User specified "custom" colormap

[blazetopher]

Is there a way to allow the client to specify a custom colormap "dynamically"? For example, by providing a series of pixel+color pairs and dynamically generating a linear gradient colormap on the fly during rendering?

If such a capability doesn't exist, would you have any advice on how it might be best implemented?

[vincentsarago]

Hey @blazetopher thanks for opening the issue.
Sadly there is no such capability, It was been in my mind for a long time now but I cannot think of a good way to do it.

I think there are 2 solutions:

1. the user provide the dynamic color_map info in each request

https://myendpoint/cog/preview?&url={myfile}&color_map={0: [0, 255, 255, 0], 2: ..., 3: ...}&color_map_type=linear|discrete 

2. user define a colormap and POST it to the backend (which store it in a DB or in a shared drive like efs)

Solution 2 is really complex and might not even be possible, so solution 1 might be better. That's say I'm not 💯 on solution 1 because it will lead to really long/complex URL 🤷‍♂️

if we add this, this is where it should happen https://github.com/developmentseed/titiler/blob/master/titiler/dependencies.py#L219-L246

ref: https://terracotta-python.readthedocs.io/en/latest/tutorials/categorical.html?highlight=colormap#in-the-frontend

[blazetopher]

@vincentsarago Thanks for the rapid response!

I was also thinking something along the lines of Option 1. Our use case is relatively straightforward: Only 3 colors and 2 splits - no gradients. So while that case theoretically wouldn't be too bad, specifying every pixel value would get complex quickly. Also sounds like there would potentially be encoding/decoding requirements (based on the terracotta reference) to take into consideration.

Thanks also for the pointer to the proper location in the codebase. I'm unsure if our current project timeline will allow for it, but if we end up trying to implement and it comes out well, I'll be sure to submit a PR so it can be folded back in!

[kylebarron]

I think it would be fine to allow a colormap argument to not include all 255 colors, as long as it's understood the colors will be split evenly. So if you had 40 numbers in a row, that could define a colormap with 10 sections, and would be less of a URL burden. But with non-categorical colormaps only definining a few colors might not be possible.