Advanced image registration for HDR (using PTGUI)

[jetmarc]

Last weekend I had time to test the PhotoMatix HDR software. It combines several

different exposures of a scene into one HDR image, and then tone-maps it to

generate a (more or less) pleasing image.

 

I captured one scene with tripod, and another one without. In both cases I used

the auto-bracket feature of my camera. In the "tripod" scene, I manually added two

more exposures.

 

After importing the scenes into PhotoMatix, it became clear that only the tripod

auto-bracket exposures are in good alignment. All other exposures are off,

including those taken with tripod (I'd moved the camera slightly).

 

The HDR images suffer from ghosting artefacts, caused by this mis-alignment. I

wasn't able to remove them. PhotoMatix offers several tools for image

registration, but they didn't work for me (good enough).

 

I have experience using PTGUI, so I gave it a try. This is what I did, and it

worked out fine:

 

A. Convert the 0EV RAW image to TIFF

B. Convert the -2EV RAW image to TIFF, using a +2EV software exposure compensation

C. Convert the +2EV RAW image to TIFF, using a -2EV software exposure compensation

 

Note that the exposure compensated images look about the same as the 0EV master

image. Only the highlights are more blown-out in one of them, and the shadows are

more noisy in the other one.

 

D. Import these three images in PTGUI.

E. Set FOV, then allow individual lens and shift parameters for the compensated

images

F. In the optimizer tab, clear all and then allow (for the compensated images) yaw

and pitch and roll and h-shift and v-shift and fov.

G. Generate control points (automatic)

H. Delete all control points off moving objects like clouds.

I. Delete all control points between the two exposure compensated images (only

leave those that refer to the master image).

J. Add some extra control points for important regions (horizon line and high

contrast details)

K. Optimize

L. Adjust panorama FOV.

 

M. save project

N. copy project file and edit copy in a text editor. replace the filenames of the

exposure-compensated images 1,2 with the real exposures 1,2

 

O. open manipulated project file

P. create 16-bit tiff at "maximum" size, individual layers only (no blending)

 

 

The resulting 3 image files were perfectly aligned versions of the 3 exposures.

Perfect alignment, that PhotoMatix integrated tools weren't capable of. With these

images I was able to generate a HDR without any ghosting.

 

The only necessary extra step in PhotoMatix is to crop the HDR image once

generated. I left a black border around the aligned images. Tone-mapping is

confused by this "contrasty" edge, so I had to remove it.

 

I hope this is of use to you.

 

Kind regards,

Marc

[bill_t__new_mexico_]

Thanks for that writeup Marc!

 

Some time ago by accident I submitted two sets of panoramic images to PTGui at the same time. They were about 10 images each of the same scene, but shot with different yaw and slightly different pitch. PTGui lined them both up perfectly, with nearly-the-same images registered over each other on individual layers, all based on 1 set of control points. In PS I could have easily separated the layers into separate panos. Haven't tried it recently, but it worked great that time!

[roger_smith4]

Thanks Marc, this is interesting. I wonder if it would make HDR from slides possible. The problem up until now has been the impossiblity of perfectly scanning two frames of film without misalignment. This could be very interesting for night photography...

[emil_ems1]

Marc,

 

Very interesting approach. I hope you permit two simple questions from an amateur (who will soon, I hope start with digital processing):

 

1. Why do you compensate the under- and overexposed files before submitting them to PTGUI?

 

2. Do you work with those compensated files in the subsequent application of PhotoMatix?

[jetmarc]

Emil,

 

1. The exposure compensation simplifies the control point generation. PTGUI can auto-align the images better, and it's easier to verify and add control points manually.

 

2. The bad formatting of my post is confusing, sorry.

 

In step N, the PTGUI project is modified with a text editor. The filenames of the exposure compensated files are replaced with the filenames of the "real" exposures.

 

Consequently, the actual panorama generation (or here: image registration) operates on the real images - and so does all further processing in PhotoMatix.

 

The compensated images play no role anymore after this point. They can be deleted, although I prefer to keep them around until verifying the successful registration. The "Average" function is a good tool to do this. It can be used after opening the images in PhotoMatix. If there's any ghosting, I can go back to step J and add more control points for the offending features.

 

I hope this clears it up.

 

Kind regards,

Marc

[emil_ems1]

Marc,

 

thanks a lot for your explanation. All is clear now. This is an excellent approach. Congratulations for your ingenuity.