glDrawPixels

OpenGL OpenGL: Basic Coding
1

Dear friends,

I want to ask you how to use the glDrawPixels function, I know that we must specify the width height format type and an array that represent the pixels .
The question is what are the coordinates of the pixels? Or where will these pixels be drawn?

Thanks in advance for your help.

2

The origin for this operation is the raster position that can be set with glRasterPos

3

Dear Zengar,

Thanks for your quick reply. The problem is that I am implementing a color cube using opengl. I am using the code in the attached file, but it is too slow. I tought that the glDrawPixels function can be fater, but it is slower.

Please can you tell me is there any way that is faster than the one I am using.

Thanks in advance for your help.


#include <GL/glut.h>
#include <stdio.h>

void redraw();
void mouseClicked(int, int, int, int);
void mouseMoved(int,int);
void reshape(int, int);

float spinX = 0.0f;
float spinZ = 0.0f;
int mousePosX = 0;
int mousePosY = 0;

void main()
{
	glutInitWindowSize(700, 700);
	glutInitWindowPosition(50, 50);
	glutCreateWindow("Test");
	glutDisplayFunc(redraw);
	glutReshapeFunc(reshape);
	glutMouseFunc(mouseClicked);
	glutMotionFunc(mouseMoved);
	glutMainLoop();
}

void redraw()
{
	glClearColor(0.0, 0.0, 0.0, 0.0);
	glClear(GL_COLOR_BUFFER_BIT );
	
	glPushMatrix();
	glRotatef(spinZ, 1, 0, 0);
	glRotatef(spinX, 0, 0, 1);
	for (int r = 0; r < 255; r++)
	{
		for (int g = 0; g < 255; g++)
		{
			for (int b = 0; b < 255; b++)
			{
				//using glDrawPixels
				//float color[3] = {(float)r / 255, (float)g / 255, (float)b / 255};
				//glRasterPos3f(r - 127.5, g - 127.5, b - 127.5);
				//glDrawPixels(1, 1, GL_RGB, GL_FLOAT, color);

				//using glVertex3f
				glColor3f((float)r / 255, (float)g / 255, (float)b / 255);
				glBegin(GL_POINTS);
					glVertex3f(r - 127.5, g - 127.5, b - 127.5);
				glEnd();
			}
		}
	}

	glPopMatrix();

	//glFlush();

	glutSwapBuffers();
}

void mouseClicked(int button, int state, int xMouse, int yMouse)
{
	if(state == GLUT_DOWN)
	{
		mousePosX = xMouse;
		mousePosY = yMouse;
	}
	glutPostRedisplay();
}

void mouseMoved(int xMouse,int yMouse)
{
	spinX += (float)(xMouse - mousePosX) * 1.0f;
	spinZ += (float)(yMouse - mousePosY) * 1.0f;
	mousePosX = xMouse;
	mousePosY = yMouse;

	glutPostRedisplay();
}

void reshape(int w, int h)
{
	glViewport(0,0,w,h);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	glOrtho((GLdouble)-w / 2, (GLdouble)w / 2, (GLdouble)-h / 2, (GLdouble)h / 2, 350.0, -350.0);
	
	//glPointSize(10.0);
	//glLineWidth(1.5);

	glEnable(GL_LINE_SMOOTH);
	glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);
	glEnable(GL_POINT_SMOOTH);
	glHint(GL_POINT_SMOOTH_HINT, GL_NICEST);
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
}
4

You seem to approach 3d graphics in a 2d-ish way: discrete points in space aka pixels (voxels).

But 3d graphics is about surface geometry. If you want to draw a color cube, you need to specify the surface of a cube with all associated attributes (color in your case), instead of your triple loop drawing discrete pixel/points in space.

A cubes surface consists of 8 vertice (corners) forming 6 faces (sides). The code to draw a cube (centered at the origin with an edge-length of 1) can look like this:


float vertice[8][3] = {	{ -0.5f, -0.5f,  0.5f},
			{  0.5f, -0.5f,  0.5f},
			{  0.5f,  0.5f,  0.5f},
			{ -0.5f,  0.5f,  0.5f},
			{ -0.5f, -0.5f, -0.5f},
			{  0.5f, -0.5f, -0.5f},
			{  0.5f,  0.5f, -0.5f},
			{ -0.5f,  0.5f, -0.5f}	};

glBegin(GL_QUADS);
// front side
glVertex3fv(vertice[0]);
glVertex3fv(vertice[1]);
glVertex3fv(vertice[2]);
glVertex3fv(vertice[3]);
// back side
glVertex3fv(vertice[5]);
glVertex3fv(vertice[4]);
glVertex3fv(vertice[7]);
glVertex3fv(vertice[6]);
// left side
glVertex3fv(vertice[4]);
glVertex3fv(vertice[0]);
glVertex3fv(vertice[3]);
glVertex3fv(vertice[7]);
// right side
glVertex3fv(vertice[1]);
glVertex3fv(vertice[5]);
glVertex3fv(vertice[6]);
glVertex3fv(vertice[2]);
// top side
glVertex3fv(vertice[3]);
glVertex3fv(vertice[2]);
glVertex3fv(vertice[6]);
glVertex3fv(vertice[7]);
// bottom side
glVertex3fv(vertice[1]);
glVertex3fv(vertice[0]);
glVertex3fv(vertice[4]);
glVertex3fv(vertice[5]);
glEnd();

For your color-cube you need to define the color for each of the eight vertice and put corresponding glColor3fv()-calls before each glVertex3fv()-call. opengl will do the interpolation (computation of in-between colors) across the surface for you.

I recommend the Red Book for an in-depth introduction to opengl, available online (not the up-to-date version, but sufficient for the basics).