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johnmarinelli

OpenGL Moving a single object around in 2D

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Hi everyone,

I know this question has been asked a few times before, but I still can't get my program to work.

 

I'm working on an Asteroids clone in OpenGL, and I'm starting off by simply getting my spaceship to move around.  I also have 4 quads set up in each respective corner so I know if the movement is working or not.

 

However, it seems like I'm moving the quads around, and not my spaceship.  Here's the code:

void renderScene()
{
	glClear(GL_COLOR_BUFFER_BIT);
	
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();

	glPushMatrix();

	gSpaceship.render();

	glPopMatrix();

	glPushMatrix();
	//move to bottom right corner
	glTranslatef(SPACE_WIDTH / 2.f, SPACE_HEIGHT / 2.f, 0.f);

	glBegin(GL_QUADS);
		glColor3f(0.f, 1.f, 0.f);
		glVertex2f(0.f, 0.f);
		glVertex2f(50.f, 0.f);
		glVertex2f(50.f, 50.f);
		glVertex2f(0.f, 50.f);
	glEnd();

	//move to bottom left corner
	glTranslatef(-SPACE_WIDTH, 0.f, 0.f);

	glBegin(GL_QUADS);
		glColor3f(0.f, 0.f, 1.f);
		glVertex2f(0.f, 0.f);
		glVertex2f(50.f, 0.f);
		glVertex2f(50.f, 50.f);
		glVertex2f(0.f, 50.f);
	glEnd();

	//move to top left corner
	glTranslatef(0.f, -SPACE_HEIGHT, 0.f);

	glBegin(GL_QUADS);
		glColor3f(1.f, 1.f, 1.f);
		glVertex2f(0.f, 0.f);
		glVertex2f(50.f, 0.f);
		glVertex2f(50.f, 50.f);
		glVertex2f(0.f, 50.f);
	glEnd();

	//move to top right corner
	glTranslatef(SPACE_WIDTH, 0.f, 0.f);

	glBegin(GL_QUADS);
		glColor3f(1.f, 0.f, 1.f);
		glVertex2f(0.f, 0.f);
		glVertex2f(50.f, 0.f);
		glVertex2f(50.f, 50.f);
		glVertex2f(0.f, 50.f);
	glEnd();
	glPopMatrix();

	glFlush();
	
	glutSwapBuffers();
}

and heres' gSpaceship.render(), and consequently, JTexture.render():

void Spaceship::render()
{
       //mGraphics is a JTexture
       //m_xPos and m_yPos start out in the center of the 4 quads
	mGraphics.render(m_xPos, m_yPos);
}

void JTexture::render(GLfloat x, GLfloat y)
{
	if (mTextureID != 0)
	{
		glTranslatef(x, y, 0.f);

		glBindTexture(GL_TEXTURE_2D, mTextureID);

		glBegin(GL_QUADS);
			glTexCoord2f(0.f, 0.f); glVertex2f(0.f, 0.f );
            glTexCoord2f(1.f, 0.f); glVertex2f(mTextureWidth, 0.f );
            glTexCoord2f(1.f, 1.f); glVertex2f(mTextureWidth, mTextureHeight);
            glTexCoord2f(0.f, 1.f); glVertex2f(0.f, mTextureHeight);
        glEnd();
	}
}

also, I'd like to eventually be able to rotate the texture and move in the direction its facing; I know how to rotate a texture, but I know I will be very lost when it comes to moving in a direction thats not axis-aligned.  How does one go about this?

 

Thanks a lot!

Edited by johnmarinelli

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An easy way to deal with position and rotation and everything else related to transforms is to use a matrix to represent an object's transformations.

 

So you can have a class with your object's data (in the case, a single Quad + Texture) in it's original state, and when you're about to draw it, you use glLoadMatrix(matrix) and all it's transformations will be done.

 

Also, a suggestion, do some research about VBOs, what you're doing is called "Immediate Mode" and is pretty outdated and usually bad on performance.

