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POWhy doesn't this midpoint line algorithm draw vertical lines (or diagonals that approach the vertical)?
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copied!<p>I implemented this algorithm I found online, and it draws horizontal lines and half of all diagonals just fine. But as the diagonal line passes the "halfway" mark to becoming vertical, it doesn't update y and only draws the line with a smaller slope. It does draw it all the way to x2, but draws a line sloped to a different y2 coordinate. And if you try to draw a vertical line, nothing is drawn. Can anyone find the bug?</p> <pre><code>void init() { glClearColor(0.0, 0.0, 0.0, 0.0); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(0.0, 500.0, 0.0, 500.0); } void midPoint(int x1, int y1, int x2, int y2) { if (x1 > x2) { midPoint(x2, y2, x1, y1); return; } int slope; int dx, dy, d, x, y; dx = x2 - x1; dy = y2 - y1; d = dx - 2 * dy; y = y1; if (dy < 0) { slope = -1; dy = -dy; } else { slope = 1; } for (x = x1; x < x2; x++) { glBegin(GL_POINTS); glVertex2f(x, y); if (d <= 0) { d += 2 * dx - 2 * dy; y += slope; } else { d += -2 * dy; } glEnd(); } } void display() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glColor3f(1.0, 0.0, 0.0); midPoint(10, 10, 110, 210); glColor3f(0.0, 1.0, 0.0); midPoint(10, 10, 210, 110); glColor3f(1.0, 1.0, 0.0); midPoint(210, 10, 10, 110); glFlush(); } int main(int argc, char *argv[]) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); glutInitWindowPosition(50, 50); glutInitWindowSize(500, 500); glutCreateWindow("Bresenham\'s midpoint line algorithm"); init(); glutDisplayFunc(display); glutMainLoop(); return 0; } </code></pre>

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