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Thread: Getting screen location of 3D point.

  1. #1
    Junior Member Newbie
    Join Date
    Apr 2017

    Getting screen location of 3D point.

    I am trying to get the screen location of a 3D point.
    I scraped together code from some online sources into this:
    Code :
    glm::vec2 ProjPoint(Cam* camera, glm::vec3 p) {
    glm::vec3 front;
    	front.x = cos(glm::radians(camera->pitch)) * cos(glm::radians(camera->yaw));
    	front.y = sin(glm::radians(camera->pitch));
    	front.z = cos(glm::radians(camera->pitch)) * sin(glm::radians(camera->yaw));
    	front = glm::normalize(front);
    	glm::mat4 projection = glm::perspective(glm::radians(fov), float(width) / float(height), 0.1f, 100.0f);
    	glm::mat4 view = glm::lookAt(camera->pos, camera->pos + front, glm::vec3(0.0f, 1.0f, 0.0f));
    	glm::mat4  viewProjectionMatrix = projection * view;
    	glm::vec4 point = viewProjectionMatrix * glm::vec4{ p.x,p.y,p.z,1 };
    	int winX = (int)(((point.x + 1) / 2.0) * float(width));
    	int winY = (int)(((1 - point.y) / 2.0) * float(height));
    	return glm::vec2(winX, winY);
    What am i doing wrong? Since i have never used matrices till now ( was just useing glTranslate and glRotate), i dont know how to debug this.

    Well, to answer my own question, i got it to work!
    I changed the ProjPoint function to:
    Code :
    	GLint viewport[4];
    	GLdouble mvmatrix[16], projmatrix[16];
    		GLdouble wx, wy, wz;
    		glGetIntegerv(GL_VIEWPORT, viewport);
    	glGetDoublev(GL_MODELVIEW_MATRIX, mvmatrix);
    	glGetDoublev(GL_PROJECTION_MATRIX, projmatrix);
    	gluProject(-p.x,p.y,-p.z, mvmatrix, projmatrix, viewport, &wx, &wy, &wz);
    	return glm::vec2{ wx ,wy };

    However what i still dont understand is the weird translation i have to do before calling it:
    Code :
    glRotatef(-camera.pitch, 1.0f, 0.0f, 0.0f);
    glRotatef(camera.yaw, 0.0f, 1.0f, 0.0f);
    glTranslated(camera.x, -camera.y, camera.z);
    Using gluProject(-p.x,p.y,-p.z, mvmatrix, projmatrix, viewport, &wx, &wy, &wz); with glTranslated(camera.x, -camera.y, camera.z); works but gluProject(p.x,p.y,p.z, mvmatrix, projmatrix, viewport, &wx, &wy, &wz); with glTranslated(-camera.x, -camera.y, -camera.z); doesnt.
    Last edited by c4ooo123; 09-27-2017 at 07:52 PM.

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