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Here is a basic bare bones multiple triangle animation using modern OpenGL 3.x with inline vertex and fragment shaders.

compile it using :     g++ -o 005_tri_mo_mu_f8_ver3_simple 005_tri_mo_mu_f8_ver3_simple.cpp  -lGL -lGLEW -lglut -lGLU
 

//		g++ -o opengl_tri_multi_move opengl_tri_multi_move.cpp -lGL -lGLU -lGLEW -lglut
//		optirun ./opengl_tri_multi_move


#include <GL/glew.h>
#include <GL/freeglut.h>
#include <vector>
#include <iostream>

#include <string.h>		//	strlen
#include <cstdio>	// printf, sprintf  in cpp c++

// ---------


#include <iomanip>	// std::setprecision
//#include <locale>	// to insert commas into large numbers


#include <sstream>



#define WINDOW_TITLE_PREFIX "move multiple triangles"
int CurrentWidth = 1100, CurrentHeight = 1000, WindowHandle = 0;
unsigned FrameCount = 0;

#define YES 1
#define NO  0

//#define DEBUG YES
#define DEBUG NO

int did_load_program = NO;
int x1_reached_edge = NO;
int y1_reached_edge = NO;

GLfloat x_max = 0.9, y_max = 0.9;
GLfloat x_min = -0.9, y_min = -0.9;
GLfloat min_incr = -0.3, max_incr = 0.3;
// GLfloat size_triangle_side = 0.007;
GLfloat size_triangle_side = 0.2;


struct struct_program {

    static GLuint Load( const char * vert, const char * geom, const char * frag ) {

        GLuint prog = glCreateProgram();
        if( vert ) AttachShader( prog, GL_VERTEX_SHADER, vert );
        if( geom ) AttachShader( prog, GL_GEOMETRY_SHADER, geom );
        if( frag ) AttachShader( prog, GL_FRAGMENT_SHADER, frag );
        glLinkProgram( prog );
        CheckStatus( prog );
        return prog;
    }

private:

    static void CheckStatus( GLuint obj ) {

        GLint status = GL_FALSE, len = 10;
        if( glIsShader(obj) )   glGetShaderiv ( obj, GL_COMPILE_STATUS, & status );
        if( glIsProgram(obj) )  glGetProgramiv( obj, GL_LINK_STATUS, & status );
        if( status == GL_TRUE ) return;
        if( glIsShader(obj) )   glGetShaderiv ( obj, GL_INFO_LOG_LENGTH, & len );
        if( glIsProgram(obj) )  glGetProgramiv( obj, GL_INFO_LOG_LENGTH, & len );
        std::vector< char > log( len, 'X' );
        if( glIsShader(obj) )   glGetShaderInfoLog ( obj, len, NULL, &log[0] );
        if( glIsProgram(obj) )  glGetProgramInfoLog( obj, len, NULL, &log[0] );
        std::cerr << & log[0] << std::endl;
        exit( -1 );
    }

    static void AttachShader( GLuint program, GLenum type, const char * src ) {

        GLuint shader = glCreateShader( type );
        glShaderSource( shader, 1, & src, NULL );
        glCompileShader( shader );
        CheckStatus( shader );
        glAttachShader( program, shader );
        glDeleteShader( shader );
    }
};

#define GLSL(version, shader) "#version " #version "\n" #shader

const char* vert = GLSL (
    400 core,
    layout( location = 0 ) in vec4 vPosition;
    void main()
    {
        gl_Position = vPosition;
    }
);

const char* frag = GLSL (
    400 core,
    out vec4 fColor;
    void main()
    {
        fColor = vec4( 0.0, 0.0, 1.0, 1.0 );
    }
);

//#define total_num_triangles  1500
//#define total_num_triangles  3
//#define total_num_triangles  10000000
// #define total_num_triangles  100000
#define total_num_triangles  10

#define index_ArrayBuffer 0
#define NumBuffers 1

//num_vertex_arrays

GLuint vPosition = 0;

//GLuint VAOs[total_num_triangles];
GLuint vertex_array_object;

GLuint Buffers[NumBuffers];

const GLuint  NumVertices = 3;

#define X 0
#define Y 1
//GLfloat tri_location[total_num_triangles][2];
GLfloat vertex_locations[ 3 * total_num_triangles][2];	//	3 vertex per triangle, X&Y per vertex
//GLfloat incr[total_num_triangles][2];

GLfloat triangle_incr[total_num_triangles][2];

// ------------------------

bool get_direction(GLfloat curr_location, GLfloat min_location, GLfloat max_location) {

	bool answer = false;
	if (curr_location < min_location || curr_location > max_location) {

		answer = true;
	}
	return answer;
}

void setup_opengl() {

    glGenVertexArrays(NumBuffers, & vertex_array_object);



    glBindVertexArray(vertex_array_object);

    glGenBuffers(NumBuffers, Buffers);
    glBindBuffer(GL_ARRAY_BUFFER, Buffers[index_ArrayBuffer]);

