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Works in Blender v2.8 onwards.
How to Install and Use the Addon
Installation:

Download the .zip file. (DO NOT extract the zip file)
In Blender, navigate to Edit -> Preferences. Then the Addons tab.
Hit the install button, locate the zip and install the zip file.
Make sure the addon is displayed in the list and enabled (search for "Poliigon" if needed)
In the 3D viewport, press ‘N’ to open the side panel in the viewport
Click on the Poliigon Addon tab.
Indepth How-To:
Background
First released in summer of 2022, this addon allows you to browse, purchase, download, and import Poliigon assets all without ever leaving Blender. It supports loading textures, models, HDRs, backplates, and even brushes.
It supports Blender 2.8+.
Installation
User Account Setup

If you haven’t already, select the Poliigon tab in the side panel.
Enter in your username and password, the same one you would use for the website.
Confirm or change default Poliigon library folder. This is where files will download to and import from*
Select Log In
* You can select add additional library folders in user preferences at any time
Navigating the Addon
Section Buttons

The addon’s UI is separated into sections, listed below from left to right.
Poliigon Assets (Whole Library)
Show My Assets (Owned Assets)
Show Imported Assets (Assets currently loaded into the scene.)
Account Details (Account-related Settings)
Addon Preferences (Shortcut to the addon in User Preferences)
Poliigon Assets Section
Searching

The search bar allows you to refine the search by category (e.g. Textures, Models) and subcategories (e.g. Bricks, Modern). Initially, it will only show one button, then more will be added as subcategories become available.
The results displayed will be identical to the website.
Asset Boxes

The buttons underneath each asset will change depending on whether or not the asset is purchased and downloaded.
Unpurchased
Eye Icon - This Quick Preview allows you to apply a low-resolution, watermarked preview to the selected object. (Only applicable for materials currently)
Download - This will download the asset at the default* resolution.
Down Arrow - This Quick Menu allows you to select a different resolution from the default*
* Default settings can be changed in the addon settings menu.
Purchased
Download - If purchased but currently not downloaded, this button will download the asset to your Addon library folder.
Import/Apply - Will import the downloaded asset into Blender, if an object is selected it will read as Apply, and will automatically apply the material to the selected object. If no object is selected, it will load the material into memory with a fake user but not apply it.
Down Arrow - This Quick Menu allows you Import/Download different resolutions of the material.
Show my Assets Section
Same functionality as the main Poliigon Assets Section, however, results are limited to assets already purchased.
Show Imported Assets Section
Similar functionality to the main Poliigon Assets Section, however, results are limited to assets currently imported to Blender (Either applied to an object, or stored in memory via a fake user). This section not offer options to download or apply new resolutions.
Account Details Section

Adding Credits
Subscription Credits - Shows the current amount of credits from your Poliigon subscription
On-Demand Credits - Shows the current amount of on-demand credits.
Add Credits - This button will open your browser at the appropriate page to purchase additional on-demand credits.
Setting Subscription
Displays Subscription Details
Subscribe Now - This button will open your browser at the appropriate page to purchase/edit a subscription.
Settings Button
Shortcut to open the Edit -> Preferences -> Addon window.
Addon Preferences

To access the addon’s preferences navigate to the Edit -> Preferences -> Addons section, and then find the Poliigon addon. Or, click the cog icon in the Poliigon 3d view panel.
Below are details of what each option does.
Library - Main root folder for the addon to use when downloading assets
Additional Libraries - Allows you to specify additional directories to scan for download assets. (Note - Only Poliigon assets are supported currently.)
Assets Per Page - How many assets to show per page in the main UI
Refresh data - (v1.2+) Force a full refresh of icons and addon internal data, also available as a refresh icon in the 3D view panel next to the search bar
Default Texture Resolution - The default resolution that materials will be downloaded in
Download + Import .blend Files - If checked, will download blend files instead of FBXs, does not affect LODs
Link .blend files - If a blend file is available and the preference to prefer importing is checked, will aim to link objects instead of appending
Default Model Resolution - The default resolution that model textures will be downloaded in.
Download Model LODs - If checked, automatically download all Level of Detail FBX files with the model
Default Model LOD - Which Level of Detail to import, if LODs are already local. If NONE is selected, the default blend file will be imported. Otherwise, the according LOD FBX file will be imported.
Default HDR Background Resolution - The default resolution for HDRs, this value will only effect the scene background, not lighting.
Default HDR Lighting Resolution - The default resolution for HDR lighting, separate from background resolution.
Default Brush Resolution - The default resolution in which brushes will be downloaded.
Auto-Download - Toggle automatic asset downloading after a purchase.
Use Micro-displacements - Set Blender to Experimental mode and adds a subdivision modifier, and adds a non-zero default displacement value to materials imported (For adaptive subdivision, note: Displacement strength will still need to be adjusted in the material shader). Does not apply to models.
Auto-check for update (daily): Checks for updates once per 24h (if Blender is open). You can also always press "Check for update".
Verbose logging to console: Enabled by default, adds additional debugging information into the output system console. Does not affect addon behavior.
Share addon errors/usage: Also available at time of log in, if enabled automatically shares errors encountered inside the addon to developers. Additionally will send key events inside the addon, such as pressing operators or viewing sections, so that we may better understand its usage and improve performance. We do not send any usage behavior outside the Poliigon Addon itself.
Using Materials
Applying a material

Select the object you would like to apply a material to.
Click the “Apply” button on the material selection within the addon.
Adjusting the imported material
To adjust the imported material, open the shading tab and locate the material node group.
Poliigon Nodegroup
Scale - Adjust the scale of the material.
Aspect Ration - Adjustment for non-square textures, will be adjusted automatically on import.
Translate X, Y - Move the texture along the X, Y access, corresponding to the UV.
Global Rotation - Rotate the material, corresponding to the UV.
Color Hue, Saturation, Value, Brightness, and Contrast Adj. - Adjustment sliders
AO Strength - Adjust the amount of influence the ambient occlusion texture has.
Roughness Adj. - Adjustment to the roughness map.
Normal Strength - Adjustment to the normal map’s strength.
Displacement Strength - Adjust displacement strength (Defaults to ZERO!)
Displacement Mid-Level - Adjust the mid-level of the displacement.
Bump Strength - Adjust the strength of the bump map (Defaults to 0.005)
Subsurface Multiplier - Adjust the effect of the SSS map.
* Certain values may be missing depending on the material imported. For example, if the material doesn’t have an SSS map, no Subsurface Multiplier value will be listed.
Using Models
Importing a model

Set the 3D Cursor to the desired location
Click Import within the addon
View the F6/redo last menu for more options on how to import, such as to import models as collections, or to change to link models instead of append (not applicable if importing LODs)
Using HDRs

Importing an HDR will automatically assign it to the active world, or create a new one if necessary. You will not see this HDR unless you go into the rendered mode, and you can see the node layout in the shader window after switching to World. HDRs are imported with two image inputs:
Light: A higher-depth EXR file used to actually light the scene.
Background: The JPG image to be visible in the background render.
Using Brushes
Importing a Brush

Select the object you would like to sculpt on
Click “Import”, this will enter Blender into sculpt mode and apply the downloaded brush with “View Plane” mapping and other appropriate defaults.