Set a frustum to a sub rect of a larger view

SetFrustumToSubRectOfLargerView.c

void SetFrustumToSubRectOfLargerView(
    float fullWidth,     // width of large 'virtual' monitor that other views will be sub rect of
    float fullHeight,    // height of large 'virtual' monitor that other views will be sub rect of
    float subRectX,      // left edge of sub rect for this view
    float subRectY,      // top edge of sub rect for this view
    float subRectWidth,  // width of sub rect for this view
    float subRectHeight, // height of sub rect fo this view
    float fovInRadians,  // field of view in radians for fullwidth x fullheight view.
    float zNear,         // z near clipping plane
    float zFar)          // z far clipping plane
{
  float aspect = fullWidth / fullHeight;
  float top    = tanf(fovInRadians * 0.5f) * zNear;
  float bottom = -top;
  float left   = aspect * bottom;
  float right  = aspect * top;
  float width  = fabs(right - left);
  float height = fabs(top - bottom);
 
  gluFrustum(
    left + subRectX * width / fullWidth,
    left + (subRectX + subRectWidth) * width / fullWidth,
    top  - (subRectY + subRectHeight) * height / fullHeight,
    top  - subRectY * height / fullHeight,
    zNear,
    zFar
  );
}