MetallicRoughness Texture - how to create?

I am playing around with the glTF2.0 materials in Shapediver. It appears we can now only use a MetallicRoughness Texture, which according to here: Metalness / Roughness

The metalness and roughness maps can be combined into a metalness-roughness map to use less textures. The blue channel is used for metalness, the green channel for roughness.

Can I ask why this complication? How can I create this map? Now I need to open Photoshop or some other tool to combine the 2 maps and even if I just want to use the roughness map I can’t just use a black and white image?

And all that to save one texture? I don’t understand the reasoning behind making it so difficult to experiment. If saving on textures is a concern, then why only combine those, but not others?

Is there no way in Grasshopper to use greyscale metalness and roughness maps like literally everybody else?

The new glTF 2.0 Material component attempts to match closely the official glTF 2 specification from Khronos. As you can read here, the metallic roughness texture is the only official way to setup metalness and roughness with textures as of now.

As a matter of fact, we shared your concern when we decided to comply with the glTF specification in our new components. Therefore, we have kept support for our Legacy Material component which allows to define the metalness and roughness textures separately, as is more common to do in many cases. Additionally, we will soon add components to the plugin which allow to easily manipulate textures, for example creating a metallic roughness map by combining two metalness and roughness maps.