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We use the following notation to describe combinations of channels in a human readable way.

Say you are computing

If you want to write a raster that has 17 channels, you need to give them a name. Let’s call them mychan. The channel spec would refers to multiple bands with the same name using a slice-like notation: mychan:17

from delayed_image import ChannelSpec
spec = ChannelSpec('mychan:17')
spec.normalize()

You can see that each “fused-band” is connected together by a pipe |.

So when I want to train a model that fuses RGB and the new mychan features, I would tell my network to run with –channels=red|green|blue|mychan:17 (which actually gets expanded out as previously shown)

We also have the idea of “Late-fused” channels. These are channels that will be input to the network via different stems, so they may be at different resolutions. We use a comma , to separate these. So if I wanted to late fuse RGB with the mychan features I might use red|green|blue,mychan:17.

Additionally, there is an extension to the channel spec called the sensorchan spec, which allows for specification of what sensor the channels belong to.

We refer to sensors using the following codes: S2=sentinel 2, WV=worldview, L8=landsat-8, PD=planet dove.

Say I have a feature that only belongs to worldview called mywvfeat:4, and I want to early fuse that with worldview RGB channels, but I want to late fuse sential2 RGB channels as well. I would specify that like:

The sensors are able to distribute over late fused sensors as well, so if I wanted to include late fused nir|swir16|swir22 with the S2 sensor I would write:

That expands like this

from delayed_image import SensorChanSpec
spec = SensorChanSpec.coerce('S2:(red|green|blue,nir|swir16|swir22),WV:(red|green|blue|mywvfeat:4)')
print(spec.normalize())

Its also possible to take an expanded code and shorten it.

from delayed_image import SensorChanSpec
spec = SensorChanSpec.coerce('S2:red|green|blue,WV:red|green|blue,L8:red|green|blue')
print(spec.concise())

Results in:

(L8,S2,WV):red|green|blue

So long story short, if you are writing hidden features as your final output, make sure you specify "channels": "<featname>:<num>" in your auxiliary dictionary.