Error adding mask as alpha channel

Problem

Calling array_to_image using the array and mask returned from tile using a 4 band tiff returns an error: WEBP driver doesn't support 5 bands. Must be 3 (RGB) or 4 (RGBA) bands.

Relevant code

# image at URL is 4 band geotiff
tile, mask = main.tile(url, int(tile_x), int(tile_y), int(tile_z), tilesize=256, nodata=None)
options = img_profiles["webp"]
buffer = utils.array_to_image(tile, mask=mask, img_format="webp", color_map=None, **options)

Possible Cause

It looks like utils:392 is just adding a band to the array without checking to see whether an alpha band already exists

[a2hill]

@vincentsarago Yes we implemented additional check’s for 4 band images and handle them differently. I think the older version of array_to_img used a has_alpha check, was that specific to PIL support?

[vincentsarago]

👋 @a2hill sorry for this bug I don’t see a fix directly to the array_to_image function but to me it seems you could pass indexes=(1,2,3) to the main.tile function and then the mask return should be the alpha band.

Handling nodata/mask/alpha band is a tricky thing (I’ve worked on the matter all the week), maybe utils.tile_read could remove the alpha band from the data array directly ?

Edits:

buffer = utils.array_to_image(tile, mask=mask, img_format=“webp”, color_map=None, **options)

Here you are also passing the mask to the function, but the alpha band is already provided with your data, so you have two solution:

• uses indexes=(1,2,3)
• don’t pass mask to the utils.array_to_image function