Free Texture Channel Packer

Red is metallic, green is ambient occlusion, blue is the detail mask and alpha is smoothness. If you have no detail mask or occlusion map, leave those channels at 255 (white). The HDRP mask map preset sets this layout and those defaults for you.

Smoothness is inverted roughness: smoothness = 1 − roughness. Drop the roughness map on the Smoothness channel and turn on Invert. If the file name contains "roughness", the packer does both steps for you.

ORM stands for Occlusion, Roughness, Metallic: ambient occlusion in red, roughness in green and metallic in blue. It is the layout glTF uses, the usual convention for Unreal Engine materials, and what Godot’s ORMMaterial3D expects.

The URP Lit shader’s Metallic Map uses the red channel for metallic and the alpha channel for smoothness; green and blue are ignored. Ambient occlusion goes in the separate Occlusion Map slot, which reads the green channel. The built-in render pipeline’s Standard shader uses the same layout.

Because the alpha channel holds smoothness, not opacity, so viewers show low-smoothness areas as see-through. The file is correct, and it is why this tool shows an RGB-only preview. In the engine, import it as a linear texture: untick sRGB (Color Texture) in Unity, or sRGB in Unreal Engine.

No. With Match input, the output takes the size of the largest map and smaller maps are scaled up to fit. You can also force 1024, 2048 or 4096 pixels. For the sharpest result, export every map at the same resolution.

Yes. Drop the packed texture on each channel tile and pick which of its channels (red, green, blue or alpha) that tile reads. That converts between layouts, for example from an ORM texture to a Unity mask map: read green into Smoothness with Invert on, red into Ambient occlusion and blue into Metallic.