Using OpenCV with OpenGL for texture mapping

[jenny_wui]

Hello,
I would like to use opencv with openGL for texture mapping. With the use of opencv, channel RGB values can be manipulated. Now, suppose, my texture XY coorodinate range is(-4, -4) to (4, 4). My image has been texture mapped to the window with dimension (500, 500). For mapping between coordinate and pixel, I have used the following relationship:
　
　
x,y (0,0) to (8,8) mapping to pixel for 0,0 to (500, 500);
now for a particular (x,y)
x += 4 ;
if ( y >= 0)
y = 4 - y;
else
y = 4-(-y);
corresponding pixel value (x*500/8) and (y*500/8);
Is this mapping ok between x,y coordinate and pixel or any better method is available?

[jenny_wui]

Hi,I need the reply to my query a bit urgently. Please let me know whether I can separate the channel RGB values using OpenGL. As I would like to touch some point of my textured plane and I am interested to know what is that RGB value of pixel at that point. I am not using mouse, I am using a haptic device to touch the plane. Thanks

[tonyo_au]

glReadPixels will allow you to read from the render buffer.

[jenny_wui]

Thanks for the reply. I have some simple question:

1) Do I need to use glOrtho to use glReadPixels; doesn't it work for gluPerspective;
2)Do I need to make the textured image mapped totally to the screen to use glReadPixels?
3) In the following formula:
void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid* data);

x, y are the position of the current pixel,
width, height is ( 1, 1) if correspondsto 1 pixel, can it correspond to more than one, then what value will be retured for different RGB value?

It would be highly appreciated if my question is answered. Thanks in advance.

[tonyo_au]

1) mapping doesn't matter
2) you need to bind the buffer you wish to read; are you rendering to a frame buffer object? Is so, it must be bound for reading or read/write before you call glReadPixels. If not
glReadBuffer(GL_FRONT) reads the front buffer and glReadBuffer(GL_BACK) reads the back buffer.

3) checkout the full description of glReadPixels there are many options.
http://www.opengl.org/sdk/docs/man/x...ReadPixels.xml

[jenny_wui]

Thanks for the reply. If some example codes are given for the different cases mention in (2), it would be easier for me to understand.

[tonyo_au]

Do you need an example for a frame buffer object or just the default render buffer?