Reply To: When normalizing the HDRI and candle, what is your goal when adjusting the exposure?

[Adrian Bellworthy]

So if the torch or candle shines outside in a scene with an HDRI, is the problem due to (a) an unnatural power setting on the digital torch/candle, (b) over or under exposure on the scene, or (c) both? Or does it just depend?

I would say (d) none of the above. 

Answer (a) – you control the power setting,  a child’s small pocket torch is not as powerful as a large torch used by search and rescue teams. So there is no unnatural power setting, it depends on the use in your scene.

Answer (b) – Exposure depends on your artistic preference and subject matter. Have you ever seen an image with bright vibrant colors, this is achieved by overexposure. Have you ever seen an image that has a dark eerie look, this can be achieved by underexposure. Try this: open blender, delete everything, switch to rendered view, go to the world settings and set the background color to fully red ( or blue or green) , slowly increase the strength, notice how the color becomes brighter and more vibrant, then the higher you go it starts losing color and eventually becomes bright and white. This is the same effect as increasing the exposure. Decreasing the exposure can also be used to reduce image noise.

Answer (c) – neither.

So, what is the answer….

There is no answer, because it is not a problem due to incorrect settings, but more to do with faking reality with a computer and artistic preference. As I understand things in Blender HDRI’s are calculated as indirect light, and a light source (point, sun, spot and area lights) are calculated as direct light. In reality the sun is a light source (direct light), but a sunny daytime HDRI is after all just a High Dynamic Range IMAGE of a scene (grass , mountains, trees, etc.), which is why it is calculated as indirect light.

You say the problem arises when you add light-casting objects to a scene with an HDRI, why is this?

As above, the method of calculation of the light in Blender.

Before you ask, it is not a Blender error or bug. If you want to point a finger at the culprit, point to the advancement in technology, but without it we would still have boring, flat, standard definition imagery.

A HDRI does a good job at simulating light based on color and brightness of different parts of the image. Direct light from a candle is in reality more powerful then indirect light because indirect light has lost some energy on reflection.

Imagine standing in a field of grass at night, shine a torch emitting white light onto the grass, white light is made up of all colors, RGB all at 1.0 in Blender, when the light hits the grass only the green light is reflected so the indirect light has decreased by two thirds, direct light cannot increase after it reflects from an object.

Thinking of a HDRI as a light source could technically be incorrect, think of it more as light that has already bounced around the scene and already lost some energy.

Therefore the problem only arises when a direct light source is added to the scene in combination to the indirect light of a HDRI and the need to increase the strength of the HDRI and decrease the exposure to correct the color of the scene. Decreasing the direct light instead would be the same as switching off the torch or blowing out the candle, but in reality you can still see the candle flame in sunlight, but not the candle light on nearby surfaces.