Java lwjgl rotating a model around its center -

i have been making project in light weight java graphics library have loaded model , rendered color per triangle fine. next step rotate model around center. reading articles , source code have general understanding of need don't understand gltranslatef() method. under standing want to

glpushmatrix()

gltranslatef(0,0,0);

glrotatef(roation values);

gltranslatef(original coords);

render model

then glpopmatrix();

my main problem manually doing math in glvertex() method calls instead of translating them believe making translation 0,0,0 it's original spot difficult. time , help.

package render;  import static org.lwjgl.opengl.gl11.glbegin; import static org.lwjgl.opengl.gl11.glrotatef; import static org.lwjgl.opengl.gl11.glpushmatrix; import static org.lwjgl.opengl.gl11.gltranslatef; import static org.lwjgl.opengl.gl11.glpopmatrix; import static org.lwjgl.opengl.gl11.glend; import static org.lwjgl.opengl.gl11.gl_triangles; import static org.lwjgl.opengl.gl11.glcolor3f; import static org.lwjgl.opengl.gl11.glclearcolor; import static org.lwjgl.opengl.gl11.glnormal3f; import static org.lwjgl.opengl.gl11.glvertex3f;  import java.io.file; import java.io.filenotfoundexception; import java.io.ioexception;  import org.lwjgl.util.vector.vector3f;  import models.faces; import models.material; import models.model; import models.objloader;  public class objrender {     material material = new material();     private int count;     model m = null;     vector3f rgb = null;     vector3f v1 = new vector3f();     vector3f v2 = new vector3f();     vector3f v3 = new vector3f();  public void load_model(string model_name){ //loads model data     try{         m = objloader.loadmodel(new file("res/obj/"+model_name+".obj"));     } catch (filenotfoundexception e) {         e.printstacktrace();     } catch (ioexception e) {         e.printstacktrace();     } }  public void model_render(vector3f coords, vector3f degrees){     count = 0; //count of triangle coloring     model_rotate(coords, degrees); //coords obj coords degrees rotation coords     glbegin(gl_triangles);     (faces face : m.faces) {         string mat = objloader.getmateriallist().get(count);         rgb = material.getmaterial(mat);         glcolor3f(rgb.x, rgb.y, rgb.z);         vector3f n1 = m.normals.get((int) face.normals.x - 1);                 v1 = m.vertices.get((int) face.vertex.x - 1);         vector3f n2 = m.normals.get((int) face.normals.y - 1);                     v2 = m.vertices.get((int) face.vertex.y - 1);                    vector3f n3 = m.normals.get((int) face.normals.z - 1);         v3 = m.vertices.get((int) face.vertex.z - 1);         glnormal3f(n1.x, n1.y, n1.z);         glvertex3f(coords.x-v1.x, coords.y+v1.y+80, coords.z-v1.z);         glnormal3f(n2.x, n2.y, n2.z);         glvertex3f(coords.x-v2.x, coords.y+v2.y+80, coords.z-v2.z);         glnormal3f(n3.x, n3.y, n3.z);         glvertex3f(coords.x-v3.x, coords.y+v3.y+80, coords.z-v3.z);         count++;     }     glclearcolor(rgb.x,rgb.y,rgb.z,0);     glend();     glpopmatrix(); }  public void model_rotate(vector3f coords, vector3f degrees){     glpushmatrix();     gltranslatef(-0, -0, -0);     glrotatef(degrees.y,0,1,0);     } } 

_{although still not entirely clear actual question (apart "what have change code want"), attempt answer:}

when load object, example, obj file, not modify vertex coordinates. use them read file. when want transform object whole, use opengl matrix operations - in case, gltranslate , glrotate.

in order rotate object center, first have know center. in doubt, can compute vertices or bounding box of object.

then, in order rotate object around center, have

• move object, using gltranslate, center of object @ origin
• rotate object, using glrotate
• move object original position, using gltranslate

note these operations applied object in opposite order appear in source code. sequence of calls be

gltranslatef( center.x,  center.y,  center.z); glrotatef(rotationangledeg,0,0,1); gltranslatef(-center.x, -center.y, -center.z); 

here mcve uses simple rectangle consisting of 4 vertices object, , rotates center. can see in glvertex calls, uses vertices (in case, read obj file). whole transformation done on gl_modelview matrix, using commands mentioned above.

import static org.lwjgl.opengl.gl11.*; import java.awt.canvas; import javax.swing.jframe; import org.lwjgl.lwjglexception; import org.lwjgl.opengl.display; import org.lwjgl.util.glu.glu; import org.lwjgl.util.vector.vector3f;  public class rotateaboutcenter {     public static void main(string[] args)     {         jframe f = new jframe();         f.setdefaultcloseoperation(jframe.exit_on_close);         f.setsize(500,500);         f.setlocationrelativeto(null);         canvas canvas = new canvas();         f.add(canvas);         try         {             display.setparent(canvas);             f.setvisible(true);             display.create();         }         catch (lwjglexception e)         {             e.printstacktrace();         }         while (!display.iscloserequested())         {             draw();             display.update();             try             {                 thread.sleep(10);             }             catch (interruptedexception e)             {                 thread.currentthread().interrupt();             }             rotationangledeg += 1.0f;         }         display.destroy();     }      private static float rotationangledeg = 0;      // vertices of model     private static vector3f v0 = new vector3f(10,10,0);     private static vector3f v1 = new vector3f(20,10,0);     private static vector3f v2 = new vector3f(20,20,0);     private static vector3f v3 = new vector3f(10,20,0);      // center of model     private static vector3f center = new vector3f(15,15,0);      private static void draw()      {         // basic setup of view etc.         int w = display.getwidth();         int h = display.getheight();         glclear(gl_color_buffer_bit | gl_depth_buffer_bit);         glviewport(0, 0, w, h);         glmatrixmode(gl_projection);         glloadidentity();         glu.gluperspective(45, (float) w / (float) h, 0.1f, 1000);         glmatrixmode(gl_modelview);         glloadidentity();         glu.glulookat(0,0,70,0,0,0,0,1,0);          glpushmatrix();          // ===================================================================         // now, model transform. remember has          // read "backwards":          // third step: move model center         // again @ original position         gltranslatef(center.x, center.y, center.z);          // second step: rotate model origin (which         // center of model)         glrotatef(rotationangledeg,0,0,1);          // first step: translate model center @ origin         gltranslatef(-center.x, -center.y, -center.z);          // ===================================================================          // draw object, original coordinates.         // transforms contained in modelview matrix.         glbegin(gl_triangles);         glcolor3f(1,0,0);         glvertex3f(v0.x, v0.y, v0.z);         glvertex3f(v1.x, v1.y, v1.z);         glvertex3f(v3.x, v3.y, v3.z);         glvertex3f(v1.x, v1.y, v1.z);         glvertex3f(v2.x, v2.y, v2.z);         glvertex3f(v3.x, v3.y, v3.z);          glend();         glpopmatrix();     } }