# OpenGL Texture Mapping [Equirectangular to Azimuthal Equidistant]

4 replies to this topic

### #1 felixrulz Posted 18 July 2012 - 09:10 PM

felixrulz

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Hey, this is my first time here... was looking for some help from some cartography savy people

I have created an almost spherical disk by joining lots of triangular/rectangular polygons together and have a equidistant image of the earth loaded as a texture map. What I am having trouble doing is converting/mapping the azimuthal equidistant points on the disk to a latitude and longitude so I can determing the texture coordinate. Here is a snippet of code where the disk is created:

```void CreateDisk (double R, double x0, double y0, double z0)

{

const double theta_inc = 2*PI/(disk_az_divs);

double latitude, longitude;

int n=0,i; // Current vertex

{

for(theta=0; theta <= PI2; theta+=theta_inc)

{

a2[0] = theta;

a2[1] = theta;

a2[2] = theta + theta_inc;

a2[3] = theta + theta_inc;

for (i=0; i<4; i++)

{

// disk coordinates

p_d[n].x = b2[i] * cos(a2[i]) + x0;

p_d[n].y = b2[i] * sin(a2[i]) + y0;

p_d[n].z = z0;

// TEXTURE COORDINATES CALCULATED HERE

n++;

}

}

}

}```

The use of b2[] and a2[] are to calculate the other points of the rectangle. And are not really relavent to the problem I am havving (I think) so this code can be interpreted like this for simplicity:

```void CreateDisk (double R, double x0, double y0, double z0)

{

const double theta_inc = 2*PI/(disk_az_divs);

double latitude, longitude; //used to calculate texture coordinates

int n=0 // Current vertex

{

for(theta=0; theta <= PI2; theta+=theta_inc)

{

// disk coordinates

p_d[n].x = b2[i] * cos(a2[i]) + x0;

p_d[n].y = b2[i] * sin(a2[i]) + y0;

p_d[n].z = z0;

// TEXTURE COORDINATES CALCULATED HERE

n++;

}

}

}```

This code starts with a radius of 0 and then will sweep through theta (the azimuth of the map). Repeating with larger radii untill reaching R. Here is an example of the resulting disk:

The problem I am havving is converting the cartesian (or polar if this is easier) coordinates to a latitude and longitude for the equirectangular map (This needs to be inserted where indicated in the code sample). I have managed managed a polar aspect by simply using a ratio of the radius and azimuth. (Note that the +0.5 and /PI2 and /R are to convert/normalise the latitude and longitude into a 0:1 range which is required by OpenGL.

```p_d[n].u = 0.5 + a2[i]/PI2; // + rotation

p_d[n].v = (b2[i])/R;```
This is the resulting image:

For the oblique projection I am struggeling. The latest equations I have been trying to use where taken from Map Projections – A Working Manual (pg 196) http://books.google....d...&pg=GBS.PR1,
the equations are reproduced here at Wolfram http://mathworld.wol...Projection.html.

I implemented them in code as follows. b2[] can simply be considered a radius and a2[] an azimuth, p_d[n].y/x are the x/y coordinates:
```if (radius == 0)

{

latitude = lat0;

longitude = long0 + theta;

}

else

{

latitude = asin( cos(b2[i])*sin(lat0) + (p_d[n].y*sin(b2[i])*cos(lat0)/b2[i]) );

if (lat0 == PI_2)

longitude = long0 + atan( -p_d[n].x/p_d[n].y );

else if (lat0 == -PI_2)

longitude = long0 + atan( p_d[n].x/p_d[n].y );

else

longitude = long0 + atan( p_d[n].x*sin(b2[i])/( b2[i]*cos(lat0)*cos(b2[i])-p_d[n].y*sin(lat0)*sin(b2[i]) ) );

}

// convert lat/long to texture coordinates

p_d[n].u = longitude/PI2;

p_d[n].v = 0.5 + latitude/PI;```

The results seem somewhat correct but I don't even get the entire azimuthal projection (for example with australia at the centre you can't see much else). I have been trying to fix this for near a week and thought I should ask for help.

Thanks.

### #2 Michael Schmeling Posted 19 July 2012 - 03:11 AM

Michael Schmeling

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• Germany

The PROJ.4 - Cartographic Projections Library (written in C) may help you with the calculations.

### #3 felixrulz Posted 22 July 2012 - 10:44 PM

felixrulz

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The PROJ.4 - Cartographic Projections Library (written in C) may help you with the calculations.

Thanks Michael, I'll take a look

Edit: Hopfully this works, seems that none of the posts I have made are showing up. See if it will accept this edit...

I think I have solved most of the mapping problems. I ended up using the method derived by T.I here: http://somedayspace....ection-and.html.

After testing formulas in MALTAB I got promising results:

I then converted to a function in C and tested. I seem to be very close now; there is just some detracting distortion where the image is joined along the date line:

I think the problem problem is due to the wrapping of texture coordinates. I have tried to illustrate the problem in the figure below. The program is currently mapping from A-->B so the distorted sectors are displaying a compressed version of nearly the entire map. What should happen instead is to map from A wrapping to B (to the right). In OpenGL I think this can be achieved by enabling GL_REPEAT and simple using that B'=B+1 to get the correct texture response.

