how to convert simple opengl line plotting code to use vertex array - c

Question

I wrote a simple opengl application in C which plots sin(x). This is my current draw function which runs very slow. How do I have to convert this code to make use of the faster 'vertex array' mode?
list of variables and functions used:
N = total number of points
x1 = min(x)
x2 = max(x)
y1 = min(y)
y2 = max(y)
func(x) = sin(x)
and here's the entire code:
/* to compile, do:
$ gcc -o out simple.c -lglut
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <GL/glut.h>
#include <time.h>
float xmin = -10, xmax = 10, ymin = -5, ymax = 5;
int nPoints = 3000;
/* function to calculate each data point */
float func(float x)
{
return sin(x);
}
/* plotting function - very slow */
void draw(float (* func)(float x), float x1, float x2, float y1, float y2, int N)
{
float x, dx = 1.0/N;
glPushMatrix();
glScalef(1.0 / (x2 - x1), 1.0 / (y2 - y1), 1.0);
glTranslatef(-x1, -y1, 0.0);
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_LINE_STRIP);
for(x = x1; x < x2; x += dx)
{
glVertex2f(x, func(x));
}
glEnd();
glPopMatrix();
};
/* Redrawing func */
void redraw(void)
{
clock_t start = clock();
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// -x, +x, -y, +y, number points
draw(func, xmin, xmax, ymin, ymax, nPoints);
glutSwapBuffers();
printf("Time elapsed: %f\n", ((double)clock() - start) / CLOCKS_PER_SEC);
};
/* Idle proc. Redisplays, if called. */
void idle(void)
{
// shift 'xmin' & 'xmax' by one.
xmin++;
xmax++;
glutPostRedisplay();
};
/* Key press processing */
void key(unsigned char c, int x, int y)
{
if(c == 27) exit(0);
};
/* Window reashape */
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 1, 0, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
};
/* Main function */
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
glutCreateWindow("Graph plotter");
glutReshapeWindow(1024, 800);
glutPostRedisplay(); // This call may or may not be necessary
/* Register GLUT callbacks. */
glutDisplayFunc(redraw);
glutKeyboardFunc(key);
glutReshapeFunc(reshape);
glutIdleFunc(idle);
/* Init the GL state */
glLineWidth(2.0);
/* Main loop */
glutMainLoop();
return 0;
}

Answer 1

In terms of LodePNG's C-style vector struct/functions:
// shared
vector pts;
vector_init( &pts, sizeof( float ) );
// whenever x1, x2, or N changes
vector_cleanup( &pts );
float x, dx = 1.0/N;
for(x = x1; x < x2; x += dx)
{
vector_resize( &pts, pts.size + 2 );
*(float*)vector_get( &pts, pts.size-2 ) = x;
*(float*)vector_get( &pts, pts.size-1 ) = func(x);
}
// whenever you want to draw
glPushMatrix();
glScalef(1.0 / (x2 - x1), 1.0 / (y2 - y1), 1.0);
glTranslatef(-x1, -y1, 0.0);
glColor3f(1.0, 1.0, 1.0);
glEnableClientState( GL_VERTEX_ARRAY );
glVertexPointer( 2, GL_FLOAT, 0, (float*)pts.data );
glDrawArrays( GL_LINE_STRIP, 0, pts.size / 2 );
glDisableClientState( GL_VERTEX_ARRAY );
glPopMatrix();
EDIT: Complete code:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <GL/glut.h>
#include <time.h>
typedef struct vector /*dynamic vector of void* pointers. This one is used only by the deflate compressor*/
{
void* data;
size_t size; /*in groups of bytes depending on type*/
size_t allocsize; /*in bytes*/
unsigned typesize; /*sizeof the type you store in data*/
} vector;
static unsigned vector_resize(vector* p, size_t size) /*returns 1 if success, 0 if failure ==> nothing done*/
{
if(size * p->typesize > p->allocsize)
{
size_t newsize = size * p->typesize * 2;
void* data = realloc(p->data, newsize);
if(data)
{
p->allocsize = newsize;
p->data = data;
p->size = size;
}
else return 0;
}
else p->size = size;
return 1;
}
static void vector_cleanup(void* p)
{
((vector*)p)->size = ((vector*)p)->allocsize = 0;
free(((vector*)p)->data);
((vector*)p)->data = NULL;
}
static void vector_init(vector* p, unsigned typesize)
{
p->data = NULL;
p->size = p->allocsize = 0;
p->typesize = typesize;
}
static void* vector_get(vector* p, size_t index)
{
return &((char*)p->data)[index * p->typesize];
}
float xmin = -10, xmax = 10, ymin = -5, ymax = 5;
int nPoints = 3000;
vector pts;
/* function to calculate each data point */
float func(float x)
{
return sin(x);
}
void update(float (* func)(float x), float x1, float x2, int N)
{
float x, dx = 1.0/N;
vector_cleanup( &pts );
for(x = x1; x < x2; x += dx)
{
vector_resize( &pts, pts.size + 2 );
*(float*)vector_get( &pts, pts.size-2 ) = x;
*(float*)vector_get( &pts, pts.size-1 ) = func(x);
}
}
/* plotting function - very slow */
void draw(float x1, float x2, float y1, float y2)
{
glPushMatrix();
glScalef(1.0 / (x2 - x1), 1.0 / (y2 - y1), 1.0);
glTranslatef(-x1, -y1, 0.0);
glColor3f(1.0, 1.0, 1.0);
if( pts.size > 0 )
{
glEnableClientState( GL_VERTEX_ARRAY );
glVertexPointer( 2, GL_FLOAT, 0, (float*)pts.data );
glDrawArrays( GL_LINE_STRIP, 0, pts.size / 2 );
glDisableClientState( GL_VERTEX_ARRAY );
}
glPopMatrix();
};
/* Redrawing func */
void redraw(void)
{
clock_t start = clock();
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// -x, +x, -y, +y, number points
draw(xmin, xmax, ymin, ymax);
glutSwapBuffers();
printf("Time elapsed: %f\n", ((double)clock() - start) / CLOCKS_PER_SEC);
};
/* Idle proc. Redisplays, if called. */
void idle(void)
{
// shift 'xmin' & 'xmax' by one.
xmin++;
xmax++;
update(func, xmin, xmax, nPoints);
glutPostRedisplay();
};
/* Key press processing */
void key(unsigned char c, int x, int y)
{
if(c == 27) exit(0);
};
/* Window reashape */
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 1, 0, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
};
/* Main function */
int main(int argc, char **argv)
{
vector_init( &pts, sizeof( float ) );
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
glutCreateWindow("Graph plotter");
glutReshapeWindow(1024, 800);
glutPostRedisplay(); // This call may or may not be necessary
/* Register GLUT callbacks. */
glutDisplayFunc(redraw);
glutKeyboardFunc(key);
glutReshapeFunc(reshape);
glutIdleFunc(idle);
/* Init the GL state */
glLineWidth(2.0);
/* Main loop */
glutMainLoop();
return 0;
}