Compositing 3D Objects - MAXON CINEMA 4D R16 Studio: A Tutorial Approach - page 349

Figure 10-4 The line created in the Perspective viewport
The Add Line button is used to add lines, whose end points can be modified. You can
reposition the line by dragging the end points of the line. When you drag any of the
endpoints, a magnifying area is displayed. It helps you in precisely positioning the end
points on the image.
8. Place the end points of the first line, as shown in Figure 10-5.
Next, you will specify the relevant axis for the first line.
9. Press and hold SHIFT and then click on the first line; the line turns red. Click again on
the same line keeping the SHIFT key pressed; the line turns green indicating the Y-axis.
The red, green, and blue colors represent X, Y, and Z axes, respectively.