The issue is that the further the mouse click is from the top left origin (0,0) the greater the height inaccuracy when the vertex is plotted. Any ideas?
int WindowWidth = 19;
int WindowHeight = 13;
int mouseClickCount = 0;
int rectPlotted;
GLint x1;
GLint y1;
GLint x2;
GLint y2;
//Declare our functions with prototypes:
void display(void);
void init (void);
void processNormalKeys(unsigned char key, int x, int y);
void on_vertex_selected(GLint x, GLint y);
void on_mouse_event(int button, int state, int x, int y);
/////////////////////////////////// MAIN ////////////////////////////////
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
//start calculation of max window size available at 19:13 aspect ratio
int screenWidth = glutGet(GLUT_SCREEN_WIDTH);
int screenHeight = glutGet(GLUT_SCREEN_HEIGHT);
screenWidth = screenWidth/WindowWidth;
WindowWidth = WindowWidth*screenWidth;
screenHeight = screenHeight/WindowHeight;
WindowHeight = screenHeight*WindowHeight;
//end calculation
glutInitWindowSize (WindowWidth, WindowHeight);
glutInitWindowPosition (0, 0);
glutCreateWindow ("Plot a rectangle!");
glutDisplayFunc(display);
glutKeyboardFunc(processNormalKeys);
glutMouseFunc(on_mouse_event);
init();
glutMainLoop();
return 0;
}
/////////////////////////////////////////////////////////////////////////
void display(void)
{
glClear (GL_COLOR_BUFFER_BIT); //clear all pixels
glFlush();
}
void init (void)
{
/* select clearing (background) color */
glClearColor (1.0, 1.0, 1.0, 0.0);
/* initialize viewing values */
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, WindowWidth, 0, WindowHeight, -1.0, 1.0);
}
void processNormalKeys(unsigned char key, int x, int y) {
if (key == 27) //esc
exit(0);
//Allows color change of most recently plotted rectangle
if (key == 98){ //b
glColor3f(0.0, 0.0, 1.0);
glBegin(GL_QUADS);
glVertex2i(x1, y1);
glVertex2i(x1, y2);
glVertex2i(x2, y2);
glVertex2i(x2, y1);
glEnd();
glFlush();
}
if (key == 114){ //r
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_QUADS);
glVertex2i(x1, y1);
glVertex2i(x1, y2);
glVertex2i(x2, y2);
glVertex2i(x2, y1);
glEnd();
glFlush();
}
if (key == 103){ //g
glColor3f(0.0, 1.0, 0.0);
glBegin(GL_QUADS);
glVertex2i(x1, y1);
glVertex2i(x1, y2);
glVertex2i(x2, y2);
glVertex2i(x2, y1);
glEnd();
glFlush();
}
}
void on_vertex_selected(GLint x, GLint y){
if(mouseClickCount == 0){
x1 = x;
y1 = y;
glColor3f(0.0, 0.0, 0.0);
glEnable(GL_POINT_SMOOTH);
glPointSize(5.0);
glBegin(GL_POINTS);
glVertex2i(x1, y1);
glEnd();
glFlush();
}
else{
x2 = x;
y2 = y;
glBegin(GL_POINTS);
glColor3f(1.0, 1.0, 1.0);
glVertex2i(x1,y1); //"clears" previous point to make way for the rectangle
glEnd();
glColor3f(0.0, 0.0, 0.0);
glBegin(GL_QUADS);
glVertex2i(x1, y1);
glVertex2i(x1, y2);
glVertex2i(x2, y2);
glVertex2i(x2, y1);
glEnd();
glFlush();
}
}
void on_mouse_event(int button, int state, int x, int y){
if(button==GLUT_LEFT_BUTTON && state ==GLUT_DOWN && mouseClickCount == 0){
//y = y+20; //adjusts for VM mouse tracking error
on_vertex_selected(x, WindowHeight - y);
rectPlotted = 0;
}
if(button==GLUT_LEFT_BUTTON && state ==GLUT_UP && mouseClickCount == 0){
if(rectPlotted == 1){
return;
}
else{
mouseClickCount++;
}
}
if(button==GLUT_LEFT_BUTTON && state ==GLUT_DOWN && mouseClickCount == 1){
//y = y+20; //adjusts for VM mouse tracking error
on_vertex_selected(x, WindowHeight - y);
mouseClickCount = 0;
rectPlotted = 1;
}
}
Your glOrtho() call is almost certainly wrong. You pass the window size, not the window client area size. The difference is the size of the borders and the height of the caption.
Be aware that your actual window size will most probably be slightly different than what you asked at init time.
