How to judge the HDR image is calibrated in absolute luminance units or relative units

[张可可]

Dear Rafal,

My name is Keke Zhang. I am a new student in HDR area. 

I noticed that you have mentioned "The images passed to the HDR-VDP-2 must be (approximately) calibrated in absolute luminance units."

(1) I have a very confused question that how to judge the collected HDR image is calibrated in absolute luminance units or relative units?    

and (2)How to convert it to absolute luminance? You mentioned "To transform images to absolute units, the RGB vales must be multiplied by an appropriate constant. Such constant will depend on an application and the brightness at which the scenes or images are going to be seen." Are there examples?

Thank you so much. 

Best regards

Keke Zhang 

[Rafal]

I am very much behind answering queries on this forum. Let me catch up in case others have similar questions.

The easiest way to calibrate an (HDR) image in absolute units is to think about the HDR display it is going to be shown on. If the display is going to have 1000 nit peak brightness, you can find a multiplier so that vast majority (say 99%) of RGB values is below 1000. This corresponds to multiplying RGB by 1000/prctile(RGB(:),99) - using Matlab syntax. 

This may not be ideal for all the scenes. A better approach is to find a white diffuse colour in an image and multiple RGB values so that it has about 200 nit.

If you work on colour-graded content, such as video encoded using BT.2100 and PQ, it is enough to convert from PQ to linear to obtain absolute displayed units. 

Please also check the first answer at: http://hdrvdp.sourceforge.net/wiki/index.php/Frequently_Asked_Questions

Best,

Rafal