HDR image False color

Pegah · January 8, 2023, 1:16pm

Hi Community,
I took some HDR 360 degree photos and measured the illumination at the time of shooting with a calibrated lux meter. now I want to make sure that the surface of the table in the photos shows the same amount of illumination in reality. so I’m trying to use HB false color. But false color legend does not show more than 120 lux in any setting of the photo (for example gamma correction of 2.2 & key value 0.01 (Reinhard, 2002) or even high exposure settings!!). While the illumination at the time of shooting was 1011 lux and it was validated in the simulation with an error of +2%.
Is the accuracy of this method low? any ideas? What should I do?

Thanks

chris · January 10, 2023, 5:51pm

Hey @Pegah ,

If your HDR image was not generated from Radiance, then it’s likely that the FalseColor component is not able to sense the units correctly. You can see in the source code of the component here that the component tries to figure out the units of the HDR image by parsing the Radiance header for certain keywords in the commands used to generate the image.

github.com

ladybug-tools/honeybee-grasshopper-radiance/blob/master/honeybee_grasshopper_radiance/src/HB False Color.py#L85-L108

    
      
          def sense_metric_from_hdr(hdr_path):
              """Sense the metric/units of a given HDR file from its properties.
          
          
    Args:
                  hdr_path: The path to an HDR image file
          
          
    Returns:
                  Text for the units of the file (either 'lux', 'W/m2', 'cd/m2', 'W/sr-m2')
              """
              with open(hdr_path, 'r') as hdr_file:
                  for lineCount, line in enumerate(hdr_file):
                      if lineCount < 10:
                          low_line = line.strip().lower()
                          if low_line.startswith('oconv') and low_line.endswith('.sky'):
                              return 'W/sr-m2'  # this is an image of a sky
                          if low_line.startswith('rpict'):
                              if line.find('_irradiance.vf') > -1:
                                  return 'W/m2'
                              if line.find('_radiance.vf') > -1:
                                  return 'W/sr-m2'

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So I wouldn’t trust that the units in your falscolor image are correct and you’ll likely need to make some tweaks to the component source code in order to get it to interpret your HDR units correctly.

Pegah · January 11, 2023, 7:11am

Hi @chris
Thanks so much for your reply.
I have noticed that the illumination results change as I change the Conversion factor. Is it possible to achieve results by changing this factor instead of changing source component? Is there a standard for this factor that I can refer to and use that number? and can this method be another way to achieve higher accuracy?
In fact, I don’t know how to create a custom component myself, so if there is any other suggestions I would be appreciated.

chris · January 12, 2023, 6:09pm

Ah, good point. I forgot that we included an optional conversion_ input there and that should give you what you need without needing to edit the source code. If your HDR is supposed to be in lux, then the conversion factor is probably 179.

I don’t want to explain the whole reason why this is the case here (it’s really just an arbitrarily-chosen conversion factor) but you can find longer explanations on the Radiance forum: