How to change the render channel for STACRender with multiple multi-band assets

[no0o0oname]

As I understand, when tiling or preview with STACRender, we can set the band indexes for each asset with asset_indexes parameter and the assets parameter could not be omitted when not setting default_assets.

With asset_indexes input like {"asset1": 1, "asset2": (1, 2,)}, the returned image would have asset1_b1 as R, asset2_b2 as G and asset3_b3 as B. Since asset_indexes is a dict that does not support order and repeated key, how to change the RGC channel for different asset_band, e.g., to configure the returned image to have asset2_b1 and R, asset1_b1 as G and asset2_b2 as B?

Thanks for any help in advance.

[vincentsarago]

Hi @no0o0oname

This is supper tricky, I think you could achieve this by using expression: asset2_b1,asset2_b1,asset2_b2

[no0o0oname]

Thank you very much for your timely reply!

I didn't know this type of expression would work. (Can you provide some doc on the expression grammar? I found it to be of numexpr but did not know the comma syntax.)

Can I use expression parameter instead of assets and asset_indexes as the default mechanism for render settings? Would expression be implemented differently than asset_indexes, like resulting in more computation or delay?

[vincentsarago]

the doc is pretty light 😬 https://cogeotiff.github.io/rio-tiler/readers/#stac-expression
It would be nice to have better documentation

Can I use expression parameter instead of assets and asset_indexes as the default mechanism for render settings? Would expression be implemented differently than asset_indexes, like resulting in more computation or delay?

we don't have default_expression as we have for default_assets sorry. I feel this is a topic for the STAC render extension (which is partially supported in titiler)

Note: if you are providing expression, you don't need to pass assets or asset_indexes

There is a downside of using expression tho, numexpr will put the data in a float32 array which mean you might need to apply some rescaling of data casting

[no0o0oname]

Thanks for your clarification.

In fact, I am currently confused about the STAC render extension, which is my original motivation of seeking answers from implementation titiler to rio-titiler. The confusion is very much similar: with only assets and bidx field in the render extension, the asset/band combination is not clear, and the mapping for asset/band to RGB (CMYK) channel based on order is hard to clarify.