[Radiance-general] Problems using hdrgen for hdri generation

[Kirk Thibault]

On Jan 6, 2005, at 12:07 PM, Despina Michael wrote:

> Hi all,
>  
> I am a student and I am new to hdri generation or processing so I need 
> your help:)
>  
> I am trying to use hdrgen (for linux) for generating an hdr image from 
> a series of photographs with different exposure each, of a static 
> scene .
>  I faced some problems and I have some questions for the whole 
> process..
>  
> 1) First of all, I would like to know which is the best way to take 
> photographs with different exposure.
>  If I understand correctly there are two ways. Either by changing 
> F-number and let fixed the shutter speed or either by varying shutter 
> speed and let the f-number fixed.
> I would like to know witch of two ways is the best?
>  

Depends on the camera you use as to which method is easiest.  I use a 
Canon 300D digital SLR with Auto Exposure Bracketing (AEB).  I set a 
fixed aperture and shoot a sequence of three images, one at the base 
shutter speed, one at the shutter speed which produces +1 stop of 
exposure and one that produces an image with -1 stop exposure.  I 
repeat this process to get the range I'm looking for (say, 7 or 8 stops 
worth.)  I use overlapping exposures so that I can compare the lighting 
conditions in identical exposures to make sure that the light is not 
changing radically throughout the picture taking process.

I use a fixed aperture so that the Depth of Field does not change 
throughout the image sequence.  I shoot "mirror ball" images and choose 
an aperture and focal length that produce sharp focus of the ball and 
very blurred focus on the distant background so that the edges of the 
mirror ball are easy to see and define.

For example:

I would shoot a mirror ball on a stand using my camera fixed to a 
tripod.  I use the timer on my camera to expose the 3 image AEB 
sequence so that the image registration is as close as possible (i.e., 
i don't touch the shutter release, to minimize camera shake) and also 
so that i do not appear in the images.  I set the desired aperture, say 
f/8 and let the metered exposure tell me which shutter speed 
"correctly" exposes the FIRST base image - here let's say it is 1/60 
second,

I would then set the AEB for +/- 1 stop, the aperture to f/8 and shoot 
the following sequence:

1) FIRST base exposure =  @ 1/60, -1 = @1/125 (faster shutter, less 
light), +1 = @ 1/30 (slower shutter, more light)

then i would manually reset the shutter speed to 1/15 and shoot
2) NEXT base exposure = @ 1/15, -1 = @1/30, +1 = @ 1/8

then set the shutter speed manually to 1/250
3) NEXT base exposure = @ 1/250, -1 = @1/500, +1 =@1/125

etc. to cover the dynamic range you want to expose.  Each image is 1 
stop away from its neighbor.  In this example I have:

MOST EXPOSURE < 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500 > LEAST 
EXPOSURE

seven exposures worth of images, which may be an acceptable dynamic 
range for certain lighting conditions.

Notice the "overlap" at 1/30 and 1/125 - I would compare the two images 
generated at identical exposures to make sure they are not drastically 
different - if they are, that means the lighting is changing and the 
exposures likely are not really 1 stop apart.


> 2) Are "Lens Aperture", "F-number" and "F-stop" same thing?
>  Are "exposure time" and "shutter speed" same thing?
> I know that these questions may seem to be "stupid" to most of you.. 
> but as I said I am not familiar at all, with all these ..
>  

Do a google on these terms or see one of the many sites about basic 
photographic principles.  These terms are not identical but in general 
describe the parameters one may vary to determine how much light hits 
the film or sensor (exposure).  A "stop" is essentially an increment of 
exposure, with successive stops being related by half or twice as much 
light as its lower or higher neighbor.

>  
> The following questions are for hdrgen software..
> I used it with different combinations of image...
>  Some times the hdr image is generated but it's not clear at all and 
> "scene" are not aligned... and some other times the hdr image is not 
> generated at all..
>  
> 3) Where is the fault
>  when there is a warning:  Trouble finding HDR patches***** ?
> when ther is a warning: Poor convergence for order 1 fit? (is 1 or any 
> other number X appear there refers to a problem with the Xth image in 
> command arguments?)
>  
> 4) Where is the fault when there is an error:
> "Cannot solve for response function" ?
> Is that because it can not generate file for response function of 
> camera, with specific series of photographs?
>  
> 5) One problem of resulting hdr images (in cases that there was a 
> generation of image)
> was that they were somehow "green". What can cause that?
>  
> 6) My photographs are not perfectly aligned.
>  As I understood, the hdrgen uses an algorithm to align the 
> photographs.
> But as I noticed when the uses of alighnment algorithm is enable (NOT 
> use of -a option) the "alignment" getting worst..
> and when I disabled it (Using -a option) the result is better.. but 
> still no good (I guess because that original photographs are not 
> aligned)
>  
> Is there anything I can do for that???
> Does the algorithm has "limits" on how much dis-aligned can the 
> original images be?
>  
> I would appreciate it if you can answer some (or all;-)  ), of these 
> questions.
> Regards,
> Despina
>  
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