OpenGL Aspect Ratio Issue - opengl

Question

I am using Qt + OpenGl for 2D rendering.
I am a beginner at OpenGL and for the life of me i am not able to figure out this aspect ratio issue. Everytime i think i have understood glOrtho and gViewPort, but very next time i am into another issue with them. While if coordinates are symmetric like between -1 and 1, my code works else it doesn't. I really want to get through these for once and all. All the suggestions i have searched and applied have gone fruitless for me.
My Problem Statement:
I am rendering a square and a triangle and i switch between them with keystroke "R". I am also zooming in and out. While square is maintaining aspect ratio, triangle is not. Coordinates for shapes are:
Square: (-10, -250), (500, -250), (500, -260), (-10, -260);
Triangle: (250, 0), (310, 0), (280, 30)
Basically I am not able to render above triangle. Here is code for same:
My Code
#include <QtGui/QMouseEvent>
#include <qdebug.h>
#include "GLWidget.h"
#include "stdio.h"
#include "qgl.h"
#include "qimage.h"
GLWidget::GLWidget(QWidget *parent) : QGLWidget(parent)
{
setMouseTracking(true);
}
void GLWidget::initializeGL()
{
glDisable(GL_TEXTURE_2D);
glDisable(GL_DEPTH_TEST);
glDisable(GL_COLOR_MATERIAL);
glEnable(GL_BLEND);
glEnable(GL_POLYGON_SMOOTH);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glClearColor(1, 1, 1, 0);
glEnable( GL_POINT_SMOOTH ); // For Circular Points
}
void GLWidget::resizeGL(int w, int h)
{
canvas_width = (double)w;
canvas_height = (double)h;
aspect_ratio = canvas_width/canvas_height;
left_plane = 250;
right_plane = 310;
bottom_plane = 0;
top_plane = 60;
z_near_plane = 1;
z_far_plane = -1;
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if( canvas_width > canvas_height ){
glOrtho(left_plane*aspect_ratio, right_plane*aspect_ratio, bottom_plane, top_plane, z_near_plane, z_far_plane);
}else{
glOrtho(left_plane, right_plane, bottom_plane/aspect_ratio, top_plane/aspect_ratio, z_near_plane, z_far_plane);
}
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void GLWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1,0,0); // red
glBegin(GL_POLYGON);
//glVertex2f(-30,0);
//glVertex2f(30,0);
//glVertex2f(0,60);
glVertex2f(250,0);
glVertex2f(310,0);
glVertex2f(280,60);
glEnd();
}
I do not see any traingle because left_plane*aspect_ratio clips my drawing (250 is minimum X, 250*1.4 > 310, 310 is maximum X).
I hope i have made myself clear.
I will try to place images as well (i guess i will have to upload images to some other site and link them here?).

Answer 1

Your problem is that the bounding box (the values assigned to left/right/top/bottom_plane) doesn't have the same aspect ratio as the viewport. If you have the bounding box for the object, you need to find viewport coordinates with the correct aspect ratio (w/h). The viewport needs to be both centered on the bounding box center, and be big enough to fit the bounding box. But the aspect ratio of the viewport has nothing to do with the size of the bounding box.
In general you have a 3D bounding box (8 corner points). You would project each corner onto the screen, then use min/max to get a rectangle that needs to be centered on screen. You then check the aspect ratio ar = wr/hr of that rectangle against the aspect ratio of the viewport a=w/r. If a < ar, you need to fit wr to w, otherwise fit hr to h.
void GLWidget::resizeGL(int w, int h)
{
// First set up the projection.
double canvas_width = (double)w;
double canvas_height = (double)h;
double a = canvas_width / canvas_height;
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-a, +a, -1, +1, -1, +1);
// Now set up the view matrix.
double leftBoundingRectangle = 250;
double rightBoundingRectangle = 310;
double bottomBoundingRectangle = 0;
double topBoundingRectangle = 60;
double widthBoundingRectangle = rightBoundingRectangle - leftBoundingRectangle;
double heightBoundingRectangle = topBoundingRectangle - bottomBoundingRectangle;
double ar = widthBoundingRectangle / heightBoundingRectangle;
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// Center on bounding rectangle center.
double tx = (leftBoundingRectangle + rightBoundingRectangle)/2.0, ty = (topBoundingRectangle + bottomBoundingRectangle)/2.0;
glTranslated(tx, ty, 0.0); // or is it -tx, -ty?
// Scale to fit bounding box.
double s;
if (ar > a)
{
s = ... // sorry, but you have to figure this one out for yourself. :)
}
else
{
s = ...
}
glScaled(s,s,s);
}