ryanocerous777

5 answers · asked @ video mark 8:48 · Lesson: HDRI World Backgrounds · Course: Fundamentals of Digital Lighting in Blender

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

So, if I'm understanding correctly, the issue is that the HDRI range isn't realistic, resulting in non-realistic phenomena like that super bright candle. So you then did the following process:

1.) Add a sun lamp for reference and set power as appropriate for your scene (here it was 750 for regular sun on a cloudless day)

2.) Set HDRI strength to 0

3.) Change exposure... <--- QUESTION: What are you looking for or tweaking here? Are you trying to get the candle to match what it would look like under an actual sun? Maybe it was just hard to see on my screen, but I saw you adjust the exposure but couldn't see the candle even a little.

4.)Delete Sun Lamp

5.) Increase HDRI strength until the candle's illumination matches what it did with the "reference sun."

Am I understanding this right?

  • Hey rryanocerous777 ,

    3.) Change exposure... <--- QUESTION: What are you looking for or tweaking here? Are you trying to get the candle to match what it would look like under an actual sun? Maybe it was just hard to see on my screen, but I saw you adjust the exposure but couldn't see the candle even a little.

    The scene was over exposed with the sun at a setting of 750, (based on a reel world value on an overcast day) So the need to lower the exposure value.

    Think of it as if you are taking a photograph outside on an overcast day, the exposure is a setting on your camera, the wrong setting will result in an over (or under) exposure, you cant change the sun's value to suit your camera but you can change the exposure setting on your camera.

    The example of taking a torch or a candle outside during the day, it would not cast light on any object, but it will in a scene using an HDRI and a lamp. You may want a lamp in your scene, if for example you had a scene where your character is standing outside a cave and shining a torch into the dark cave, you want to cast light into the darkness but the light should not be visible on the cave entrance as it would be without adjusting the HDRI value and lowering the exposure.

    Remember your dealing with digital lighting not a real sun or lamps.

    the issue is that the HDRI range isn't realistic

    this is not the case, the HDRI will do a great, realistic job of illuminating a scene. The problem arises when you add lamps to the scene which cast light on nearby objects.

    I hope you understand my explanation, I know a little about photography which helps me, jlampel maybe able to explain it better.

  • Thank you for answering!

    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?

    You say the problem arises when you add light-casting objects to a scene with an HDRI, why is this? Because the candle is suddenly brighter than what it would realistically be in the "daylight" conditions simulated by the HDRI?

    • 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.

  • crew

    I think Adrian did a great job answering, but just to clarify my thoughts the original question:

    What are you looking for or tweaking here?

    I'm not being super exact when I'm changing the exposure and there's plenty of room for creative interpretation - I'm just making sure that we can actually see the scene when using such high values. 

     Are you trying to get the candle to match what it would look like under an actual sun? Maybe it was just hard to see on my screen, but I saw you adjust the exposure but couldn't see the candle even a little. 

    Exactly! A candle in the sun doesn't really do a whole lot of good. It's not about getting exact values but getting used to how vastly different the values really are. 

  • very good information here!! Thank you Adrian for the articulation...