I am trying to map screen pixels to openGL coordinates. Basically I have a 3D sphere drawn at position Vector3f(0,0,0) and radius=1 using glut.
glutSolidSphere(radius/*1*/,50,50);
This sphere, rather than appearing at the center of the screen appears in the top left quadrant of the screen. My goal is to convert a mouse click's location to openGL coordinates to find out what object on screen was clicked. I followed the link here to calculate the openGL coordinates using the formula
converted_x = 2.0f/screen_width*mouse_x - 1.0f
converted_y = 2.0f/screen_width*mouse_y - 1.0f
When I try to click on the center of the sphere, I get the click position (converted_x,converted_y) as (-0.62185,-0.607500) i.e., pixel location (mouse_x, mouse_y) is (242, 157). My screen resolution is 1280*800. I am not able to figure how I can use this information to figure out that the sphere was clicked. Any help is much appreciated.
Currently this is the code I have in reshape(), taken from an existing project.
void reshape(int w, int h)
{
// Adjust the viewport if the user resizes the window
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if (h==0)
gluPerspective(80,(float)w,1.0,5000.0);
else
gluPerspective (80,( float )w /( float )h,1.0,5000.0 );
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
Related
I am not able to understand the correct way of transforming primitive coordinate values to the screen coordinates.
If I use the following code (where w and h are width and height of my window 640 X 480)
glViewport(0,0,w,h);
// set up the projection matrix
glMatrixMode(GL_PROJECTION);
// clear any previous transform and set to the identity matrix
glLoadIdentity();
// just use an orthographic projection
glOrtho(0,w,h,0,1,-1);
and my primitives are
glBegin(GL_TRIANGLES);
glColor3f(1,0,0);
glVertex3f(-10,-10,0);
glColor3f(0,1,0);
glVertex3f(10,-10,0);
glColor3f(0,0,1);
glVertex3f(0,10,0);
glEnd();
The triangle becomes too big to fit the window. Most of the tutorials have the primitives in the range[-1,1] and their ortho projection between [-1,1], so the triangle comes correctly at the centre.
So, if the coordinates are generated by a 3rd party software (or lies above the range [-1,1], how would I transform them correctly so that the coordinates fit the screen?
So a lot of questions online about resizing have been about maintaining the right ratios and avoid stretching etc. From what I understand, this would be done by setting the new ratio with gluOrtho2D.
However, I wasn't sure exactly how to go about showing MORE and LESS of the world upon resize. E.g. you have a plane that could travel from 0 to 100 along the x axis. Upon resizing, it should now (still same size) travel from 0 to 200.
EDIT: so what I mean is, I want everything in my game to stay the same size as before, but the "sky" if you will, should be bigger upon the resize, and my plane should be able to fly into that sky (since currently I have code that limits it to within the screen).
Similarly, if my screen is smaller, then the plane should no longer be able to fly to the section of the 'sky' that no longer exists
Initially, I'm setting up my program using the following lines, where everything about the game is stored in 'game', and XSize, YSize returns the size of the screen.
void init(void) {
glClearColor(0.0, 0.0, 0.3, 0.0); /* set background color to a dark blue */
glColor3f(1.0, 1.0, 1.0); /* set drawing color to white */
glMatrixMode(GL_PROJECTION);
glEnable (GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glLoadIdentity();
gluOrtho2D(0, game.getXSize()*game.getAspect(), 0, game.getYSize() / game.getAspect()); /* defines world window */
}
int main(int argc, char *argv[]) {
game = GameManager(GAMENAME, 1000, 750, 60);
/*SETUP*/
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(game.getXSize(), game.getYSize());
glutCreateWindow(GAMENAME);
/*Other GLUT main function lines here */
glutReshapeFunc(resize);
}
When I try to set up the gluOrtho2D in resize, however, the program sets up the background and stops drawing anything at all.
void resize(int w, int h){
game.setScreenSize(w,h);
glViewport(0,0,width,height)
const GLfloat aspectRatio = (GLfloat)game.getXSize() / (GLfloat)game.getYSize();
gluOrtho2D(0, game.getXSize()*game.getAspect(), 0, game.getYSize() / game.getAspect());
}
I have, of course, managed to just use glViewport(0,0,w,h) by itself, but that's pretty much the same as not doing anything at all (the graphics just stretch, and functions I'm using to move objects to the mouse position no longer work properly), since glViewport is called by default if I don't create a Reshape function.
The general way world coordinates get mapped to screen in OpenGL is:
world coordinates --> clip space coordinates --> device coordinates
The "world coordinates" are just whatever you feed to OpenGL as vertex data. They can be whatever you want, there are no rules.
The vertex shader (or matrix stack, if you are time traveling to the 1990s) is what transforms world coordinates to clip space coordinates.
The clip space coordinates go from –1…+1. So (–1,–1) is the lower-left corner of the window, (–1,+1) is the top left, (+1,+1) is the top right, etc. This is the same no matter what size your window is. So if your window gets larger, the picture will also get larger, unless you scale down the clip space coordinates at the same time.
So if you want to keep the same world coordinates and keep the same size in pixels, you have to change the way world coordinates are transformed to clip space. In general, this means that if your window gets twice as big, your clip space coordinates should get half as big, in order to keep everything the same size.
Typically, to achieve this, you'll end up multiplying in a matrix that looks something like this:
int windowWidth = ..., windowHeight = ...;
double matrix[2][2] = {
{ 1.0 / windowWidth, 0.0 },
{ 0.0, 1.0 / windowHeight },
};
That's if you're using a 2D matrix. Change this appropriately if you are using glOrtho or for your particular vertex shader. Or just read the manual for glOrtho.
By using:
gluOrtho2D(-1.0f, 1.0f, -1.0f, 1.0f);
Which would be the same as:
glOrtho(-1.0f, 1.0f, -1.0f, 1.0f, -1.0f, 1.0f);
Then I'm assuming your problem is that when you scale a scene like this, then it ends up looking like this:
As you say this can be fixed by taking the aspect ratio into account. Given the width and height of your screen. Then you can calculate the aspect ratio and set the proper orthographic projection:
const GLfloat aspectRatio = (GLfloat)width / (GLfloat)height;
gluOrtho2D(-aspectRatio, aspectRatio, -1.0f, 1.0f);
This now results in everything scaling in relation to the aspect ratio, and subsequently allowing you to see more.
Since the above is actually a sphere in 3D, setting the near and far values is also needed:
glOrtho(-aspectRatio, aspectRatio, -1.0f, 1.0f, 1.0f, 100.0f);
I have made some shapes like this :
// Triangle
glBegin(GL_TRIANGLES);
glVertex3f(0.0,0.0,0);
glVertex3f(1.0,0.0,0);
glVertex3f(0.5,1.0,0);
glEnd();
// Cube using GLUT
glColor3f(0.0,0.0,1.0);
glutSolidCube(.5);
// Circle
glPointSize(2);
glColor3f(1.0,0.0,1.0);
glBegin(GL_POINTS);
float radius = .75;
for( float theta = 0 ; theta < 360 ; theta+=.01 )
glVertex3f( radius * cos(theta), radius * sin(theta), 0 );
glEnd();
Initially I keep my window size as 500x500 and the output is as shown :
However, if I change the width and height (not in proportion) of my widget, the shapes get distorted (Circle looks oval, Equilateral triangle looks isosceles) :
This is the widget update code :
void DrawingSurface::resizeGL(int width, int height)
{
// Update the drawable area in case the widget area changes
glViewport(0, 0, (GLint)width, (GLint)height);
}
I understand that I can keep the viewport itself with same width and height, but then lot of space will get wasted on sides.
Q. Any solution for this ?
Q. How do game developers handle this in general, designing OpenGL game for different resolutions ?
P.S. : I do understand that this isn't modern OpenGL and also there are better ways of making a circle.
They solve it by using the projection matrix, both the perspective matrix and ortho projection traditionally have a way of getting the aspect ratio (width/height) and use that to adjust the result on screen.
