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NWN supports 32bit TGA files with alpha. They must be powers of 2, such as 32x32 or 128x1024. They can be uncompressed or compressed with RLE compression (using Bottom Left, not sure about Top Left). GIMP for instance can output compressed RLE which is up to half the size for some icons/textures.
Note that in some contexts, NWN does not appear to support grayscale TGA's; this has been tested in PBR maps (roughness, height, specular) loaded via MTR on creature and tile models. If affected by this issue you can simply convert the TGA to RGB format and it should work.
| Image Dimensions | Uncompressed TGA File Size | Notes |
|---|---|---|
| 32x32 | 4KB | Game's default (now awful) icon size (eg; feats, spells). For new content use x2 (64x64) or x4 (128x128) due to UI scaling now being a thing. |
| 64x64 | 16KB | |
| 128x128 | 64KB | |
| 256x256 | 256KB | Maximum limit of certain shiny water effects. Retaining TGA for this might be advisable to improve quality slightly. But DDS works fine. |
| 512x512 | 1MB | |
| 1024x1024 | 4MB | |
| 2048x2048 | 12MB |
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