Simply said, implement a glutReshapeFunc()
I was compiling and running my code within Parallels VM v.6. This was the issue leading to inaccurate mouse coordinates being returned. I compiled and ran the exact same code on OS X without problem.
Related
I need to make an animation where the box starts from (-15, 0) then going up to (-8, 15) and when it reach that, then it came down. and so on, just like a DVD screen saver, but i can't seem to find the formula for it to work.
Here's the code:
#include<windows.h>
#include<GL/glu.h>
#include<GL/glut.h>
#ifdef APPLE
#include<GLUT/glut.h>
#else
#include<GL/glut.h>
#endif
#include<stdio.h>
#include<stdlib.h>
#include <GL/freeglut.h>
//Variable menampung sudut awal
float xpos = -15.0;
float deltax = 0.5;
float ypos = 0.0;
bool balik = true;
bool turun = true;
//Variable mengatur perubahan sudut
void myInit(void) {
glClearColor(0.0, 0.0, 0.0, 1.0);
glColor3f(1.0, 1.0, 1.0);
glPointSize(2.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-15.0f, 15.0f, -15.0f, 15.0f);
}
void myDisplay(void) {
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_LINES);
glColor3f(1.0, 1.0, 1.0);
glVertex2f(-15.0, 0.0);
glVertex2f(15.0, 0.0);
glEnd();
glBegin(GL_LINES);
glColor3f(1.0, 1.0, 1.0);
glVertex2f(-8.0, 15.0);
glVertex2f(-8.0, -15.0);
glEnd();
glBegin(GL_LINES);
glColor3f(1.0, 1.0, 1.0);
glVertex2f(8.0, 15.0);
glVertex2f(8.0, -15.0);
glEnd();
glBegin(GL_LINES);
glColor3f(1.0, 1.0, 1.0);
glVertex2f(0.0, 15.0);
glVertex2f(0.0, -15.0);
glEnd();
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_POLYGON);
glVertex2f(-0.2 + xpos, 0.2 + ypos);
glVertex2f(-0.2 + xpos, -0.2 + ypos);
glVertex2f(0.2 + xpos, -0.2 + ypos);
glVertex2f(0.2 + xpos, 0.2 + ypos);
glEnd();
glutSwapBuffers();
}
void Timer(int) {
glutPostRedisplay();
glutTimerFunc(100, Timer, 0);
if (ypos < 15 && xpos <-8 && turun == true){
xpos = xpos + deltax;
ypos = (2.143*xpos) + 32.143;
}
else {turun = false;};
if (ypos > -15 && xpos <8 && turun == false){
xpos = xpos + deltax;
ypos = -1,875*xpos + ;
}
else turun = true;
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(500, 500);
glutInitWindowPosition(100, 100);
glutCreateWindow("Animasi");
glutDisplayFunc(myDisplay);
myInit();
glutTimerFunc(0, Timer, 0);
//Perulangan
glutMainLoop();
return 0;
}
If you want the box to bounce you can just reverse the x or y velocity depending on which axis you hit. If you want the box to bounce with Gravity you will use vector parametric equations like so:
y = -16t^2 + vsin(x) + s.y and x = vcos(x) + s.x where t is time v is magnitude(velocity) and s is the starting position of the box which will be reset on every collision with the sides. Keep the magnitude the same to infinitely bounce. If the equation is confusing look up vector pre-calculus online.
#include <stdio.h> // this library is for standard input and output
#include "glut.h"// this library is for glut the OpenGL Utility Toolkit
#include <math.h>
float squareX = 0.0f;
float squareY = -0.3f;
float squareZ = 0.0f;
static int flag = 1;
void drawShape(void) {
glTranslatef(squareX, squareY, squareZ);
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(162, 50);
glVertex2f(162, 10);
glVertex2f(220, 10);
glVertex2f(220, 50);
glVertex2f(162, 50);
glEnd();
}
void initRendering() {
glEnable(GL_DEPTH_TEST);
}
// called when the window is resized
void handleResize(int w, int h) {
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, (float)w, 0.0f, (float)h, -1.0f, 1.0f);
}
void drawScene() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
drawShape();
glutSwapBuffers();
}
// make the square go up
void update(int value) {
if (flag) {
squareY += 1.0f;
if (squareY > 400.0) {
flag = 0;
}
}
glutPostRedisplay();
glutTimerFunc(25, update, 0);
}
// make the square go right
/* void update(int value) {
if (flag) {
squareX += 1.0f;
if (squareX > 400.0) {
flag = 0;
}
}
glutPostRedisplay();
glutTimerFunc(25, update, 0);
} */
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(400, 400);
glutCreateWindow("Moving Square");
initRendering();
glutDisplayFunc(drawScene);
glutReshapeFunc(handleResize);
glutTimerFunc(25, update, 0);
glutMainLoop();
return(0);
}
I have uploaded this code before but this time I made the square go all the way up. The code just moves the square up, but I don't know how to position it on the left once it reaches the top, so then I can make it move to the right. I have uploaded a demonstration on how I want it to look below.
