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Material swatches can be updated automatically, whenever a material attribute is changed, or on demand by pressing the Preview button below the material tree. You can select the update mode in the plug-in preferences, along with the preview scene and quality settings. Depending on the hardware configuration, material complexity and number of materials in the scene, updating the swatches can become a time consuming operation, especially when opening MEdit, which tries to display many materials at once. In that case, switch to on demand updating. The swatch images are saved to disk when saving the scene, so in on demand mode they don't need to be recomputed every time they need to be displayed.