    // ----------


    glVertexAttribPointer(vPosition, 2, GL_FLOAT, GL_FALSE, 0, (void*)(0) );
    glEnableVertexAttribArray(vPosition);
    glBindVertexArray(vertex_array_object);
}

void display(void) {

	++FrameCount;

    glClear(GL_COLOR_BUFFER_BIT);

//    glGenVertexArrays(total_num_triangles, & vertex_array_object);
//    glGenVertexArrays(NumBuffers, & vertex_array_object);

//	std::cout << "----- top of display ------ " << std::endl;

    // ----------------

    int curr_vertex = 0;
    int curr_triangle = 0;
    for (; curr_triangle < total_num_triangles; curr_triangle++) {

    	if (get_direction(vertex_locations[curr_vertex    ][X], x_min, x_max) ||
    		get_direction(vertex_locations[curr_vertex + 1][X], x_min, x_max) ||
    		get_direction(vertex_locations[curr_vertex + 2][X], x_min, x_max)) {

    		triangle_incr[curr_triangle][X] *= -1.0;
    	}

    	if (get_direction(vertex_locations[curr_vertex    ][Y], y_min, y_max) ||
    		get_direction(vertex_locations[curr_vertex + 1][Y], y_min, y_max) ||
    		get_direction(vertex_locations[curr_vertex + 2][Y], y_min, y_max)) {

    		triangle_incr[curr_triangle][Y] *= -1.0;
    	}

    	// triangle vertex 1
    	vertex_locations[curr_vertex][X] += triangle_incr[curr_triangle][X];
    	vertex_locations[curr_vertex][Y] += triangle_incr[curr_triangle][Y];

    	if (YES == DEBUG) {
        	std::cout << "tri " << curr_triangle;
			std::cout << " X " << vertex_locations[curr_vertex][X]
					  << " Y " << vertex_locations[curr_vertex][Y];
    	}

    	curr_vertex++;

    	// triangle vertex 2
    	vertex_locations[curr_vertex][X] += triangle_incr[curr_triangle][X];
    	vertex_locations[curr_vertex][Y] += triangle_incr[curr_triangle][Y];

    	if (YES == DEBUG) {
			std::cout << "        X " << vertex_locations[curr_vertex][X]
					  <<        " Y " << vertex_locations[curr_vertex][Y];
    	}

    	curr_vertex++;

    	// triangle vertex 3
    	vertex_locations[curr_vertex][X] += triangle_incr[curr_triangle][X];
    	vertex_locations[curr_vertex][Y] += triangle_incr[curr_triangle][Y];

    	if (YES == DEBUG) {
			std::cout << "        X " << vertex_locations[curr_vertex][X]
					  <<        " Y " << vertex_locations[curr_vertex][Y] << std::endl;
    	}

    	curr_vertex++;
    }

    // ----------

    glBindVertexArray(vertex_array_object);

//    glGenBuffers(NumBuffers, Buffers);
//    glBindBuffer(GL_ARRAY_BUFFER, Buffers[index_ArrayBuffer]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertex_locations), vertex_locations, GL_STATIC_DRAW);
//    glBufferData(GL_ARRAY_BUFFER, sizeof(vertex_locations), vertex_locations, GL_DYNAMIC_DRAW);
//
//    glVertexAttribPointer(vPosition, 2, GL_FLOAT, GL_FALSE, 0, (void*)(0) );
//    glEnableVertexAttribArray(vPosition);
//    glBindVertexArray(vertex_array_object);

    glDrawArrays(GL_TRIANGLES, 0, 3 * total_num_triangles);
//    glDrawArrays(GL_TRIANGLES, 0, total_num_triangles);

    // ----------

    glutSwapBuffers();
}

// ---------------

void process_SHIFT_ALT_CTRL(unsigned char key, int x, int y) {

    switch(key) {

    	case 27: {

//    		printf ("just hit ESC key\n");
    		exit(8);
    	}
    }
}

// --------------

const char * insert_commas_into_int(int given_integer) {

	std::ostringstream stm ;
	stm << given_integer;
	std::string str_num(stm.str());

	int size_str = str_num.size() ;

