2 Simple Methods of Channel Packing for Unreal 4

Can't seem to get your head around what channel mapping is? I didn't understand it at first either, but it's actually very simple, and also very efficient.

I came across channel packing through Clinton Crumpler's lvl 80 blog post. Check it out if you haven't seen it.

Channel packing is basically taking multiple textures and packing each one into the individual channels of an image. Each channel can then be separately used in Unreal. 

Channel packing helps avoid the issue of reaching the texture samplers capacity (the amount of textures that can be used in a material, the maximum is 16) by having 4 textures packed into 1. Minimising texture files also reduces memory usage, which is great for optimising your Unreal scenes.

It's up to you which texture goes in which channel, but I usually do the following:

• Red - Roughness
• Green - Metallic
• Blue - Ambient Occlusion
• Alpha - I use this for alpha, emissive, or even height if I know that it will not be being used for anything else.

My albedo and normal maps are separate textures. I must credit Clinton Crumpler for some of these preferences.

Now, here are two simple methods for channel packing textures in preparation for importing into Unreal 4.

01. Substance Designer

Channel packing in Substance Designer is simple when you've done it once or twice.

Create a new graph and delete all output nodes.

1. Import or link your textures into a new graph. Linking will automatically update the graph when the linked bitmap is updated.
2. Create an RGBA merge node, connect the textures into each channel of the node as required.
3. Create custom outputs for exporting as required. You can do this by pressing space bar and selecting output to create a new output node. Connect the bitmaps to the corresponding output nodes.

Next, right click on your graph and select 'export outputs as bitmaps'. Choose your file destination, format and naming convention and then hit 'save all'. Your textures should now be exported.

That's all you need to do! Below you can see my graph for the complete texture set of my ceiling fan model. I've created an extra output for the height texture. There is no specific formulae to this so feel free to customise to suit your own preferences. 

A useful tip to speed up this process is to create a custom graph template. That way you won't have to set up the same graph every time. Follow the link above to watch a great tutorial on how to set this up yourself.

02. Photoshop

You can also use Photoshop to pack your textures. Open each texture individually in Photoshop. We'll start with the roughness map, but feel free to start with whichever texture you want to use in the red channel.

Select all (ctrl + a) and then copy selection (ctrl + c).

1. Create a new photoshop file, make it whatever size is required, this example is 2048 x 2048.
2. Ensure that only the red channel is visible and selected in the channels menu.
3. Go to Edit - Paste Special - Paste in Place (ctrl + Shift + v). This will paste the image into the selected channel.

Repeat for the green, blue and Alpha channels as required.

Finally, save the image in whichever file format is required. I used 24 bit Targa for this tutorial but PNG is another common format for use in Unreal 4.

It is also beneficial to use a PSD file while making iterations, and then later changing to a more efficient file type when all necessary changes have been made.

Your textures should be ready for importing into Unreal now. The image below shows the RMA texture from this tutorial with each channel connecting to the corresponding material input.

There you have it, your textures should now be ready for use in Unreal 4. These are the two methods I use for channel packing, but I'm sure there are many more. If you know of a more effective way then please leave a message in the comments or get in touch with me through my website. It would be great to hear methods that other people have for doing this!