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OpenGL Points Selection

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Can someone help me with selection? I am drawing a big cloud of points (over 5000) and want to select the points with mouse and highlight them (nearest single point). The example with planets from opengl books doesn't help me much, because I am not defining any names to the points and I not using SWITCH function (too much data). Thanks

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Quote:
Original post by backspin
Can someone help me with selection? I am drawing a big cloud of points (over 5000) and want to select the points with mouse and highlight them (nearest single point). The example with planets from opengl books doesn't help me much, because I am not defining any names to the points and I not using SWITCH function (too much data). Thanks
I would use a 'picking volume' for this. Using this method, you would build a list of points contained by the volume, and then select the one closest to the camera view plane.

The details kind of depend on what tools you're using. Are you using a math library of some sort? What language are you programming in?

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I am writing an application in Visual C++ 6.0 in MFC. I am not using any complicated tools or methods. I was trying to use GLUT library, but the effect was not so good. I just want to draw point (already done) and select them with mouse click.

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Ok, but what with the Names? Should I give every single point a name? I was also thinking about reading OpenGL coordinates and then after mouse click save one(x,y) and compare with the rest, but somethings wrong with my points translation and also I get huge numbers after rotating, moving or zooming.

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It seems likely that your transformation problems stem from getting confused between object space, camera space and screen space.

Between the world and the screen a vertex is subjected to a shearing (perspective) and projection transformation. It gets 'flattened' into screen x,y coordinates and given a depth value. These screen coordinates are not the same as the world coordinates, even if your screen and world axes are aligned, and the depth value is not even the same type of value as a world z value... it is more like a layer number in photoshop. The original 3D data about the vertex is discarded, which is why GL employs the technique of redrawing objects to a selection buffer and giving them names so you can tell what it is that you picked.

If you want each individual point to be selectable then you give them each a name (an integer value)... if they are all in one array then you can use the index value as the name, then when your picking result is the name '27' you know that vertex[27] was selected.

[Edited by - ElectricVoodoo on February 13, 2008 5:49:17 AM]

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thats my drawing function:

for(int i=0; i<points_limit; i++)
{
glBegin(GL_POINTS);
glLoadName(i);
glVertex3f(xpoint, ypoint, zpoint);
glEnd();

if(i<points_limit && line_link[i+1]=="" && lines_or_dots==0)
{
glBegin(GL_LINES);
glLoadName(i);
glVertex3f(xpoint, ypoint, zpoint);
glVertex3f(xpoint[i+1], ypoint[i+1], zpoint[i+1]);
glEnd();
}
}

my question is the LoadName() function correct? Then I get no errors or warnings in my code by still the hits value is zero. I know that the point are very small and it is hard to hit one but I have a big cloud of point, so I think I shoud hit something. Is there something to set the range around a point?

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I found out when I enable glSwapBuffers() the application crashes. I included glut.h and also glup32.lib

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Have you created a suitable buffer and passed it to GL with glSelectBuffer()?
Have you properly initialized the name stack by calling glInitNames and glPushName(0)?
Have you enabled glRenderMode(GL_SELECT)?
Have you taken into account that GL screen coordinates start from the bottom left although window coordinates start from the top left?

To increase the size of the hit region:

gluPickMatrix(mousex,
mousey,
PICK_REGION_SIZE, PICK_REGION_SIZE,
viewport);

PICK_REGION_SIZE is the size of your square hit region.

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It is much faster if each point is rendered with another color into the back buffer (16m or 2³² should be enough) After that read the pixel below the mouse with glReadPixel. The opengl picking is not supported in hardware on many cards...

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Thats the function I wanted to read pixels. Values winX and winY are correctly transfered from OnLButtonDown(UINT nFlags, CPoint point) funtion, but the output values objX and objY are far away from correct results. As I wrote before, after zoom, move or rotation is much worse.

GLdouble winX;
GLdouble winY;
GLdouble winZ;
GLdouble objX;
GLdouble objY;
GLdouble objZ;


GLdouble modelMatrix[16];
glGetDoublev(GL_MODELVIEW_MATRIX,modelMatrix);
GLdouble projMatrix[16];
glGetDoublev(GL_PROJECTION_MATRIX,projMatrix);
int viewport[4];
glViewport(0, 0, mx, my);
glGetIntegerv(GL_VIEWPORT,viewport);
winX = (float)point.x; //MousePosX
winY = (float)viewport[3] - (float)point.y;
glReadPixels( winX, int(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ );
gluUnProject(
winX,
winY,
winZ,
modelMatrix,
projMatrix,
viewport,
&objX,
&objY,
&objZ
);

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...good idea oc2k1. Although that method will hit problems in complex picking scenarios such as group selections, for simpler problems like this one it's great. I'm sure I'll find a use for this myself. Thanks!

[Edited by - ElectricVoodoo on February 13, 2008 6:32:54 AM]

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glSelectBuffer, glPushName(0), GL_SELECT are set as in exemple from link and coordinates are gluPickMatrix(xPos, viewport[3] - yPos, 1.0, 1.0, viewport); But I still get value zero in hits. Could be because of this glutSwapBuffers() ???

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That's the function is use and get no hits:

void COpenGLControl::ProcessSelection(int xPos, int yPos)
{
const int BUFFER_LENGTH = 64;
GLuint selectBuff[BUFFER_LENGTH]={0};
GLint hits, viewport[4];
glSelectBuffer(BUFFER_LENGTH, selectBuff);
glGetIntegerv(GL_VIEWPORT, viewport);
glRenderMode(GL_SELECT);
glInitNames();
glPushName(0);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
float PICK_REGION_SIZE = 20.0f;
//gluPickMatrix(xPos, viewport[3] - yPos, 1.0, 1.0, viewport);
gluPickMatrix(xPos, viewport[3] - yPos, PICK_REGION_SIZE, PICK_REGION_SIZE, viewport);
gluPerspective(35.0, mx / my, 0.01f, 2000.0f);
glMatrixMode(GL_MODELVIEW);
//glutSwapBuffers(); //when enable application crashes
oglDrawScene(); //my drawing function
glMatrixMode(GL_PROJECTION);
glPopMatrix();
hits = glRenderMode(GL_RENDER);
if(hits == 1)
{
AfxMessageBox("HIT");
MakeSelection(selectBuff[3]);
}
glMatrixMode(GL_MODELVIEW);
}

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Certainly you should not swap buffers in selection mode. glutSwapBuffers is a framebuffer function and we have been rendering to our own selection buffer, not the frame buffer.

In your draw function you should move your glLoadName() calls to OUTSIDE the glBegin()...glEnd() blocks as I don't think they work inside these blocks.

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For what it's worth, I have created modules for color picking and selection picking, and the speed differences are minimal. My app displays large amounts of primitives, so I would think the original poster would have the same results.

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