The multi value assets query param order affects the output of the stac/tiles endpoint.

[raster-blaster]

I have noticed that the order of the assets query parameter affects the output of the stac/tiles endpoint in Titiler. For example, the following two requests have the same parameters, except for the order of the assets parameter, but they produce different outputs:

• https://api.cogeo.xyz/stac/tiles/WebMercatorQuad/13/4794/3394@1x?url=https%3A%2F%2Fearth-search.aws.element84.com%2Fv0%2Fcollections%2Fsentinel-s2-l2a-cogs%2Fitems%2FS2B_36RTT_20190102_0_L2A&resampling_method=nearest&assets=B04%2CB03%2CB02&bidx=1&rescale=0%2C1000
• https://api.cogeo.xyz/stac/tiles/WebMercatorQuad/13/4794/3394@1x?url=https%3A%2F%2Fearth-search.aws.element84.com%2Fv0%2Fcollections%2Fsentinel-s2-l2a-cogs%2Fitems%2FS2B_36RTT_20190102_0_L2A&resampling_method=nearest&assets=B02%2CB03%2CB04&bidx=1&rescale=0%2C1000

In the rio-tiler utility render function, the provided assets are stacked as a 3-dimensional NumPy array and passed to rasterio to create an RGB image with the [1,2,3] color indexes. The assets are stacked in the same order as we provide in the stac/tiles endpoint.

Now, I have a few questions:

1. Is preserving the order of the assets parameter the expected behavior in Titiler?
2. Since HTTP requests do not guarantee the order of query parameters or array query parameters, isn't this problematic? If the order of the assets parameter changes on the fly, different color combinations will be produced, resulting in images with different channel arrangements.
3. Is there a way to specify the order of indexes or which asset corresponds to a particular color channel in the stac/tiles endpoint?
4. Does Titiler, underneath FastAPI, preserve the order of the assets query parameter?
5. Is it the responsibility of the client to ensure the correct order of the assets query parameter in the stac/tiles endpoint?

[vincentsarago]

Hi @raster-blaster

Multiple things:

• api.cogeo.xyz is deprecated and use a really old version of titiler
• https://titiler.xyz is a official demo endpoint
• in your case passing assets=B04,B03,B02 and assets=B02,B03,B04 is expected to gives different result, titiler will respect the order it was given

Since HTTP requests do not guarantee the order of query parameters or array query parameters, isn't this problematic?

Is that true? if yes this is problematic and we have then a bug in titiler, but TBH for now I never had any problem with parameter order 🤷

Does Titiler, underneath FastAPI, preserve the order of the assets query parameter?

It should

Is it the responsibility of the client to ensure the correct order of the assets query parameter in the stac/tiles endpoint?

Yes, TiTiler won't decide things for users 😄