//	std::cout << " num digits in " << given_integer << " is " << size_str << std::endl;
	for( int i = size_str ; i > 0 ; i -= 3 ) {

		if (i != size_str)
			str_num.insert( i, 1, ',' ) ;
	}
//	std::cout << str_num << '\n' ;

	return str_num.c_str();
}

// --------------

void TimerFunction(int Value) {

	if (0 != Value) {
		char* TempString = (char*)
			malloc(512 + strlen(WINDOW_TITLE_PREFIX));

		sprintf(
			TempString,
			"%s: %s triangles doing %d Frames Per Second @ %d x %d",
			WINDOW_TITLE_PREFIX,
			insert_commas_into_int(total_num_triangles),
			FrameCount * 4,
			CurrentWidth,
			CurrentHeight
		);

		glutSetWindowTitle(TempString);
		free(TempString);
	}

	FrameCount = 0;
	glutTimerFunc(250, TimerFunction, 1);
}

GLfloat get_random_in_range(GLfloat minimum, GLfloat maximum) {

	return(minimum + (float)rand()/((float)RAND_MAX/(maximum - minimum)));
}
void IdleFunction(void)
{
	glutPostRedisplay();
}


static void define_callbacks() {

    glutDisplayFunc(display);
    glutKeyboardFunc( process_SHIFT_ALT_CTRL );
	glutIdleFunc(IdleFunction);
	glutTimerFunc(0, TimerFunction, 0);
}

int main(int argc, char** argv) {

	// initialize random seed
	srand ( time(NULL) );

	glutInit(&argc, argv);
	glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
 	glutInitWindowSize(CurrentWidth, CurrentHeight);

     glutInitContextVersion(4, 0);
     glutInitContextProfile(GLUT_CORE_PROFILE);
     glutCreateWindow(argv[0]);

     glewExperimental = GL_TRUE;
     if( GLEW_OK != glewInit() )
         exit(EXIT_FAILURE);

     int curr_vertex = 0;
     int curr_triangle = 0;
     for (; curr_triangle < total_num_triangles; curr_triangle++) {

    	 triangle_incr[curr_triangle][X] = get_random_in_range(min_incr, max_incr) / 50.0;
    	 triangle_incr[curr_triangle][Y] = get_random_in_range(min_incr, max_incr) / 50.0;


     	if (YES == DEBUG) {
			 std::cout << " triangle_incr X " << triangle_incr[curr_triangle][X]
					   << " triangle_incr Y " << triangle_incr[curr_triangle][Y] << std::endl;
     	}

    	 GLfloat x_curr = get_random_in_range(min_incr, max_incr);
    	 GLfloat y_curr = get_random_in_range(min_incr, max_incr);

    	 // triangle vertex 1
    	 vertex_locations[curr_vertex][X] = x_curr;
    	 vertex_locations[curr_vertex][Y] = y_curr;

     	if (YES == DEBUG) {
			 std::cout << "tri " << curr_triangle << std::endl;
			 std::cout << " vertex 1 X " << vertex_locations[curr_vertex][X]
					   << " Y " << vertex_locations[curr_vertex][Y] << std::endl;
     	}

    	 // triangle vertex 2
    	 vertex_locations[curr_vertex + 1][X] = x_curr;
    	 vertex_locations[curr_vertex + 1][Y] = y_curr - size_triangle_side;

     	if (YES == DEBUG) {
			 std::cout << " vertex 2 X " << vertex_locations[curr_vertex + 1][X]
					 << " Y " << vertex_locations[curr_vertex + 1][Y] << std::endl;
     	}

    	 // triangle vertex 3
    	 vertex_locations[curr_vertex + 2][X] = x_curr - size_triangle_side;
    	 vertex_locations[curr_vertex + 2][Y] = y_curr - size_triangle_side;

     	if (YES == DEBUG) {
			 std::cout << " vertex 3 X " << vertex_locations[curr_vertex + 2][X]
					 << " Y " << vertex_locations[curr_vertex + 2][Y] << std::endl;
     	}

    	 curr_vertex += 3;	// 3 vertex per triangle
     }

     // -----------

     GLuint program = struct_program::Load( vert, NULL, frag );
     glUseProgram(program);

     // -----------

     setup_opengl();

     define_callbacks();

     glutMainLoop();
}


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A tip:

 

Use Shaders instead of the fixed pipeline. If you are thinking that's from another world, just read 5 minutes about it.

A awesome site : http://research.ncl.ac.uk/game/mastersdegree/graphicsforgames/ (where i start)

Edited by irlanrobson

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