understaning how where expression works

[iferencik]

Hello folks,

I am facing some difficulties understanding how the rendering works when using where expressions.

I have a raster layer with values going from 1 to 6 (inclusive) info

the nodata value is set to 0

for the image above i use following parameters to render the layer and it works well, the nodata(0) is transparent

scale: 1
TileMatrixSetId: WebMercatorQuad
url: https://undpngddlsgeohubdev01.blob.core.windows.net/zero-hunger/Crop_Trends/Categorical_maize.tif?c3Y9MjAyMS0xMC0wNCZzcz1iJnNydD1vJnNlPTIwMjMtMTEtMjRUMTklM0EzOSUzQTQxWiZzcD1yJnNpZz10Rk45cWdZOE5ZTVdUR3ozSHg5RGw4SGt0ZGE2aTVBMzdvU1duJTJCc1hMTjglM0Q=
bidx: 1
unscale: false
resampling: nearest
rescale: 1,6
return_mask: true
colormap_name: viridis

if i apply a where expression

scale: 1
TileMatrixSetId: WebMercatorQuad
url: https://undpngddlsgeohubdev01.blob.core.windows.net/zero-hunger/Crop_Trends/Categorical_maize.tif?c3Y9MjAyMS0xMC0wNCZzcz1iJnNydD1vJnNlPTIwMjMtMTEtMjRUMTklM0EzOSUzQTQxWiZzcD1yJnNpZz10Rk45cWdZOE5ZTVdUR3ozSHg5RGw4SGt0ZGE2aTVBMzdvU1duJTJCc1hMTjglM0Q=
bidx: 1
unscale: false
resampling: nearest
rescale: 0,6
return_mask: true
colormap_name: viridis
expression: where(b1 > 3, b1, 0);
nodata: 0

The this is now the pixels having a value of 0 are no longer transparent, as I expected.
I am just curious if this behavior is correct.Is it possible to render the 0's transparent?

Thanks in advance

[vincentsarago]

@iferencik mask is not updated on data expression! https://github.com/cogeotiff/rio-tiler/blob/main/rio_tiler/models.py#L430-L431

Hopefully this is the kind of use case for custom algorithm we are going to introduce in #542

[vincentsarago]

No, because the dataset might not use Nodata and the ImageData class has no idea about it anyway!

As I said, this will be quite trivial with the custom algorithm!

[iferencik]

do you have any docs/link relate to this algos? how are they supposed to work?

[iferencik]

guess the PR explains it :)

[vincentsarago]

guess the PR explains it :)

well you have to dig a bit

• https://github.com/developmentseed/titiler/blob/CustomAlgorithm/docs/src/advanced/Algorithms.md
• https://github.com/developmentseed/titiler/blob/CustomAlgorithm/docs/src/examples/code/tiler_with_custom_algorithm.md
• https://github.com/developmentseed/titiler/blob/CustomAlgorithm/docs/src/examples/notebooks/Working_with_Algorithm.ipynb

The PR is still WIP, but if you want to go through it, feel free to ask question/comment directly in #542

[iferencik]

thanks

[iferencik]

this actually looks fantastic :)

thanks @vincentsarago