OffTopic:Photography/BC Parliament Buildings HDR mosaic

From HarmReductionProductions
Jump to: navigation, search

See w:Image:British Columbia Parliament Buildings - Pano - HDR.jpg

By HighInBC, feel free to discuss or ask questions about this page at Talk:OffTopic:Photography/BC Parliament Buildings HDR mosaic. You may edit this page if you sincerely feel it will improve it. HighInBC 21:51, 12 June 2007 (UTC)


Source images

I used the CHDK add-on for my Canon PowerShot S3 IS to allow me to shoot in RAW mode, and to load a script allowing me to bracket off a fixed setting, as opposed to bracketing off of metering which is all the camera normally allows. I used a Panoramic tripod head to make sure the point of view of the camera did not change when I rotated.

I took a 10x2 set of the buildings, each with 3 shutter speed bracketed exposures for a total of 60 images. Images were taken in RAW mode at 2x zoom using a focal range from 2.9 meters to infinity. The Images are stepped 10 degrees horizontally centered on the center of the building, the top set has +20 degrees pitch, and the bottom set is level. Each shot covered about 17 degrees horizontally and more vertically.

The resulting area covered is 105 x 63 degrees.

I shot so that I got both RAW and jpeg files. I converted the RAW images into DNGs using "DNG For PowerShot 1.1.4".

Convert to 32 bit

Created pixel accurate 32 bit HDR images for each of the 20 camera positions by loading the three DNGs for each point of view in Photoshop and combining them using the "Merge to HDR" command. This took me many hours.

Creating 16 bit sets

I converted the 32 bit HDRs into 16 bit LDRs by creating a tone curve using Photoshop's "Local Adaptation" filter. I then applied the same tone curve to all the 32 bit images to create equally exposed 16 bit files.

A few problems exist in the source files that needed to be addressed.

Objects moved while taking the images

Moving objects such as flags, birds and the two guards caused artifacts in the 32 bit version that need to be manually repaired in each 16 bit set. In order to fix this I took a section from the source image that best matches the exposure in the HDR. I took a square of, for example a flag, leaving plenty of room around it. Then I placed it over the damaged flag in the HDR image, and adjusted its contrast and brightness to match as best I could. Then I selected just the flag with the lasso tool and feathered the selection(by 20 pixels in the case, depends on size of object), then inverted the selection and deleted. This creates a copy of the flag with outward graduating transparency, which helps blend the repaired version into its surroundings.

Polarizing filter changed lighting

Some of the sections of sky are darker than the others despite being shot with identical settings and treated the same. This is due to the polarizer filter I used filtering different depending on the angle. Next time I must not do that. I was able to reduce, but not eliminate this effect in post production.

Creating mosaic

Creating the mosaic is time consuming, but not difficult.

I used PTgui. I ran the set of 16 bit tone mapped images into it and entered the pitch and yaw I used when photographing. I automatically generate control points, optimize, and use a rectilinear projection. In the alignment editor I center the image horizontally(it was not perfectly aligned to 0 when I took the shots). The vertical axis is already centered because I leveled my tripod to 0.

PTgui does a much better job stitching when you use a panoramic tripod head to keep you point of view stationary, and you enter the precise values of the cameras pitch and yaw. I used a "Panosoarus" panoramic tripod head. It is not the best one, but it is cheap and it works just fine if you use it right.

Post processing

I then applied contrast, brightness, and saturation adjustments to the image. I also removed some litter from the grass using clones from other parts of the lawn.