz); this line showns an error www.socbay.com - Best data center - search engine in Vietnam
 Re: I really upset! andalmeida13-Nov-07 0:56 andalmeida 13-Nov-07 0:56
 Re: I really upset! Mr. Cencious15-Nov-07 16:15 Mr. Cencious 15-Nov-07 16:15
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