Combining multiple diffuse textures into one leaves strange distortions?

Question

I made 3 models, each has own material. And after texture painting, I combined each their texture to single textures for optimization.

Currently making Mobile Game with Unity 2018 and using Mobile/BumpedSpecular shader for rendering, which is actually simplified BlinnPhong lighting model.

However after checking the result, I get weird result in some spots with Diffuse texture.

Note that Mobile/BumpedSpecular shader in Unity uses alpha channel of diffuse texture as glossiness. So after I combined those three different diffuse textures, I exported with no background(transparent background) like this:

UV mapped correctly, diffuse texture has alpha, importing alpha channel as well.

Everything seems fine, but why do I have this wrong result? First time I thought that I did something mistake, so I repeated whole process again, but still have same result, not this one, with other models also. So I thought that it was bug from Unity, actually but it wasn't, I tried with Blender and have same result.

So one thing I can know that there might be something missed in my integrated diffuse/glossiness texture. What am I missing?

Answer 1

DMGregory's suggestion to use a different format is simple and straight forward. So perhaps try that first and see if it solves your issue.

However, if you for some reason want to keep using PNG, you can try disable compression or use a plugin like SuperPNG for Photoshop - I can't tell if you're using Photoshop, but it looks like an older version.

Reasons for sticking with PNG can be compatibility with other tools in your pipeline, where formats like TIFF and TGA can sometimes lead to different results depending on the application.

If you're comfortable with command-line tools, ImageMagick may also be an option to convert your file to a PNG.

Answer 2

This often happens when saving in PNG format. As Morten Andersen points out, those streaky stripes are a telltale sign of PNG compression in zero-alpha areas.

The reason is that PNG exporters assume that the alpha channel is transparency, and thus zero-alpha regions will be completely invisible. So, they figure they're free to do whatever weird stuff they want with the colours in those regions in order to get better compression ratios - like extruding stripes of the same colour so adjacent pixels are really easy to predict.

To avoid this, you can save your source assets in a format that does not assume that an alpha channel with a value of zero will be invisible, like TGA "Targa" or PSD itself. Rather than storing the alpha in the transparency of each layer, you'd go into your channels window in Photoshop and paste your gloss map on a new alpha channel.

(This same reason is why Unity has that "alpha is transparency" checkbox on its importer, so it knows how to interpret zero-alpha regions)

These formats tend to be bigger on disc than png, but ultimately your assets get baked to GPU-friendly formats when you build your game anyway, so it shouldn't impact your published game's size or memory footprint.