I need to draw a triangle in 2d at the center of the screen and rotate it at the center only using a passive mouse actions any reference code?
void myDisplay(void)
{
glClear(GL_COLOR_BUFFER_BIT); // clear the screen
glBegin(GL_TRIANGLES);
glVertex2i(100, 50); // draw three points
glVertex2i(100, 130);
glVertex2i(150, 130);
glEnd();
glFlush();
This is the image and the triangle:
Assume we have a triangle with one vertex lying in the center of the screen (NDC=[0,0]), that is symmetric along the x-axis. The problem of letting the triangle point to the mouse is than the problem of finding the angle between the vector going from the center of the screen to the mouseposition and the x-axis ([1,0]). A solution for doing this can look like the following:
Vec2 mousePosition = queryMousePositionAsDoneInYourAPI();
float angle = acos(dot(normalize(mousePosition - [width/2, height/2]), [1,0]));
glLoadIdentity();
glRotatef(angle, 0,0,1);
glBegin();
drawTriangle
glEnd();
Most of this code should be fairly self explanatory. I got an display function and my view port function. There are two modes which is 4 small view ports in the window or one large.
I got one camera which can be moved and if in 4 view port mode just 3 fixed angles. The thing is I want the free moving cameras position to be displayed in the 3 other view ports. I tried doing it by drawing spheres using opengl but the problem is that then the position gets draw in the free roaming camera too as it shows the same scene.
It doesn't have to be a sphere, just something simple that represents the cameras spacial position in these three other views.
Drawing the scene once with camera object showing for the three viewports, render to texture. Clear and draw scene without camera object render to texture and then stitch these together before actually drawing the scene seems like a lot o work for something that should be easy.
void display(int what)
{
if(what==5){
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
camControll();}
if(what==1){
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(75,15,-5,0,5,-5,0,1,0);}
if(what==2){
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0,110,0,0,0,0,1,0,0);}
if(what==3){
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, float(320) / float(240), 0.1f, 100.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
camControll();}
if(what==4){
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(185,75,25,0,28,0,0,1,0);}
glClearColor(0, 0, 0, 1);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
drawScene();
drawCamera();
glutSwapBuffers();
}
void viewport(){
glEnable(GL_SCISSOR_TEST);
if(!divided_view_port)
{
glViewport(0, 0, w, h);
glScissor(0,0,640,480);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, w / h, 0.1f, 100.0f);
display(5);
}
else
{
////////////////////// bottom left - working
glViewport(0, 0, w/2, h/2);
glScissor(0,0,w/2,h/2);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, w / h, 0.1f, 300.0f);
display(1);
//////////////////////
////////////////////// top right - working
glViewport(w/2, h/2, w/2, h/2);
glScissor(w/2,h/2,w/2,h/2);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, w / h, 0.1f, 300.0f);
display(2);
//////////////////////
////////////////////// bottom right -working
glViewport(w/2, 0, w/2, h/2);
glScissor(w/2,0,w/2,h/2);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, w / h, 0.1f, 300.0f);
display(3);
////////////////////////
////////////////////////// top left
glViewport(0, h/2, w/2, h/2);
glScissor(0,h/2,w/2,h/2);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, w / h, 0.1f, 300.0f);
display(4);
///////////////////////////
}
glDisable(GL_SCISSOR_TEST);
glMatrixMode(GL_MODELVIEW);
}
So what I basically need is to hide this object in specific viewport.
Why not make that single Sphere object (or the entity responsible for drawing the sphere) aware of the "current viewport" (which happens to be the what variable in your code) and let it be invisible if it's the given viewport?
This solution exactly corresponds to the logic involved here sounds both simple and correct.
A more general solution would be to give each "camera" a GUID and make it available for the entity responsible for drawing Cameras to check the GUID of the "camera" bound to the viewport which is being rendered at the moment. If they happen to be equal, ignore the camera object during this draw pass.
I think that should be easy if you would just draw point, because if you want to see point in viewport, its center have to be in viewport, otherwise nothing of it is displayed even if you set huge point size. Then you have 2 options to eliminate flickering effect (as when you put 2 squares in the very same possition they will flicker one over another). You can just move that point little behind camera, or use nonzero value for near clipping plane in glFrustrum/gluPerspective call.. and well if you update point position every time you move camera you have no chance of seeing that point in your moving camera and you can use single scene.
And second option, I don't know if you can update just single viewport, but maybe just setting scene, displaying it to moving camera, drawing camera position and displaying it for other 3 viewports should be easy also..
Why don't you draw the sphere behind the moving camera's near plane? That should ensure that the moving camera doesn't see the sphere at all, but its position is clearly marked for the others.