How To Draw a line with variables

Question

for example,In the following code, in the function “glVertex3f”, a constant between 1 and-1 is used to represent a point

glBegin(GL_LINES); { glColor3ub(255, 0, 0); glVertex3f(-1,0,0);glVertex3f(1,0,0); }

But most of the time, we use ‘variables’ more often. Suppose the width of the window is 450, and half of it is 225. The parameter input range is between 0 and 225, and when we enter an integer according to this interval, I expect to get a proportional decimal value and use this decimal value to draw a straight line.

void DrawLine(GLint ix, GLint iy,GLint ia,GLint ib) { GLfloat width = 225; GLfloat x, y, z; GLfloat a, b, c; x = (GLfloat)(ix / width); y = (GLfloat)(iy / width); z = 0.2; a = (GLfloat)(ia / width); b = (GLfloat)(ib / width); c = 0.2; glBegin(GL_LINES); { glColor3ub(255, 0, 0); glVertex3f(x, y, z); glVertex3f(a, b, c); } glEnd(); }

The above program doesn’t work at all. Please tell me what the problem is.

Rabbid76 · Accepted Answer · 2021-05-25 14:20:35Z

ix / width is an integral division You have to cast ix to a float, to do a floation point division:

x = (GLfloat)(ix) / (GLfloat)(width);

Normalized device coordinates are in range [-1.0, 1.0]. There for the conversion from NDC to window coordinates is:

x = (GLfloat)(ix) / (GLfloat)(width) * 2.0f - 1.0f;

However, I recommend to use an Orthographic projection which maps window coordinates to NDC:

glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0, width, 0, width, -1, 1);

With this projection matrix you can use the integral window coordinates (ix, iy, ...) directly.

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