void ThickLine(point_t origin, point_t p)
{
    double angleN = 0;
    loc_t dy = 0;
    loc_t dx = 0;
    int thickness = 0;
    point_t s = { 0, 0 };
    point_t e = { 0, 0 };

    lcd.line(origin,p, rgb);
    INFO("   End @ (%3d,%3d) - (%3d,%3d) [%d]", origin.x, origin.y, p.x, p.y, pensize);
    #define PI 3.14159
    dy = p.y - origin.y;
    dx = p.x - origin.x;
    INFO("delta (%+3d,%+3d)", dx,dy);
    angleN = atan2((double)(p.y-origin.y), (double)(p.x-origin.x));
    thickness = pensize;
    for (int l=-thickness; l<=thickness; l++) {
        s.x = origin.x + l * cos(angleN+PI/2);
        s.y = origin.y + l * sin(angleN+PI/2);
        e.x = p.x      + l * cos(angleN+PI/2);
        e.y = p.y      + l * sin(angleN+PI/2);
        lcd.line(s, e, rgb);
        INFO("     %+d @ (%3d,%3d) - (%3d,%3d) a:%+3.2f:%+3.2f",
                  l, s.x,s.y, e.x,e.y, angleN, angleN+PI/2);
    }
}

The following code fragment reports 2 warnings - both dx and dy are reported {Variable 'dy' was set but never used "loc_t dy = 0;"}. Before I added the "= 0;" to these the warnings were reporting that the values might be used before initialized, but now if I remove the "= 0" is remains "set but never used". Quite confusing to me. I also changed from my typedefs (loc_t) back to int and this did not affect the warnings. And point_t is another typedef of a pair of loc_t. The code runs correctly, but I always aim for a zero-warnings. <<code>> void ThickLine(point_t origin, point_t p) { double angleN = 0; loc_t dy = 0; loc_t dx = 0; int thickness = 0; point_t s = { 0, 0 }; point_t e = { 0, 0 }; lcd.line(origin,p, rgb); INFO(" End @ (%3d,%3d) - (%3d,%3d) [%d]", origin.x, origin.y, p.x, p.y, pensize); #define PI 3.14159 dy = p.y - origin.y; dx = p.x - origin.x; INFO("delta (%+3d,%+3d)", dx,dy); angleN = atan2((double)(p.y-origin.y), (double)(p.x-origin.x)); thickness = pensize; for (int l=-thickness; l<=thickness; l++) { s.x = origin.x + l * cos(angleN+PI/2); s.y = origin.y + l * sin(angleN+PI/2); e.x = p.x + l * cos(angleN+PI/2); e.y = p.y + l * sin(angleN+PI/2); lcd.line(s, e, rgb); INFO(" %+d @ (%3d,%3d) - (%3d,%3d) a:%+3.2f:%+3.2f", l, s.x,s.y, e.x,e.y, angleN, angleN+PI/2); } } <</code>> Any suggestions?

dx and dy are given values and then used only in what seems to be a macro #define named INFO which is probably for debugging. Check the macro. It may be disabled currently and not use dx and dy at all. That would explain the warnings.

angleN = atan2((double)(dy), (double)(dx));

As you are calculating dx and dy, you might as well use them here: <<code>> angleN = atan2((double)(dy), (double)(dx)); <</code>>

Thanks Wim and Chris. Some form of code-blindness apparently had set in. INFO was indeed a disabled macro, and placing dy,dx into the atan2 was something I apparently thought I already did - no matter how many times I looked directly at it I couldn't see it... One of those forehead slapping moments. This also supports the other warning when they were not initialized.