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QuickBird imagery has a dynamic range of 11 bits per channel, in a 16-bit file. The Version 7 of Photoshop will open 16-bit images but cannot do very much with them.Imagery first has to be converted to 8 bits before any processing can be done. The current version - Photoshop CS - can perform most operations on 16-bit images but files sizes are much larger and operations can be very slow.
On opening the image it will appear to be completely black, or nearly. If it is a four-band image the Channels palette should look like this:
Sometimes Photoshop will instead open the file as a greyscale image with three Alpha channels, like this:
Open the Levels dialogue (Image\Adjustments\Levels…) and put a 7 in the third Input Levels box, as below, them click OK:
This needs to be repeated for each Alpha channel present.
You will now have an image which is visible, though it is still very dark and the colours may be all wrong. If you plan to crop the image this is a good time to do so (even if only roughly) as it will make subsequent operations much faster.
Convert to 8-bit (Image\Mode\8 Bits/Channel). Save this file.
Three-band imagery should open with the bands in the required order, but with four-band imagery the channels need to be re-ordered.
Photoshop channels |
or |
QuickBird bands |
Red |
Gray |
1 |
Green |
Alpha 1 |
2 |
Blue |
Alpha 2 |
3 |
Alpha 1 |
Alpha 3 |
4 |
From the Channels palette, choose the command Split Channels. (If the command is not available make sure that you are dealing with an 8-bit image and that Layers have been flattened). Then choose the command Merge Channels, and the option RGB Color. The normal Band combinations are 3 2 1 (for natural colour) and 4 3 2 (for infrared). For example, to make a 3 2 1 image we need to assign the Red, Green and Blue channels to the original Blue, Green and Red, respectively – that is, to reverse the order of the channels. To make a 4 3 2 image we need to assign the Red, Green and Blue channels to the original Alpha 1, Blue and Green.
Once the image has the bands in the correct order, you need to use the Levels command again to adjust the brightness and contrast of each colour. If the image is fairly uniform, try the Auto setting to start with. Images which contain large bright or dark areas (for example, images with large amounts of water or snow, or tiles with 100% black surrounds) will give poor results with the Auto setting.
A useful trick is to use the Selection tool to select just a part of the image and use the Levels command on this. When it looks right, without clicking OK, save the levels setting, cancel the Levels dialogue, deselect the part of the image and use the Levels command again. You can then load the saved settings which will be applied to the whole image.
Photoshop CS includes a new filter, (Image/Adjustments/Shadow/Highlight...) which can dramatically improve midrange contrast.
It is possible to improve the image by resampling and sharpening, but much depends on what the final image is intended for. As resampling and sharpening produce irreversible effects, they should only be done at the end of the processing. Always save a version at the original resolution with no filters applied.
If the image is to be printed, to preserve the full detail of the original the size should be close to that seen on the monitor, with an original resolution between 72 and 96 dpi. The quality of the print can be improved by then resampling (Image\Image Size…) the image to 150 – 200 dpi, even 300 if the image is to be printed typographically. Always use the Bicubic setting. After resampling, apply the Unsharp Mask Filter (Filter\Sharpen\Unsharp Mask…) to bring out additional detail. This filter enhances the edges of the images (do not ask why it is called Unsharp Mask…) and a degree of sharpening that looks excessive on screen may produce good results when printed - experiment.
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Pan sharpened image, no resampling, no sharpening |
 |
Unsharp mask: Amount 50% Radius 1 Threshold 0 This is visbly sharper, with |
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Resampled to 200 dpi Unsharp mask: Amount 125% Radius 1 Threshold 0 This much enhancement is OK for printing, although
it looks |
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Resampled to 200 dpi Unsharp mask: Amount 250% Radius 1 Threshold 0 This shows the effect of excessive enhancement, with visble noise around edges |
This guide covers the following procedures:
Converting 16-bit imagery to 8-bit
Note: Photoshop is not the ideal software for processing QuickBird imagery, which is normally processed with GIS software. This guide is for those who want to prepare images for printing or other display purposes. Using the imagery in this way implies a good deal of trial-and-error and subjective judgement. This guide assumes a basic working knowledge of Photoshop.
QuickBird imagery typically involves very large files. Photoshop 7 has a limit of 30,000 pixels in each direction - this limit has increased to 300,000 with Photoshop CS. You will need a powerful computer with plenty of disk space (at least four times the image size), and in any case it will usually be better to work on small parts of the imagery.