GLUT individual pixel manipulation produces unexpected result - c++

Question

For the sake of learning, I'm accessing individual pixel data using GLUT and manually setting pixel color by going through all pixels in the window, like this (some non-related code omitted):
void init() {
glClearColor(0.0, 0.0, 0.0, 0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0.0, WIDTH, 0.0, HEIGHT);
}
void display() {
glClear(GL_COLOR_BUFFER_BIT);
for (int i = 0; i < WIDTH; i++) {
for (int j = 0; j < HEIGHT; j++) {
glPointSize(1.0f);
glColor3f(255, 0, 0);
glBegin(GL_POINTS);
glVertex2i(i, j);
glEnd();
}
}
glutSwapBuffers();
}
void timer(int obsolete) {
glutPostRedisplay();
glutTimerFunc(16, timer, 0);
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowPosition(100, 100);
glutInitWindowSize(WIDTH, HEIGHT);
glutCreateWindow("GLUT Test");
init();
glutDisplayFunc(display);
timer(0);
glutMainLoop();
return 0;
}
I'm expecting to get a fully red pixels window, but I'm getting something different - a window with black vertical stripes, as if horizontal for loop suddenly skipped some lines.
Any ideas what am I doing wrong here? I have a suspicion it might be related to float to int conversion somewhere "inside", but I'm not sure what to search for.
Edit: I've found out that if I resize the window in runtime to be one pixel less in width, then these black stripe tears disappear.

Answer 1

You set up the projection such that the left edge is at 0, and the right one at WIDTH. Note that your pixels are small squares with an area, and this means that 0.0 maps to the left edge ot the left-most pixel, and WIDTH maps to the right edge of the right-most pixel. Integer coordinates will lie exactly in the middle between two pixels. And with some numerical precision loss during transformation, you might end up with two neighboring points beeing rounded to the same pixel.
You can either add 0.5 to x and y when drawing your points, or just shift your orth projection by half a pixel so that integers are mapped to pixel centers:
Ortho(-0.5f, WIDTH-0.5f, -0.5f, HEIGHT-0.5f, ...);