OpenGL example

Linux KDE Ksnapshot of 512x512 OpenGL window produced by glDrawPixels on Ox with NVIDIA GeForce 8800 GTX graphics card

glDrawPixels.cpp

/* W B Langdon at MUN 10 May 2007
 * Program to demonstarte use of OpenGL's glDrawPixels
 */

#ifdef _WIN32
#include <windows.h>
#endif

#include <GL/gl.h>
#include <GL/glext.h>
#include <GL/glut.h>

#include <iostream>
#include <sstream>
#include "math.h"

unsigned int window_width = 512, window_height = 512;
const int size=window_width*window_height;

struct rgbf {float r; float g; float b;};
//WBL 9 May 2007 Based on
//http://www.codeguru.com/cpp/w-d/dislog/commondialogs/article.php/c1861/
//Common.h
void toRGBf(const float h, const float s, const float v,
	    rgbf* rgb)
{
  /*
RGBType rgb;
	if(!h  && !s)
	{
		rgb.r = rgb.g = rgb.b = v;
	}
  */
  //rgbf* rgb = (rgbf*) out;
double min,max,delta,hue;
	
	max = v;
	delta = max * s;
	min = max - delta;

	hue = h;
	if(h > 300 || h <= 60)
	{
		rgb->r = max;
		if(h > 300)
		{
			rgb->g = min;
			hue = (hue - 360.0)/60.0;
			rgb->b = ((hue * delta - min) * -1);
		}
		else
		{
			rgb->b = min;
			hue = hue / 60.0;
			rgb->g = (hue * delta + min);
		}
	}
	else
	if(h > 60 && h < 180)
	{
		rgb->g = max;
		if(h < 120)
		{
			rgb->b = min;
			hue = (hue/60.0 - 2.0 ) * delta;
			rgb->r = min - hue;
		}
		else
		{
			rgb->r = min;
			hue = (hue/60 - 2.0) * delta;
			rgb->b = (min + hue);
		}
	}
	else
	{
		rgb->b = max;
		if(h < 240)
		{
			rgb->r = min;
			hue = (hue/60.0 - 4.0 ) * delta;
			rgb->g = (min - hue);
		}
		else
		{
			rgb->g = min;
			hue = (hue/60 - 4.0) * delta;
			rgb->r = (min + hue);
		}
	}
}


//Convert a wide range of data values into nice colours 
void colour(const float data, float* out) {
  //convert data to angle
  const float a = atan2(data,1)/(2*atan2(1,1)); // -1 .. +1
  const float angle = (1+a)*180; //red=0 at -1,+1

  const float saturation = 1;

  const float h = (data<-1||data>1)? 1 : fabs(data);

  toRGBf(angle,saturation,h,(rgbf*)out);
}

void display()
{
  //Create some nice colours (3 floats per pixel) from data -10..+10
  float* pixels = new float[size*3];
  for(int i=0;i<size;i++) {
    colour(10.0-((i*20.0)/size),&pixels[i*3]);
  } 

  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  //http://msdn2.microsoft.com/en-us/library/ms537062.aspx
  //glDrawPixels writes a block of pixels to the framebuffer.

  glDrawPixels(window_width,window_height,GL_RGB,GL_FLOAT,pixels);

  glutSwapBuffers();
}

int main(int argc, char** argv) {
  glutInit(&argc, argv);

  glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
  glutInitWindowSize(window_width, window_height);
  glutCreateWindow("OpenGL glDrawPixels demo");

  glutDisplayFunc(display);
  //glutReshapeFunc(reshape);
  //glutMouseFunc(mouse_button);
  //glutMotionFunc(mouse_motion);
  //glutKeyboardFunc(keyboard);
  //glutIdleFunc(idle);
  
  glEnable(GL_DEPTH_TEST);
  glClearColor(0.0, 0.0, 0.0, 1.0);
  //glPointSize(2);

  glutMainLoop();
}

Makefile

############################################################################
#
#       Project		OpenGL demo make file with GNU gcc $Revision: 1.00 $ 
#	    
#       Author		W.B.Langdon at MUN 10 May 2007
#
############################################################################

NAME 	= glDrawPixels
OBJS 	= glDrawPixels.o

LIBS 	= -lm 
#LIBS 	= -lm -lC
CC 	= g++
#CC 	= cc
COMPILE = $(CC) -c $(CFLAGS)
#CFLAGS 	= -O5
CFLAGS 	= -g
LINK 	= $(CC) -o 
GL_LIBS = -lGL -lglut -lGLU


$(NAME):$(OBJS) 
	$(LINK) $(NAME) $(LDFLAGS) $(OBJS) $(LIBS) $(GL_LIBS) 

#C++ source files

glDrawPixels.o:	glDrawPixels.cpp
	$(COMPILE) glDrawPixels.cpp