Picking glReadPixels, gluUnProject and resize

[Steff]

I work in the moment on a c++ SDL/OpenGL 3D application and run into a problem with picking using glReadPixels and gluUnProject.
Everything works fin till I resize the window.

The problem is that glReadPixels don´t give me a right value for the new screen reagions. I get always the value 8.96831e-044. in this reagions. Else I get back 0.0f till 1.0f.

Here my resize function :

void Video::resize( int xres_, int yres_ )
{
_xres = xres_;
_yres = yres_;
_ratio = static_cast<float>( xres() ) / static_cast<float>( yres() );
 
SDL_Rect rect = { 0, 0, xres(), yres() };
SDL_SetClipRect( _primary, &amp;rect );
 
glViewport( 0.0f, 0.0f, xres(), yres() );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
gluPerspective( fov(), ratio(), nearclip(), farclip() );
glMatrixMode( GL_MODELVIEW );
}

The rendering works fine after resize only the picking fails. The reagion from the left lower screen corner work fine. The new reagions above and right beside don´t work.

[mobeen]

Are you getting the current modelview and projection matrices when you call glReadPixels?

[Steff]

Yes I do. In the old parts of the screen it works only the new areas don´t give back a working Z value

Here my picking part of the code:

screenX + screenY = mouse positions;

GLdouble modelview[16];
glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
 
GLdouble projection[16];
glGetDoublev( GL_PROJECTION_MATRIX, projection );
 
GLint viewport[4];
glGetIntegerv( GL_VIEWPORT, viewport );
 
GLfloat winX, winY, winZ;
GLdouble posX, posY, posZ;
 
winX = (float)screenX;
winY = (float)viewport[3] - (float)screenY;
 
glReadPixels( screenX, int(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ );
 
// winZ hase NOW wrong value!
 
gluUnProject( winX, winY, winZ, modelview, projection, viewport, &posX, &posY, &posZ);

I think it could have something to do with missing depth informations for the new reagions. (GL_DEPTH_COMPONENT)

[mobeen]

Hmm. Could u recheck that the depth test is enabled and you are clearing the depth buffer along with the color buffer in the render function that is

glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);

and that the picking is after the rendering.

[Steff]

Yes everythings ok.
depth test is enabled.
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT); called in render function.
Picking is after the rendering.

I search for this now over 5 houres...
Hope someone have a hint that helps.

[mobeen]

Hi just noted this

SDL_Rect rect = { 0, 0, xres(), yres() };
SDL_SetClipRect( _primary, &rect );

why do u set the clip rect? and are xres() and yres() returning the current width and height?

[Steff]

Yes xres() and yres() are inline functions returning the values.
The application is not realy smale and not started by me.

I have commented out this lines and it works all like before.
But picking is buged.

[mobeen]

Your code looks fine to me. My suspicion is probably some code path is modifying you modelview/projection matrices. Could u recheck using debugger whether these matrices have their original values?