Before starting work on a building such as the Pantheon, you must first poses an understanding of perspective. Let's start at the very beginning (if you already have a good grasp of how to use perspective to draw buildings, you can skip ahead to step 10). Let's start with using one point perspective to create cubes. Using a ruler and a 2H or harder pencils, very lightly draw the horizon line (blue line) and the vanishing point (red X).
Next, draw a few squares floating in various places on the page, but not too close the vanishing point.
Draw lines from the vanishing point to the closest corners of each square (red lines). Then close off your cubes by drawing the back line of the cubes (green line), but be sure to stay within the red lines leading to the vanishing point.
Erase your unneeded guidelines leading back to the vanishing point and darken the lines of the cubes. You now have a cube created in one point perspective.
Now let's work on two point perspective. After drawing your horizon line, place two different vanishing points on opposite sides of the horizon line. This time, instead of drawing squares, just draw a few vertical lines floating on the page in various places. These lines will become the front edge of our cubes.
From the tops and bottoms of each line (cube edges), draw lines leading to both vanishing points on the page. It's ok if some of your lines run through each other because they can be erased later.
Now close off the sides of your cube in the same manner as step 3 above. But we are not done, since we still need to draw one more side of each cube.
This is where it gets a tad tricky. To finish off the cubes, we have to draw the third side on all necessary cubes. For any cube floating above the horizon line, we will draw the bottom side of the cube by drawing a lines from the bottom tips of the side edges of the cube leading to the opposite vanishing point. Similarly, for any cube floating beneath the horizon line, we will draw the top side of the cube by drawing a lines from the top tips of the side edges of the cube leading to the opposite vanishing point. Note that any cube who's front edge runs through the horizon line will not require a third edge, as it is hidden from our perspective.
Erase unneeded lines and darken up your cubes if you wish. You now know how to utilize two point perspective, and this is a good point to begin working on the Pantheon.
Let's start by laying down the horizon line and two vanishing points on either end.
Next, using the methods we learned for two point perspective, create a cube for the portico (entryway) of the building. It's also a good idea to draw a line near the top to get started on some of the trim.
The portico of the Pantheon has a pointed peak, which means we have to find the center of the cube to determine where the peak will be. As illustrated her in green, draw an X from corner to corner inside the face of your cube. The center of the X marks the very center of the cube. Draw a vertical line through the center of the X that protrudes out the top of the cube. The top of your peak will hit this line. Make sure to add the side of the roof and leave a lip for the trim of the roof.
Continue adding detail to the trim of the roof. Make sure the trim lines within the cube recede back to their appropriate vanishing points, and also make sure that the trim lines following the triangular roof are parallel to the roofs edge.
Next, we can work on the Corinthian columns that support the rooftop of the portico. The portico is 8 columns wide, and 4 columns deep, but you will notice that there are gaps inside the portico where columns do not stand. Make sure that your columns are perfectly vertical, but that they are slightly thinner at the tops than they are at the bottom. The top of the columns are rather decorative and elaborate, but because of our distance from the structure, we do not have to fill in every last detail. Be sure to follow the vanishing points when creating your rows of columns.
Some of the walls at the back of the portico are visible, as is one of the doorways. Draw these in, but watch your vanishing points!
Let's knock out the foundation and the floor of the portico. Notice how the wall on the far side of the building does not align with the vanishing point (as demonstrated with green lines, which you do not need to draw), but the rest of the foundation does in fact align with the appropriate vanishing points. Don't forget the steps on the side and also notice the geometric tile work on the floor.
There are a couple more levels behind the portico that we need to draw. For the peak, make sure that if falls just behind our first peak and that the lines run parallel to one another.
Now we can work on the round part of the exterior, although not too much is shown at this angle. Draw a perfectly straight vertical line for the contour of the building, and then draw the curved ledges that wrap around the side. The ledge at the top is larger than the lower two.
The dome of the structure should just peak out over the top. There is a series of rings making up the base of the dome, almost resembling steps.
Let's go back and draw in the ancient stones that form the remainder of the foundation. There are a series of large blocks running along the side of the building. Between the large blocks and the short wall along the outer parameter is a gap that we will later fill in with shadow.
There are a couple of small windows along the top level of the round structure. Take this time to also draw in some of the granite blocks that make up the portico. Not every block needs to be drawn, but keep your eye on the vanishing point. The round part of the building is made of concrete, so no stone or brick patterns are noticeable there.
Continue adding detail by giving the trim decorative edges.
Finish drawing the structure by including the words at the top of the portico. Draw two guide lines (green lines) as guides so that the letters are the proper size as they recede back towards the vanishing point. The words read: M•AGRIPPA•L•F•COS•TERTIUM•FECIT, meaning "Marcus Agrippa, son of Lucius, made this building when consul for the third time." Marcus Agrippa was a Roman general who commissioned the building in about 126 A.D.
Finally, use Micron markers and a ruler to carefully ink the image. Be sure not to ink any unneeded guide lines. Erase your pencils marks with a kneaded erases and then add some shadow under the roof of the portico and under the ledges. And that's how you draw the Pantheon!