Preview:
What I want it to do next:
I recommend to initialize the variables squareX, squareY and squareZ with the start position of the rectangle:
float squareX = 162.0f;
float squareY = 0.0f;
float squareZ = 0.0f;
Do not draw a rectangle specific position, but draw a rectangle on the position (0,0) with a length (width, height). Let the model matrix (set by glTranslatef), do the job of the positioning:
void drawShape(void)
{
float width = 58.0f;
float height = 40.0f;
glTranslatef(squareX, squareY, squareZ);
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(0, 0);
glVertex2f(width, 0);
glVertex2f(width, height);
glVertex2f(0, height);
glVertex2f(0, 0);
glEnd();
}
Use a variable state, which has stated the direction of the current movement:
int state = 1; // 0: stop; 1: move up; 2: move right
If the rectangle a certain position has reached, then the state has to be changed and a the new start position can be set. At the final position, the rectangle can stop or the process can even be restarted:
void update(int value)
{
if (state == 1) // 1 : move up
{
squareY += 1.0f;
if (squareY > 400.0)
{
state = 2;
squareX = 0.0f;
squareY = 180.0f;
}
}
else if (state == 2) // 2 : move right
{
squareX += 1.0f;
if (squareX > 400.0)
{
state = 0;
// restart
//state = 1;
//squareX = 162.0f;
//squareY = 0.0f;
}
}
glutPostRedisplay();
glutTimerFunc(25, update, 0);
}
I am unable to understand why I am not getting any output for the given program. This problem has been occurring to me for my last 2 openGL programs, the previous one being DDA algorithm, again in which I didnt get any output. Is there a problem with the algorithm, or in my understanding how openGL works internally ?
#include <iostream>
#include <GL/glut.h>
using namespace std;
float X1 = 0, X2 = 100, Y1 = 0, Y2 = 400, X11, Y11, X22, Y22, slope, dely, delx, c, intercept, pi;
float absl(float x) {
if (x >= 0) return x;
return -1*x;
}
void drawScene() {
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, 500, 0, 500);
glClearColor(1.0, 1.0, 1.0, 0);
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
//start bresenham algo
glBegin(GL_POINTS);
glVertex2f(X1, Y1);
while(X1 <= X2) {
X1++;
if(pi < 0) {
glVertex2f(X1, Y1);
}
else {
Y1++;
glVertex2f(X1, Y1);
}
float flag = (pi >= 0);
pi = pi + 2*dely - 2*delx*flag;
}
glEnd();
//end DDA algo
glFlush();
}
int main(int argc, char **argv) {
//cin >> X1 >> Y1 >> X2 >> Y2;
// dely = Y2 - Y1;
// delx = X2 - X1;
// pi = 2*dely - delx;
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500, 500);
glutInitWindowPosition(0,0);
glutCreateWindow("DDA");
glutDisplayFunc(drawScene);
glutMainLoop();
return 0;
}
gluOrtho2D defines a 2D orthographic projection matrix. But it also multiplies the current matrix on the matrix stack with the orthographic projection matrix and replaces the current matrix with the product.
Note, in OpenGL fixed function pipeline all matrix operations work like this (except glPushMatrix, glPopMatrix, glLoadMatrix and glLoadIdentity).
This means you have to replace the current matrix with the identity matrix (glLoadIdentity), befor you apply the orthographic projection matrix:
glLoadIdentity();
gluOrtho2D(0, 500, 0, 500);
Or you do gluOrtho2D only one time before you start the main loop in the main function:
gluOrtho2D(0, 500, 0, 500);
glutMainLoop();
Since the display function is called (drawScene) continuously, you should use local control varibales and not modify X1 and Y1:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, 500, 0, 500);
glClearColor(0.0, 0.0, 0.0, 0);
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
//start bresenham algo
int x = X1, y = Y1;
glBegin(GL_LINES);
glVertex2f(100, 400);
glVertex2f(400, 100);
glEnd();
glBegin(GL_POINTS);
glVertex2f(x, Y1);
while(x <= X2) {
x ++;
if(pi < 0) {
glVertex2f(x, y);
}
else {
y ++
glVertex2f(x, y);
}
}
int flag = (pi >= 0);
pi = pi + 2*dely - 2*delx*flag;
glEnd();
I want to receive a small animation for four triangles. I wrote a several versions of the code, but none of these versions works. In result I have a blank black screen.
