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Used for blend texture previews in Hammer's texture browser.

When using a preview tooltexture in a material, the material is scaled up or down to the size of the tooltexture. If the material is a blend of two 1024x1024 textures but your tooltexture is only 256x256, the 1024x1024 textures will be scaled down to the much lower quality of 256x256. Unfortunately, this means that the tooltexture cannot be made smaller to save space (since nobody will see it in-game). You'll need a preview tooltexture as large as you want the texture to display within the engine.

Clarify: When using a smaller tooltexture, does the basetexture actually get downscaled, or does it just result in a denser texture scale? The latter could actually be useful in some situations.
The texture to the left
    $basetexture    dirt/blend1_mountain_dirt_a1
    $basetexture2   dirt/blend2_mountain_dirt_a1
    %tooltexture    preview/blend_mountain_dirt_a1
    $bumpmap        dirt/blend_mountain_dirt_a1-ssbump
    $ssbump         1
    %detailtype     desert1
    $detail         dirt/blend_mountain_dirt_a1_DM
    $surfaceprop    dirt
    $surfaceprop2   rock

Without a tooltexture, Hammer will only show your first $basetexture, making it impossible to see the blend in the texture browser.

Icon-Bug.pngBug:VTF files without mipmaps will not display correctly in the texture preview, regardless of resolution). Fixed in Hammer++ build 8866.
Icon-Important.pngImportant:Hammer++ build 8866 (and newer) do not use %tooltexture on WorldVertexTransition, as it renders the blends in real time.
Confirm.pngConfirm: Is the resolution of the %tooltexture still read? The scale of the %tooltexture takes priority over the scale of the $basetexture in-game.