how can I get the right RGB values from a HDR image ?

Hi everyone Im working on HDR image analysis. I use terminal and radiance software to get the RGB value as a text in mac book and then work on them with matlab. but I have noticed that even for white area in the photo RGB values are (17, 18, 16)(or number in this range). what color space is it using ? or what is wrong with this conversion ? next question is what is the code to get the RGB value for a HDR image in Matlab ? Thanks !

Answer 1

[Image Analyst]

Those numbers describe a dark gray color. White values are usually brighter than 200 or so. I've never heard of "terminal and radiance software" so I can't really say what it's doing. And there is no "right" values for RGB for HDR images. Two points could have similar RGB but have vastly different radiances. For example, in an image of your desk and also looking out the window, the sheet of paper on your desk may have an illuminance of 500 lux but the white car that you can see through the window has an illuminance of 20,000 lux, and yet they both might appear in the image with an RGB of (200,200,200) or whatever. So you can't really say that any object in your scene has a "right" or "correct" range of RGB values. I mean the whole concept of HDR goes against that - HDR is meant to adjust the RGB so that everything looks good despite vastly different brightnesses of the subject, far more than a single exposure can handle.

[farfar]

I am projecting a white light source to a diffusive white panel and taking HDR photo of that panel.I knew that HDR is supposed to change some data due to those adjustments,but i was wondering why a white scene in the photo should have a dark gray color's RGB value while both panel and light sources are white ? am I missing something here ?

[Image Analyst]

You're not showing your image so we're just guessing here. If you took a photo of a diffuse white panel with HDR, it might try to maximize the dynamic range of that scene so that some of the darker parts of the panel show up as dark gray and the very brightest parts show up as bright white and everything in between is scaled linearly (or perhaps not linearly) in between. HDR is not meant for taking photos of uniform scenes. A uniform scene is a low dynamic range scene, not a high dynamic range scene.

[farfar]

[farfar]

[Image Analyst]

It looks like your flash is at or on the camera and you're getting flash bounceback (specular reflection). To eliminate, put a polarizer over the flash and another rotatable one on the camera lens. Rotate the lens polarizer until the flash bounceback disappears. Or else use a long enough exposure such that you don't need a flash.

[farfar]

the second image is the first image , i just reduced the exposure, im trying to get CCT of the light source but when I calculate x, y ,none of the points (x,y) are inside quadrangles on MacAdam ellipses on CIE 1931 (on the black body curve).and i guess it is saying the light is not white. so i guess the problem is with RGB values which I use them to calculate X,Y.Z.

[Image Analyst]

I don't understand what you're saying. I know they were the scene scene at two different exposures. I don't know what your CCT is. Can you spell it out. I don't know what x,y points you're getting and why that has anything at all to do with Macadam ellipses, which are regions of indistinguishable color and are spread over the chromaticity diagram (not just in white where there is only one ellipse). To convert from RGB to XYZ you need to use known standards, such as the X-rite ColorChecker Chart, and as shown in my attached tutorial. Finally, the center of your picture does not seem to have gray levels of 16, but more like 250, but when I save the images, I can't open them because it says they're corrupted.

[farfar]

I took a screen shot of all photos to attach it here, thats why. is there a way that I can email you these photos plus some details about what im doing? I searched for your email here but i did not find anything. Thanks !

[Image Analyst]

No, sorry.

[farfar]

[farfar]

Oh okay no problem. this is a HDR photo made of 18 exposures. even here the extracted RGB values are around 4,4,1. why it is giving me all grey and black RGB value ? the scene is dynamic enough, i guess.

[Image Analyst]

I have no idea. It looks like the gray levels span the range from 0,0,0 to 255,255,255 or so. What do your histograms look like?

[farfar]

[farfar]