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Issues creating accurate color ramps based on grayscale image (is it possible in photoshop without the Avenza plugin?)

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#1
jetboots

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Hello forum members,

I have been trying to create accurate color ramps based upon grayscale images, however, any route I take seems to be a dead end. I was hoping some of you may have some thoughts on my problem.

ArcMap can create color ramps, but is tedious to customize and in addition does not retain the correct colors upon export (no idea why). Therefore, i decided to try to create the color ramp in Photoshop. Seemed to be the logical choice, however I have run into unexpected issues.

In preparation for photoshop work, I converted my raw data into an 8byte grayscale image. However, i noticed that photoshop actually displays the an 8byte DEM image wrong (it performs some sort of stretching). Some adjustments to the photoshop's color settings seemed fixed this (well, one small issue remained, but that is a different topic). I then added a gradient map adjustment layer (only possible with 8byte images) to create my final color ramp product. Sadly, it seems that the gradient mapping adjustment layer is not very accurate on two levels:

1) it seems to read the original gray values (which i changed in the color settings). This makes me think that the color setting in photoshop only affect the display, and not any algorithms that rely upon color information such as gradient mapping.

2) gradient mapping looses information. try this: paint a white to black gradient box, then add a gradient mapping adjustment layer with the same white to black gradient. The result is less color steps and a slightly less faded look.

I assume that the Avenza Geographic Imager would make all these problems (except for the gradient mapping informaiton loss?). However, at the moment this would be my only use for it, so it seems like an expensive solution just to make a color ramp. Are there any other routes i could take in photoshop? If not, i suppose the best (free) route would be working with GDAL coding. This seems viable, but since i do not have much experience using GDAL, it will be a steep learning curve. Do any of you have good resources for color ramps + GDAL + python?

Thanks for any advice!

#2
s hubbard

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whoa, let's back up first.
first address why your colors aren't the same when exporting your map....
what are you exporting them to, and how?
they should export out nicely from ArcMap depending on the settings >>
s hubbard
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#3
frax

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With your tools, the most exact in ArcMap would be reclass the DEM into separate R, G and B grids, with the actual color values in there.
Hugo Ahlenius
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#4
ivar

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My guess is that you are not using correct color profiles in Photoshop; note that TIFF rasters exported from ArcGIS do not have the ICC profile emebdded and this makes using the correct profile in Photoshop particularly important for these images.
It is of course hard to tell w/o actually seeing the original DEMs and the exported images... do you have them posted somewhere?
ivar
PS: and yeah, of course with Geographic Imager you could just import the DEMs right into Photoshop :)

#5
jetboots

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hey, thanks for the replies -

s hubbard:

well, photoshop does read it correctly (i figured it out). Now I'm left with the same problem I described earlier - although i can get photoshop to display the DEM correctly, i cannot get it to READ it correctly, which becomes apparent when applying a gradient map adjustment later. I am using a grayscale 8 byte of a DEM that I converted from a 32 byte tif using GDAL.

frax:

im not sure which settings you are referring to? The only way I know to export a color ramp image from ArcGIS is through ArcMap. (make the color ramp using properties>symbolozation, then right click, export, and check user renderer).Using this method, ArcMap does export a tif, but the colors change, and depending how what they where (and i assume what algorithms were used to create the ramp), the result is sometimes a bit or a lot off from the original colors. Even if i could get this to work, i would still rather define the color ramp in a photoshop like program where i have finer and easier control over the color transitions.

cheers

#6
frax

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jetboots - the hardcore ArcGIS way to do it would be to create a color map with each class 0-255 with a separate RGB color. Reclass and stretch your DEM in an 8-bit space (values 0-255) then create separate value grids for red, blue and green. With these you map the 0-255 e.g. blue values to what you want for that channel. Finally you assemble it into a stack (a final RGB 24-bit image)
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