LucaFr

04-13-2011, 04:19 AM

Hi,

suppose i have a texture of WxH (not a power of two texture).

I use it to texture a quad with the same aspect ratio. This quad can be rotated and scaled before it is rendered on screen.

Now, i have a screen pixel (x,y) and i want to know to which texture pixel it correponds (in my application, i am sure that (x,y) is a pixel that belongs to the textured quad).

I use gluUnproject using the x,y coordinates of the pixel in this way (the screen has resolution ScreenW X ScreenH):

// This is the rendering of the quad

// w and h is the width and height of the texture

// refImageScale = w/h

glBindTexture(GL_TEXTURE_2D, texture[0]);

glBegin(GL_QUADS);

glTexCoord2f(0.0f, 1.0f);

glVertex3f (-1*refImageScale, -1, 0);

glTexCoord2f(1.0f, 1.0f);

glVertex3f (1*refImageScale, -1, 0);

glTexCoord2f(1.0f, 0.0f);

glVertex3f (1*refImageScale, 1, 0);

glTexCoord2f(0.0f, 0.0f);

glVertex3f (-1*refImageScale, 1, 0);

glEnd();

// This code will get model view, projection and viewport

// matrices

GLdouble mvMat[16], prjMat[16];

GLint viewport[4];

glGetDoublev(GL_MODELVIEW_MATRIX, mvMat);

glGetDoublev(GL_PROJECTION_MATRIX, prjMat);

glGetIntegerv(GL_VIEWPORT, viewport);

// Read back the depth buffer

glReadPixels(0, 0, ScreenW, ScreenH, GL_DEPTH_COMPONENT, GL_FLOAT, depthBuffer);

// Suppose we have a pixel on the screen with coordinates (x,y)

// Unproject screen pixel to retrieve object coordinates

gluUnProject(x, viewport[3] - y,(double)depthBuffer[(viewport[3]-y) * ScreenW + x],

mvMat,

prjMat,

viewport,

&x1, &y1, &z1);

After that, i want to compute the coordinates of the texture pixel that corresponds (at least approximately) to the screen pixel:

// w and h is the width and height of the texture

// refImageScale = w/h

x1 = (x1 + refImageScale)/(2.0*refImageScale);

y1 = (-y1 + 1.0)/2.0;

x1 = ROUND(x1 * (double)(w-1));

y1 = ROUND(y1 * (double)(h-1));

It does not seems too precise. Am i doing things in the right way??

Thank you,

Luca

suppose i have a texture of WxH (not a power of two texture).

I use it to texture a quad with the same aspect ratio. This quad can be rotated and scaled before it is rendered on screen.

Now, i have a screen pixel (x,y) and i want to know to which texture pixel it correponds (in my application, i am sure that (x,y) is a pixel that belongs to the textured quad).

I use gluUnproject using the x,y coordinates of the pixel in this way (the screen has resolution ScreenW X ScreenH):

// This is the rendering of the quad

// w and h is the width and height of the texture

// refImageScale = w/h

glBindTexture(GL_TEXTURE_2D, texture[0]);

glBegin(GL_QUADS);

glTexCoord2f(0.0f, 1.0f);

glVertex3f (-1*refImageScale, -1, 0);

glTexCoord2f(1.0f, 1.0f);

glVertex3f (1*refImageScale, -1, 0);

glTexCoord2f(1.0f, 0.0f);

glVertex3f (1*refImageScale, 1, 0);

glTexCoord2f(0.0f, 0.0f);

glVertex3f (-1*refImageScale, 1, 0);

glEnd();

// This code will get model view, projection and viewport

// matrices

GLdouble mvMat[16], prjMat[16];

GLint viewport[4];

glGetDoublev(GL_MODELVIEW_MATRIX, mvMat);

glGetDoublev(GL_PROJECTION_MATRIX, prjMat);

glGetIntegerv(GL_VIEWPORT, viewport);

// Read back the depth buffer

glReadPixels(0, 0, ScreenW, ScreenH, GL_DEPTH_COMPONENT, GL_FLOAT, depthBuffer);

// Suppose we have a pixel on the screen with coordinates (x,y)

// Unproject screen pixel to retrieve object coordinates

gluUnProject(x, viewport[3] - y,(double)depthBuffer[(viewport[3]-y) * ScreenW + x],

mvMat,

prjMat,

viewport,

&x1, &y1, &z1);

After that, i want to compute the coordinates of the texture pixel that corresponds (at least approximately) to the screen pixel:

// w and h is the width and height of the texture

// refImageScale = w/h

x1 = (x1 + refImageScale)/(2.0*refImageScale);

y1 = (-y1 + 1.0)/2.0;

x1 = ROUND(x1 * (double)(w-1));

y1 = ROUND(y1 * (double)(h-1));

It does not seems too precise. Am i doing things in the right way??

Thank you,

Luca