My code:
#define GLUT_DISABLE_ATEXIT_HACK
#define TIMERSECS 20
#include <windows.h>
#include <GL/glut.h>
#include <GL/gl.h>
#include <GL/GL.H>
#include <stdlib.h>
float arc = 0.0f;
void draw_traingle(float x0, float y0, float x1, float y1, float x2, float y2) {
glColor4f(0.0f, 0.0f, 1.0f, 1.0f); //Blue
glVertex2f(x0, y0);
glVertex2f(x1, y1);
glVertex2f(x2, y2);
}
void draw(void) {
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT);
glRotatef(arc, 1.0f,0.0f,0.0f);
glBegin(GL_POLYGON);
float center_x = 300.0f;
float center_y = 300.0f;
float up_x = center_x;
float up_y = 250.0f;
float down_x = center_x;
float down_y = 350.0f;
float right_x = 350.0f;
float right_y = center_y;
float left_x = 250.0f;
float left_y = center_y;
glPushMatrix();
draw_traingle(up_x, up_y, right_x, right_y, center_x, center_y);
draw_traingle(right_x, right_y, down_x, down_y, center_x, center_y);
draw_traingle(down_x, down_y, left_x, left_y, center_x, center_y);
draw_traingle(left_x, left_y, up_x, up_y, center_x, center_y);
glPopMatrix();
glEnd();
glFlush();
glutSwapBuffers();
}
void handleKeypress(unsigned char key, int x, int y) {
switch (key) {
case 27:
exit(0);
}
}
void init(void) {
glClearColor(0.0, 0.0, 0.0, 0.0);
glViewport(0, 0, 600, 600);
glMatrixMode(GL_PROJECTION);
glLoadIdentity(); //=1
gluOrtho2D(0.0, 600.0, 0.0, 600.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity(); //=1
}
void update(int value) {
arc += 2.0f;
if (arc > 360.f) {
arc -= 360;
}
glutPostRedisplay();
glutTimerFunc(TIMERSECS, update, 0);
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); //single buffer and RGBA
glutInitWindowSize(600, 600);
glutInitWindowPosition(100, 100);
glutCreateWindow("Window");
init();
glutDisplayFunc(draw);
glutTimerFunc(TIMERSECS, update, 0);
glutKeyboardFunc(handleKeypress);
glutMainLoop();
return 0;
}
I want to create an animation where my rectange builded from traingles will rotate for 2 degrees per some small time. I want to rotate it like clock works. i know that the problem is not in time (not to small) but in glRotatef - I don't know what parameters it should takes to give me a proper effect.
Thanks in advance! :)
#include <GL/glut.h>
#include <GL/gl.h>
#include <stdlib.h>
//#define TIMERSECS 200 prefer not to use preprocessor macros
//instead you can use:
static const int TIMERSECS = 200;
float arc = 0.0f;
void draw_traingle(float x0, float y0, float x1, float y1, float x2, float y2) {
glBegin(GL_POLYGON);
glColor4f(0.0f, 0.0f, 1.0f, 1.0f); //Blue
glVertex2f(x0, y0);
glVertex2f(x1, y1);
glVertex2f(x2, y2);
glEnd();
}
void draw(void) {
glClear(GL_COLOR_BUFFER_BIT);
glTranslatef(300, 300, 0);
glRotatef(arc, 0.0f,0.0f,1.0f);
glTranslatef(-300, -300, 0);
float center_x = 300.0f;
float center_y = 300.0f;
float up_x = center_x;
float up_y = 250.0f;
float down_x = center_x;
float down_y = 350.0f;
float right_x = 350.0f;
float right_y = center_y;
float left_x = 250.0f;
float left_y = center_y;
draw_traingle(up_x, up_y, right_x, right_y, center_x, center_y);
draw_traingle(right_x, right_y, down_x, down_y, center_x, center_y);
draw_traingle(down_x, down_y, left_x, left_y, center_x, center_y);
draw_traingle(left_x, left_y, up_x, up_y, center_x, center_y);
glutSwapBuffers();
}
void handleKeypress(unsigned char key, int x, int y) {
switch (key) {
case 27:
exit(0);
}
}
void init() {
glClearColor(0.0, 0.0, 0.0, 0.0);
glViewport(0, 0, 600, 600);
glMatrixMode(GL_PROJECTION);
glLoadIdentity(); //=1
gluOrtho2D(0.0, 600.0, 0.0, 600.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity(); //=1
}
void update(int value) {
arc += 2.0f;
if (arc > 360.f) {
arc -= 360;
}
glutPostRedisplay();
glutTimerFunc(200, update, 1);
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA); //single buffer and RGBA
glutInitWindowSize(600, 600);
glutCreateWindow("Window");
init();
glutDisplayFunc(draw);
glutTimerFunc(TIMERSECS, update, 1);
glutKeyboardFunc(handleKeypress);
glutMainLoop();
return 0;
}
Rotation is always relative to the coordinate system you are using. Transforimations are always applied in the reverse order. When we use rotatef function to rotate an object it's always relative to the center of the coordinate system. So first we need to translate it to to the center which is done by translatef(-300,-300,0). Now the object is on the center. Then we use rotatef to rotate the object to x,y or z axis. Then we translate the object again to the position that was before the transformation. Finally we write the functions in reverse order. You are good to go :)
I'm trying to make a simple openGL application which creates shapes using mouse clicks. The shape required is selected using a pop-up menu opened using the right mouse button. Currently I have rectangle working and also a simple paint-esque function using points. They disappear once redrawn but that's a problem I'll worry about in the future.