But when I added this check:
```if (p_d[n].u < p_d[n-1].u)

p_d[n].u += 1;```
only the problem from one of the polar aspects was fixed (and not at all from oblique aspects).

Any ideas? Here is a link to the program I have so far if it will help with diagnoses: http://dl.dropbox.co.....ery.close].7z
You will need the data folder with the image (and maybe some dll files I haven't included). The arrow keys control the centre of the projection, 't' toggles the texture, 'up-pg' 'dn-pg' zoom in and out.

[EDIT]
The problem is no longer cartography related so I i'm not sure If I should keep posting here.
I have solved the majority of the problems using the method I have illustrated in this short video:
http://dl.dropbox.co...[EvenCloser].7z

Edited by felixrulz, 28 July 2012 - 02:30 AM.

### #4 felixrulz Posted 23 July 2012 - 08:58 PM

felixrulz

Newbie

• Validated Member
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Thanks Michael, I'll take a look

Ok, so there is a function for Azimuthal Equidistant: "PJ_aeqd.c" it seems to include many variations I didn''t even know about...
This seems to be the section i am interested in:

```INVERSE(s_inverse); /* spherical */

double cosc, c_rh, sinc;

if ((c_rh = hypot(xy.x, xy.y)) > PI) {		[color="#FF0000"][b]// for some reason the radius cannot be larger than PI[/b][/color]

if (c_rh - EPS10 > PI) I_ERROR;   [color="#FF0000"] [b]// EPS10 is a small (1.e-10) used to prevent problems with rounding errors[/b][/color]

c_rh = PI;

} else if (c_rh < EPS10) {						   [color="#FF0000"] [b]// if at the centre of the projection just use centre lat/long[/b][/color]

lp.phi = P->phi0;

lp.lam = 0.;

return (lp);

}

if (P->mode == OBLIQ || P->mode == EQUIT)

{

sinc = sin(c_rh);								[color="#FF0000"][b]//precalculate for speed[/b][/color]

cosc = cos(c_rh);							   [color="#FF0000"] [b]//precalculate for speed[/b][/color]

if (P->mode == EQUIT)

{

[color="#FF0000"][b]//calculate latitude[/b][/color]

lp.phi = aasin(P->ctx,	 xy.y * sinc / c_rh);

xy.x *= sinc;

xy.y = cosc * c_rh;

}

else

{

[color="#FF0000"][b]//calculate latitude

a bit different to what they have, I'll try to find where they define aasin (haven't been able to yet)

[/b][/color]

lp.phi = aasin(P->ctx,cosc * P->sinph0 + xy.y * sinc * P->cosph0

c_rh);

xy.y = (cosc - P->sinph0 * sin(lp.phi)) * c_rh;

xy.x *= sinc * P->cosph0;

}

[color="#FF0000"][b]//calculate longitude

for the oblique case I used: longitude = long0 + atan2( p_d[n].x*sin(b2[i]),( b2[i]*cos(lat0)*cos(b2[i])-p_d[n].y*sin(lat0)*sin(b2[i]) ) );

[/b][/color]

lp.lam = xy.y == 0. ? 0. : atan2(xy.x, xy.y);

}

else if (P->mode == N_POLE)

{

lp.phi = HALFPI - c_rh;

lp.lam = atan2(xy.x, -xy.y);

}

else

{

lp.phi = c_rh - HALFPI;

lp.lam = atan2(xy.x, xy.y);

}

return (lp);```

I'll try and work out why their function is different.

Cheers

### #5 felixrulz Posted 25 July 2012 - 08:33 AM

felixrulz

Newbie

• Validated Member
• 6 posts
• Australia

Ok, so I think I have solved most of the mapping problems. I ended up using the method derived by T.I here: http://somedayspace....ection-and.html.

After testing formulas in MALTAB I got promising results:

I then converted to a function in C and tested. I seem to be very close now; there is just some detracting distortion where the image is joined along the date line:

I the problem (at least I think) is due to the wrapping of texture coordinates. I have tried to illustrate the problem in the figure below. The program is currently mapping from A-->B so the sector has a compressed version of nearly the entire map; what we want is to map from A to the right wrapping to B. In OpenGL I think this can be achieved by enabling GL_REPEAT and simple using that B'=B+1 to get the correct texture response.

```if (p_d[n].u < p_d[n-1].u)

p_d[n].u += 1;```
but this only fixed them problem from one of the polar aspects.

Let me know if you have any ideas on hot to fix this. Here is a link to the program I have so far: if it helps diagnose (or if you just want a look).
You will need the data folder with the image (and maybe some dll files I haven't included). The arrow keys control the centre of the projection, 't' toggles the texture, 'up-pg' 'dn-pg' zoom in and out.

Problem solved:
See hereProblem Solved

Not sure why but new replies won't work so i added info in this one

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