Jun 1, 2009

HDR - Tips & Tricks

Candyland, Arizona, early evening

©Joe Bridwell
Late afternoon Hoodoos encapsulate beauty while crying subtle patriotism. In a remote Arizona corner, one can imagine a candymaker ladling chocolate in ropy swirls; in a fantasy land, maybe chocolate is red! Perhaps Mother Nature was the candymaker; she can choose to color any desert as she wishes.
For me, Candyland is also a delicately gorgeous high dynamic range (HDR) image which became a challenge during development.

Photomatix vs. CS4 HDR
Five HDR images ranging from -2, -1, 0, +1, +2 in EV (exposure value) are processed into a 32-bit image to capture the subject’s full color range. Then, one might choose to modify 32-bit image before saving it as a 16-bit tiff. I tried the same process in CS4 and Photomatix; each of the results contains major problems.


CS4 contains less chromatic aberration and a very distracting feature I will call 'edge grunge'. Edge grunge is a 4-5 pixel wide disruption - darker edge on dark colors and lighter edge at the sky (each about 2 pixels wide). It's usually noticeable at 100% magnification. Unfortunately, CS4 didn't achieve very much success in reproducing the rich colors of rocks and sky.

Photomatix, chromatic abberations, edge grunge

Photomatix tends to produce more appropriate colors. Unfortunately, it really emphasizes edge grunge and leaves red or blue chromatic aberrations much more strongly; not only on edges between sky and land, but within the rocks as well.
Scrooge would have said, “Bah…Humbug!”
Rather than use sliders provided by Photomatix, I've learned to take the 32-bit file at default settings, convert it to 16-bit. For best over all results, I use default values ~ then Tone Map in CS4.

CS4’s New OpenGL Ability as a Preferred Working Tool
CS4 does graphics calculations to a GPU (graphical processor unit). If your computer contains an appropriate graphics card, CS4 will let you blow your image to magnifications of 800-1200% for high-resolution touch up. At that scale, you're able to see individual pixels, scale your clone or repair brush to 3,4, or 5 pixels, and Clone these irritating scenes away.
To prepare CS4 for OpenGL, you go to Edit> Preferences> and choose Enable OpenGl Drawing for an appropriate graphics card. If you don't have an appropriate graphics card, the Enable button will not allow a checkmark (and you can't take advantage of additional features from the next section). An NVIDIA GeForce 7300 LT is about the minimum card to be of reasonable support.

Rotate, Smooth Pan and Zoom, Pixel Grid in CS4
A nice aspect of 'normalizing' edge grunge and chromatic aberrations is the speed CS4, OpenGL, and a good graphics card provide when Tone Mapping an HDR image.
I always evaluate the entire image at 1-to-1 (100% zoom) several times. I may be looking for dust spots, edge grunge, chromatic aberrations, or other eye-catching issues.
CTRL-+ scales your image up; above 600%, a pixel overlay is provided. For me, 800% quickly helps evaluate each problem and Clone corrections. ALT-click on a ‘good’ color lets me create a Clone source to replace bad colors. Holding down Space Bar turns the Clone Tool into a Hand; simply flick the zoomed image to a new position containing another repair area to swiftly proceed.
For those of you who do Paths (sharp vector outlines of specific objects to create masks), check out the Rotate tool! Instead of making corner points and bending lines during the creation of a path, you can use rotate the tool, use your Wacom pen tablet, and draw a straight line around the object. Wow; what a marvelously skilled Photoshop tool!

Although longer than usual, this HDR tip will certainly put you in a better position to submit your final images for judging.

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