• Advertisement
Sign in to follow this  

OpenGL getting the coordinates of the world when picking

This topic is 4778 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

hi, i have a project with matrix based camera i.e. - no glulookat, and i render a city like world with huge amouts of polygons and allow user to pick the individual objects, but i would like to know where did the user actualy click (XYZ) coordinates, and if there is an easy solution for this, basicly all i have is the camera matrix and the hit distance that i get from opengl picking rutines but i need the coordinates of the hit :(

Share this post

Link to post
Share on other sites
Hi there, you can try to read the pixels.
This is how I do it.

Vector3 WorldCoords;

void GetGLPos(int x, int y)
static GLint viewport[4];
static GLdouble modelview[16];
static GLdouble projection[16];
static GLfloat winX, winY, winZ;

double posX, posY, posZ;

glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
glGetDoublev( GL_PROJECTION_MATRIX, projection );
glGetIntegerv( GL_VIEWPORT, viewport );

winX = (float)x;
winY = (float)y;

glReadPixels(int(winX), int(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ );
gluUnProject( winX, winY, winZ, modelview, projection, viewport, &posX, &posY, &posZ);

WorldCoords.x = (float)posX;
WorldCoords.y = (float)posY;
WorldCoords.z = (float)posZ;

the GetGLPos function takes the mouse x,y 2D coords, then return the 3D coords xPos, yPoz, zPos to the WorldCoords vector.

hope it helps.

Share this post

Link to post
Share on other sites
this seem to do something, i used this pascal equalent:

function TGLView.getdepthhitat(X, Y: LongWord): TVector3f;
viewport: TVector4i;
modelview: TMatrix4d;
projection: TMatrix4d;

winX, winY, winZ: GLfloat;
posX, posY, posZ: double;

glGetDoublev( GL_MODELVIEW_MATRIX, @modelview );
glGetDoublev( GL_PROJECTION_MATRIX, @projection );
glGetIntegerv( GL_VIEWPORT, @viewport );

winX:= x;
winY:= y;

glReadPixels( X, Y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, @winZ );
gluUnProject( winX, winY, winZ, @modelview, @projection, @viewport, posX, posY, posZ);

result[0]:= posX;
result[1]:= posY;
result[2]:= posZ;


but the coordinates are all wrong, sometimes it looks like that they are
relative to something else?

Share this post

Link to post
Share on other sites
How do you call the function?

for example how to get the coords on an terrain, you do something like this.


GetGLPos(cursor.x, cursor.y);


Now you get the right coords for the terrain, so you dont call the GetGLpos
before the render.

Share this post

Link to post
Share on other sites
why do i have to pop the matrixes?
my state of renderer when i call the func is in the
modelview and all stacks are popped back to zero already :/

Share this post

Link to post
Share on other sites
this is what i do on my terrain when i want to get the world coordinates. first you have to pass in the mouse position before converting such as:

void myGlutMouse(int button, int button_state, int x, int y)
if ( button == GLUT_LEFT_BUTTON && button_state == GLUT_DOWN )
if(current_terrain->world_coords(x, y, world_pick))
current_terrain->get_world(world_pick[0], world_pick[1], world_pick[2]);


Now in my class those two functions:

bool myClass::world_coords(GLuint x, GLuint y, GLdouble *wpos)
GLdouble model[16], proj[16];
GLint view[4];
GLfloat z;
glClear(GL_DEPTH_BUFFER_BIT); //clear the depth buffer for reading

glGetDoublev(GL_MODELVIEW_MATRIX, model);
glGetDoublev(GL_PROJECTION_MATRIX, proj);
glGetIntegerv(GL_VIEWPORT, view);

y = view[3] - (GLfloat) y - 1;

glReadPixels(x, y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &z); //get the pixel(in 2D) w/respect to 3D Coordinates

if(z == 1.0) //depth buffer max depth 1.0 is the default, PIXEL at mouse pointer is below the heightmap plane(terrain)
return FALSE;

gluUnProject(x, y, z, model, proj, view, wpos, wpos+1, wpos+2);

return TRUE;


if you clicked on the terrain successfully the gluUnProject returns to you the XYZ world coords back to main if you look carefully. the get_world function from main simply assigns the XYZ world coordinates to a private array in my class and is used for things like drawing a sphere at the place i picked on my terrain or simply some information from the data that is displayed from the screen. hope this helps!

Share this post

Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
    • By QQemka
      Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
      Let's go:
      Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
      Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
      Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
      What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
      There were several more but i forgot/solved them at time of writing
      Thanks in advance
    • By RenanRR
      Hi All,
      I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
      I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
      Vertex Shader:
      #version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
      ..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);  
      So, some doubts:
      - Why use it like that?
      - Is it okay to manipulate the camera that way?
      -in this way, are not the vertex's positions that changes instead of the camera?
      - I need to pass MVP to all shaders of object in my scenes ?
      What it seems, is that the camera stands still and the scenery that changes...
      it's right?
      Thank you
    • By dpadam450
      Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

      int rgbValue = int(textureSample.w);//4 bytes of data packed as color
      // algorithm might not be correct and endianness might need switching.
      vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
      extractedData /= 255.0f;
    • By Devashish Khandelwal
      While writing a simple renderer using OpenGL, I faced an issue with the glGetUniformLocation function. For some reason, the location is coming to be -1.
      Anyone has any idea .. what should I do?
  • Advertisement