so, what is HDR?
HDR is short for High Dynamic Range Images. HDR images are a special type of image that contains more information per pixel than simply color values. each pixel contains a range of values depending on the number and exposures of input images. this range can then be expressed as lighting, and the range will extend from the brightest white to the darkest black in the image. a .jpg image has a contrast ratio of 256:1, most dynamic range images available here are between 1,000:1 to 50,000:1.
a typical HDR image is created in a computer by combining at least 2 images taken with a digital camera. these images need to be taken with different exposure settings, but from the same vantage point, preferably on a tripod so that when laid on top of each other in the computer they will line up. then when the computer combines the mutilple images into a single image, each pixel now contains a "range" of values intead of a single RGB value. this "range" represents the lighting intensity values per pixel that the multiple images captured.
most modern mid-high end digital cameras have a RAW image format, which is a type of HDR image. most RAW formats capture roughly 5 aperature stops in a single image. this is usually enough light information to make adjustments to your average photgraphs highlights and shadows.
for our purposes, we are going to want a much higher dynamic range than 5 stops, so we will need to take more photos to build up the HDR image. to do this precisely we need to control the camera with a laptop, and a bit of Canon software. the computer allows us to mount the camera on a tripod, control all of the camera settings, and take sequential photos without moving. typically we take at least 10 photos spaced 3 stops apart, starting at 1/4000th sec to however long is needed to have a completely blown out image. this gives us the maximum exposure range for that capture, although not every image may be used, it is nice to have them all in case they are needed. just like clarence says, "better to have a gun and not need it, than need a gun and not have it.", not so much life or death here, but the principle applies.
so, what is a light probe?
a light probe is basically a small chrome sphere either mounted or set on top of a tripod. by taking photos of the sphere in an interesting setting it is possible to capture a full 360 degree view of the environment. if sequential images are taken, the images will not only gather the environment, it will capture the lighting information as well. since each pixel represents a range of color information, and the image contains a 360 degree view, we can capture the lighting and reflections for a particular time and place.
so, what is it used for?
since these images contain a 360 degree view of the world and are in HDR space, they can be used by computer rendering programs to recreate lighting from that particular time and place. it will correctly cast shadows, reflections, and correct ambient lighting without any real effort by the artist. if the capture is high enough (none of mine are) they could also be used as background textures.
so, how do you make one?
the pioneer in this area is Paul Debevec, from USC, and everything i am doing here is derived from his work. that being said, these are the items needed to make an HDR light probe:
required items:
-
laptop, smaller and lighter is better
-
digital camera, controllable through USB
-
chrome sphere
-
2 tripods
-
HDRshop
-
Photoshop CS2 or another HDR editor
items suggested from experience:
-
a good Canon DSLR with telephoto lens
-
90 degree laser line tool
-
2 plumb bobs, one per tripod
-
string and small weights
so, what's the capture process?
-
begin by setting up the chrome sphere on one of the tripods in the capture environment. it should be setup at least 6in above the tripod, to minimize the tripods inclusion in the photos. the sphere should be at least waist high, perhaps even chest high. usually the height is dictated by the length of the USB cable, since it has to stretch from the camera to the laptop, which eventully needs to be sat on the ground. also remove or hide any handles, knobs, or levers from tripod, it just saves time later.
-
setup the camera on the other tripod with the optimum distance being "as far away as possible keeping the sphere full frame" using the camera's strongest optical zoom. the camera should be level with the sphere, set to manual focus, a wide aperature setting, and setup for PC controlled use.
-
now use the laptop to begin taking photos. start at 1/4000th sec and take a picture every 3rd stop until the image is completely burned out.
-
after the first set of photos are done, leave the sphere alone, and rotate the camera around the sphere to 90 degrees out from the original location. imagine looking at this setup from above like a clock, with the sphere in the middle and the first photo set taken from the 12 o'clock position. the second set will be taken from either the 3 or 9 o'clock positions.
-
take a second set of images like the first, start at 1/4000th sec and take a picture every 3rd stop until the image is completely burned out.
-
this is enough to complete the probe, but to be super anal retentive, take another set from the opposite side. so if the second set was taken at the 3 o'clock position, do another set at the 9 o'clock position. this just minimizes the chance of taking the images, and then not having them line up enough to complete the probe, or if there is glare, scratch or hair on the ball, etc..
-
that is about all for the capture process, to do the final creation, follow the HDRshop tutorial process outlined by Paul Debevec.
so, any tips and tricks?
-
a 90 degree laser level tool from any hardware store. i got mine from Home Depot and i ended up with a Strait-Line laser level tool for about $60. it will shoot laser beams in 3 directions at once, 90 degrees apart . it gets placed directly under the chrome sphere tripod and from that point it is easy to see where to line up the camera for the 2 sets of photos. the laser is usually faint enough to not show up in the final composites, but it makes aligning the photos much easier.
-
a couple of plumb bobs from the hardware store. i attach one to each of the tripods middle column, so that when the legs are open its easy to know where the direct center of the legs are. it makes lining up the shots with the laser easier since you know where the center of the tripod is, and the silver plumb bob will reflect a bit of the laser making it easy to see.
-
the next trick i stole from a jc penny portrait photographer. know your exact distance from the sphere to the camera so the distances always match. if your focus is locked then the distance shouldn't be too hard to match, but just to make it that much easier, i suggest the string trick. setup the camera the correct distance from the ball, and cut a piece to string to the distance from tripod center to tripod center. i tied a slipknot on one end and a small keyring on the other. its not attached to the camera tripod, just the sphere tripod, so its just used to measure the correct distance.