For this month’s Geospatial Frequently Asked Question (G-FAQ), I shift gears slightly from my normal approach with a primer on using a data in a commonly used software application. We have seen an increased interest in using 5-meter natural color RapidEye imagery as a back drop in movies, ads and apps for large-area coverage; however working with this data in Photoshop, a very common graphic design software package, is not as straight forward as it may seem. And when you need to get your project done yesterday, even a five minute delay is too long to wait!
As such, this G-FAQ will address this core set of questions:
Why is RapidEye imagery challenging to work with in Photoshop CS5? How can I open RapidEye imagery in Photoshop CS5? Are there any suggestions you can offer to color balance the data once it is opened properly?
While RapidEye is easy to work with inside of GIS and remote sensing software, it is not as easy to work with when you move over to a graphics editing package such as PhotoShop CS5. Here are the three key challenges you will face in Photoshop and very likely other graphics packages:
- RapidEye imagery has 5 spectral bands and as such Photoshop is unable to load properly as red, green and blue natural color data.
- RapidEye imagery starts with 16-bit depth so it will need to be color balanced. See our recent G-FAQ series for more details on the implications of and working with 16-bit data. Color balancing imagery with clouds can be particularly difficult as clouds are extremely bright white which can wash out areas without cloud cover in automated color balancing techniques.
- A single 5,000 pixel x 5,000 pixel raw 16-bit RapidEye imagery tile is about 240 megabytes (MB) making them more challenging to work with than smaller 8-bit files.
Now that I have given you sense of the challenges of working with RapidEye imagery in Photoshop CS5, let me explain the solutions! What follows below is a high level summary of the steps you will need to take in order to load, view, color balance and export your processed RapidEye imagery. Given how finicky Photoshop can be, for more exact visual instructions, you will want to check out the two in-depth video tutorials that follow these instructions.
- Load the TIFF RapidEye file you downloaded in Photoshop. It will be named with a long string of numbers and letters. When it loads, you will see five Channels called: Grey and Alpha 1, Alpha 2, Alpha 3, and Alpha 4.
- Select the entire Grey Channel and copy it. Now you will need to create a File -> New workspace and use the parameters from your Clipboard. It will set most everything correctly (i.e. 5000 x 5000 pixels and 72 DPI for a single 3A RapidEye ortho tile), but you will need to change the Color Mode to RGB.
- In the new workspace you have created, make sure you turn on the single Blue Channel (by pressing on the Eyes in the Channel sidebar) and copy the Grey Channel from your 5-band RapidEye raw data here.
- Go back to the raw RapidEye imagery, select the Alpha 1 Channel, copy it and then paste it in your new workspace as the Green Channel.
- Repeat Step 4 by copying the raw Alpha 2 Channel and pasting into your new Red Channel.
- You have now created a RGB (natural color) layer stack from your 5-band raw RapidEye data. The next step is color balancing the data. See the section the follows the video tutorials for more advice on color balancing data in Photoshop CS5.
- Once you have color balanced your RapidEye imagery as you see fit, the final step is changing your Image -> Mode to 8 Bits/Channel and saving it as a new TIFF file. When you do this, you reduce the file size and ‘lock’ in the color balance. Now you can open the new TIFF file in any program that works with this format and the colors will look exactly as you just set them.
Click the image above to watch part 2 of a two part video tutorial on working with RapidEye imagery in Photoshop CS5. In Part 2, I show you how to color balance and export the RGB layer stack created in Part 1. (Data Credit: RapidEye)
Tips to Color Balance RapidEye Natural Color Imagery in Photoshop CS5
To conclude this edition of G-FAQ, I will provide you with a list of tips to color balance your RapidEye imagery in Photoshop CS5. As colors are extremely subjective, I suggest you experiment with multiple filter combinations to get the exact effect you are looking for. And for many of you out there, your skill set with Photoshop is much more extensive than my own so I apologize up front if all of these tips are mundane.
- The easiest set of filters to apply to your RapidEye imagery are the Auto Tone, Auto Contrast and Auto Color options found under the Image menu. I have found that the order you apply the three Auto filters matters, so that is something to experiment with on your own. For many users, these filters might be sufficient for color balancing your RapidEye imagery.
- If the auto filters are not enough, the next place to go is the list of options found under Image -> Adjustments. In the Levels, Exposure, Color Balance and Hue/Saturation options, you will find a list of preset options in a pull down menu – give them a try to start. Be sure you have the Preview option on so you can see your changes in real time. I zoomed into the full pixels before I tested these filters out to see exactly what my changes were doing to the colors in the RapidEye imagery.
- Under the same Image -> Adjustments menu, you can try out the Vibrance and Brightness/Contrast options. These are slider bars that you can test out and preview in real time.
- And finally, a word to the wise, if there are clouds in your RapidEye imagery, the auto filters will more than likely create a stretch that is too dark over areas with no cloud cover. In this instance, I suggest you work with the Brightness filters, making it brighter in the dark areas and accepting that your clouds might be over-white or even completely saturated and odd looking.
Until our next edition of G-FAQ, happy GIS-ing!
Do you have an idea for a future G-FAQ? If so, let me know by email at firstname.lastname@example.org.
Find Out More About This Topic Here:
- Apollo Mapping – The RapidEye Constellation
Brock Adam McCarty