What is HDR?
High dynamic range (or HDR) photography is a technique that allows a photographer to take the same image at different exposures and then blend them together to produce one image that features the best exposed parts from each of the images. For a virtual tour this is particularly useful as most Virtual tours rely on a 360 degree perspective so often you will be shooting both away from and into a light source. Usually this would mean that you would have to compromise between the two and potentially have dark patches that are underexposed on one side and light patches that are over exposed on the other. Using HDR you can shoot at 3 or more exposure levels, low, medium and high and then blend them together to create the perfect panorama.
What are the disadvantages?
Although this technique is very useful it does has its disadvantages: First, processing time: Using HDR imagery effectively triples the amount of images you are dealing with, unless you are using top end equipment (i5+ processors) then this is going to greatly increase time spent during post processing. Second; ghosting. HDR images are 3 or more images taken one after the other. If you have movement in your images (for example trees moving in the wind or someone slipping on a humorously placed banana) then when you come to piecing your images together the differences in the images can cause a grey ghosting effect that can ruin your scene. Finally you will have to work with software that supports HDR imagery; most top end software will but it is still a consideration, especially if you are used to one that does not.
So should I use HDR imagery to create my 360 virtual tours or not?
If you are considering using HDR then my advice is that you should first invest in a tripod and Panohead, the ghosting caused by holding your camera by hand will cause you no end of trouble during post processing and can even make your scene unstitchable. If you do invest in these pieces of equipment then with a bit of experimentation and post production patience you can create some beautiful HDR imagery for you virtual tour. If you are looking to sell the 360 virtual tours you produce HDR is now an industry standard and an essential tool to creating professional, vibrant and well lit panoramas.
Exposure Bracketing
To take HDR images technically all you need is a camera where you can manually change the exposure. You can then set the camera in a fixed position on a tripod and take several images at different exposures for use to create one HDR image. However this process is time consuming, especially for a 360 virtual tour where you will need to take several images for each scene to stitch together later. When your virtual tour contains 10+ scenes this can become a massive time sink. If you are serious about virtual tours then I highly recommend investing in a high end DSLR camera that features exposure bracketing. Exposure bracketing is a feature on a camera that allows you to set the exposures at 3 or more levels (usually low middle and high, then once you press the shutter button the camera will take the images in quick succession. This has the added benefit of reducing ghosting due to the greatly shortened time between taking the images saved by not having to manually adjust the exposure level.
Taking HDR Images
To take HDR images for your virtual tour you need to have the camera set to manual exposure and white balance. Once I have my camera and tripod set up I set my exposures by pointing the camera at the most over exposed part of the panorama (the sun if outdoors, otherwise the brightest light source). I then adjust the exposure until the image is just on the dark side of clear (you may want to be careful here if the sun is fully exposed as pointing the lens of your camera directly into the sun can cause sensor damage. Try not to have it pointing directly at the sun and if you do then only for a few seconds). Once I have the base level set to my satisfaction I adjust exposure bracketing to at least +2 and -2 (some cameras are unable to stretch this far and you might have to take 2 bracketed images). Now your camera should be set and ready to take exposure bracketed images for HDR. You can test it by taking a sample picture, if the camera takes 3 images for each press of the shutter button then exposure bracketing is set and you are ready. Now just take the images you would normally take for a panorama and depending on how many you usually take (I vary between 6 and 12) you should now have a set of three pictures for each one, one light, one normal and one dark. For extra stability set your camera to a 2 second time delay, this will combat ghosting or blurring due to tripod shake.
HDR and post processing
Now you have your HDR panoramic images for your virtual tour you are ready to stitch them together. This can be done in a number of ways; you can use your raw images to create HDR images before stitching, you can stitch your images together using software that accommodates HDR and allow it to fuse your HDR images for you, or you can create separate panoramas at each exposure level and fuse them together afterwards. Each of these ways has merits and I highly recommend experimenting with each to find which one suits you best. Whichever way you choose there is software out there to make your life a lot easier. Technically you can use high end stitching software such as PTGui or Panoweaver to stitch the images, adjust the exposure and output an HDR image ready for a virtual tour however I have found that the HDR fusing mechanisms within stitching software to be not as good as dedicated HDR software. My personal HDR magic formula is to use PTGui to stitch the images together then output three separate panoramas at different exposures. I then use dedicated HDR software to fuse them together into one fully HDR beautifully exposed panorama ready for use in my Virtual Tour. There is a lot of software on the market but for sheer professionalism of results I have to recommend Photomatix Pro, a steal for 瞿60. Good luck and happy shooting.
No comments:
Post a Comment