Sometimes architecture and nature come together to produce the perfect marriage of sunlight and building. That is the case in the following image (courtesy of GMT Architecture, Campinas, Brazil).
The back of the building, with a drop of 16 meters, faces west so owners can enjoy the sunset every day. In real life, it is necessary to render other faces that are not touched by so much sunlight. Let's see how we can make sunlight-coherent images using HDRI and Enscape.
The location of an object placed by the sun can be determined by its shadow, the north direction, date, and time. Enscape can reproduce geographic sunlight with remarkable accuracy. But if the purpose of your render is to be sunlight coherent, you must be careful to let the sun go in its natural path.
Note: You may be interested in understanding a process that Greek scholar Eratosthenes (c.276 BC - c.194 BC) used to evaluate the Earth's circumference by measuring the shadow of a stick stuck in the ground. We will refer to this technique in this article.
In the following image, Revit shows the sun's path simulation for a whole year.
The main entrance of this house faces south which is the least illuminated face by the sunlight. That said, it is necessary to choose a day, a time, and an HDRI for the rendering of that face. I have chosen the summer solstice (December 21), which is the day that the sun best illuminates that face.
For the HDRI, I have picked a midday one from Poly Haven. Now it is necessary to find where this HDRI's sun matches the physical sun. Following Eratosthenes's idea, I model a disc with a perpendicular cylindrical profile on it and locate it over the building, as shown in the image below.
Then I observed the shadow of this profile on the disk in two situations, defined by Enscape's Visual Settings tab:
- "Brightest Point as Sun Direction" - checked
- "Brightest Point as Sun Direction" - unchecked
The idea is to find a time of the day and an HDRI's rotation angle to make shadows match.
This can be achieved by pressing "i" or "u" (without control) when the Brightest Point as Sun box is unchecked (situation 2) to find the time of the day and by scrolling "Rotation" when the box is checked for situation 1, to find the HDRI rotation angle. The image below shows the accuracy I have achieved with such alignment.
Once shadows are aligned, I discarded the disk and profile and rendered my image, which is shown below.
If you use "control + i" or "control + u", you could get a better illumination of the scene, as shown in the following image. Still, it would not be sun coherent with respect to its geo-referenced location.
There are many situations where this feature is very useful, for instance, to define an ideal position for the building on the site or in a render contest.
Some people place rectangular lights in windows to artificially enhance the sunlight for internal scenes. They use this technique due to the lack of diffuse illumination provided by renders. As effective as this technique can be, by doing this, you may miss interesting opportunities to reflect on the possible lack of illumination in the project. If used, the intensity of light of such rectangles must not be very high to preserve sunlight coherence.
The ideal HDRI
You may want to render the same scene at sunset, for instance. Then you will have to choose another HDRI and proceed as described above. The result may be a little odd if you compare the two images since the building's surroundings would have changed.
The easiest solution for that issue is to use Enscape's native sky sources instead of HDRI, but they may not fulfill your rendering expectations. The ideal HDRI would then be composed of two concentric images: the external one for the sky and the internal one for the background. The background should be both zoomed in and out and illuminated by the sky's sun.