Getting the color of a pixel

[dholcomb9711]

I am working with weather data - this is "super resolution" doppler wind velocity data. My data exporter writes geo-TIFs and colorizes them, but sometimes (for an unknown reason) it fills in the background of the image with a random color from the color table it uses.( In this case, bright green... it should all be transparent!) Since I cannot figure out why, I need to somehow use IM to find what color this is, and make it transparent. Example image is below. The very bottom left corner is always going to be out of range of the radar, so if a script can somehow determine whatever color that pixel is (I pointed out the location that is always best to use), that would work just great.

If I can get this figured out - I can make a simple script to make it transparent. I am just lost as to how to determine what color it is.

[dholcomb9711]

Disregard this reply, I had trouble with my internet connection. I edited my question above. I still need help.

[snibgo]

For integer formats such as JPEG and PNG, "hex:" is useful, eg:

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magick in.png -format %[hex:p{0,99}] info:

Instead of 99, use the height minus one.

This gives the colour in hex format. If the image has transparency, the fourth quarter is the alpha channel, which you need to test whether the pixel is already transparent.

[dholcomb9711]

Okay, forgive my lack of skills here as I am trying to follow along, it's a new command for me.

My filename is "KEWX_L2_VEL_colorized.gif" and the height is 3770px. So my command was :

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magick KEWX_L2_VEL_colorized.gif -format p{0,3769}] info:

It returned :

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p{0,3769}]

I was expecting a hex value. Can you explain if I am missing something?

Also, if I do get the hex value... how can I then launch the command to make it transparent? My 2nd command would be :

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magick KEWX_L2_VEL_colorized.gif -transparent #5BD13D KEWX_L2_VEL_colorized.gif

That part is easy, but how do I get it TO that point? (I am making this automated in the windows command prompt). I am trying to follow along as I know you know what you are talking about! You are very helpful here. I just struggle sometimes with my commands.

[snibgo]

Compare your command to mine. Mine included characters in the format that yours doesn't.

You haven't said what shell you use (bash, Windows CMD, or whatever). When your magick command works, use shell facilities to put the result in a variable, then you can use that variable in your 2nd command.

[dholcomb9711]

Ah, I had edited my post to let you know I am using the windows command line, but I think it was slow to update the edit. I am still struggling to get this working. What is the name of the variable in the command you showed me? Am I thinking too hard?

[snibgo]

Here's the command I showed you:

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magick in.png -format %[hex:p{0,99}] info:

You should put in your own filename, and height minus 1:

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magick KEWX_L2_VEL_colorized.gif -format %[hex:p{0,3769}] info:

Does that work okay?

[dholcomb9711]

It does not work. My command is exactly as follows (I used notepad to save it as a batch file and ran it):

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magick KEWX_L2_VEL_colorized.gif -format %[hex:p{0,3769}] info:

When I launch it in the windows 10 command prompt, the command seems to be different than what I actually told it to do. (screenshot) The '%' an the first '[' are gone.Not sure what is going on here.




Edit : In case it helps - Version: ImageMagick 7.0.7-35 Q16 x64 2018-05-21

[snibgo]

(I used notepad to save it as a batch file and ran it):

In a BAT file, you need to double the %.

[dholcomb9711]

That worked! Output is "4BCB2AFF".

How do I get this into the next command? My command should be -

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magick KEWX_L2_VEL_colorized.gif -transparent #4BCB2AFF KEWX_L2_VEL_colorized.gif

This works when I manually type it, but I should be able to inject the variable in there to automate it somehow. I can't thank you enough for your help. I am learning in this process.

[snibgo]

Like this, Windows BAT syntax:

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for /F "usebackq" %%L in (`magick ^
  KEWX_L2_VEL_colorized.gif ^
  -format "mycol=%%[hex:p{0,3769}]\n" ^
  info:`) do set %%L

magick KEWX_L2_VEL_colorized.gif -transparent #%mycol% KEWX_L2_VEL_colorized.gif

[GeeMack]

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magick KEWX_L2_VEL_colorized.gif -transparent #4BCB2AFF KEWX_L2_VEL_colorized.gif

This works when I manually type it, but I should be able to inject the variable in there to automate it somehow.

Since you're using IM7, in many cases you can do this sort of thing as a single command by reading the pixel's color and applying it directly in the operation where you need it. This should give you a result like your example...

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magick KEWX_L2_VEL_colorized.gif -transparent %[pixel:p{0,3769}] KEWX_L2_VEL_colorized.gif

Again, if you use it in a BAT script you need to double the percent "%" signs.

[snibgo]

-transparent %[pixel:p{0,3769}]

Ah, I didn't know we could do that. Very useful. Thanks, GeeMack.

[fmw42]

Nice. Note that you can also use hex:p{0,3769}], but you have to preface it with #. So it would be in Unix syntax

magick KEWX_L2_VEL_colorized.gif -transparent "#%[hex:p{0,3769}]" KEWX_L2_VEL_colorized.gif

or I presume in Windows, this will work without the quotes, but someone should check that out.

magick KEWX_L2_VEL_colorized.gif -transparent #%[hex:p{0,3769}] KEWX_L2_VEL_colorized.gif

[GeeMack]

or I presume in Windows, this will work without the quotes, but someone should check that out.

magick KEWX_L2_VEL_colorized.gif -transparent #%[hex:p{0,3769}] KEWX_L2_VEL_colorized.gif

That works in Windows with or without the quotes. Either way requires doubling the percent sign if used in a BAT script, of course.