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.
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 [email protected]/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.
On Jan 6, 2005, at 12:07 PM, Despina Michael wrote:
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.
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