Currently I'm trying to also implement a Line function as a menu option. I have the below code in place but when I select the Line option from the pop-up menu and click two points it doesn't seem to draw a line as expected. Can anyone point out where I'm going wrong here?
GLfloat x1, x2, y1, y2;
GLfloat x_Src, y_Src, x_Dest, y_Dest;
//used to change colour (To be implemented)
static int colour;
void menu(int);
void display(void)
{
glClearColor(0.0, 2.0, 1.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glPointSize(3.0);
glColor3f(1.0, 0.0, 0.0);
//Rectangle vertices
glBegin(GL_POLYGON);
glVertex2f(x1, y1);
glVertex2f(x1, y2);
glVertex2f(x2, y2);
glVertex2f(x2, y1);
glEnd();
//Line vertices
glColor3f(0.0, 0.0, 1.0);
glBegin(GL_LINE);
glVertex2f(x_Src, y_Src);
glVertex2f(x_Dest, y_Dest);
glEnd();
glFlush();
return;
}
void MyRect(GLint button, GLint state, GLint x, GLint y)
{
static int first = 1;
if (state == GLUT_DOWN && button == GLUT_LEFT_BUTTON)
{
if (first)
{
//first point in terms of window size
x1 = (x - 250.0) / 250.0;
y1 = -(y - 250) / 250.0;;
}
else
{
//second point in terms of window size
x2 = (x - 250.0) / 250.0;
y2 = -(y - 250) / 250.0;
glutPostRedisplay();
}
first = !first;
}
return;
}
void MyLine(GLint button, GLint state, GLint x, GLint y)
{
static int first = 1;
if (state == GLUT_DOWN && button == GLUT_LEFT_BUTTON)
{
if (first)
{
x_Src = (x - 250.0) / 250.0;
y_Src = -(y - 250) / 250.0;
}
else
{
x_Dest = (x - 250.0) / 250.0;
y_Dest = -(y - 250) / 250.0;
glutPostRedisplay();
}
first = !first;
}
return;
}
//allows drawing of un-fixed line using mouse
void MyPaint(GLint x, GLint y)
{
glColor3f(0.0, 0.0, 0.0);
glPointSize(3.0);
glBegin(GL_POINTS);
glVertex2f((x - 250.0) / 250.0, -(y - 250.0) / 250.0);
glEnd();
glFlush();
return;
}
void MainMenu(int item)
{
//Remove motion function if exists and go to rectangle function
//prevents two functions running at once
if (item == 1)
{
glutMotionFunc(NULL);
glutMouseFunc(MyRect);
}
else if (item == 2)
{
glutMotionFunc(NULL);
glutMouseFunc(MyLine);
}
else if (item == 5)
{
glutMouseFunc(NULL);
glutMotionFunc(MyPaint);
}
glutPostRedisplay();
return;
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitWindowSize(500, 500);
glutInitWindowPosition(500, 200);
glutCreateWindow("Draw Shapes");
glutDisplayFunc(display);
glutCreateMenu(MainMenu);
glutAddMenuEntry("Rectangle", 1);
glutAddMenuEntry("Line", 2);
glutAddMenuEntry("Circle", 3);
glutAddMenuEntry("Triangle", 4);
glutAddMenuEntry("Paintbrush", 5);
glutAddMenuEntry("Background colour", 6);
glutAddMenuEntry("Clear screen", 7);
glutAttachMenu(GLUT_RIGHT_BUTTON);
glutMainLoop();
}
You need to use glBegin(GL_LINES) instead of glBegin(GL_LINE).
GL_LINE is an argument to the unrelated function glPolygonMode.