Some combinations of Base Color and Metallic map values can lead to incorrect rendering, no longer physically based.
The rules to avoid those problems are quite simple:
To simplify the task of respecting these rules, a PBR Validate setting is available below the Base Color and the Metallic editor areas.
To see how PBR Validate works, let's create a set of PBR maps: click the New Maps button located above the Editor area.
In the New Maps window that appears, from the Presets... drop menu at the top-right, select the Silver preset, near the middle of the list.
Click Create to initialize the new maps with physically accurate Silver material values. In the 3D Preview rotate the model to see how it reflects the environment.
Selecting the Base Color editor, enable the PBR Validate switch at the bottom: a green overlay is displayed above the map meaning there are no validation problems.
Before we continue, please set PBR Validate switch back to disabled.
Change to the Metallic map so that we create a dielectric area in the fully metallic original map.
To do this, select the Paint Tool. Choose its Paint operation and set the Metallicness slider to 0 (meaning dielectric) and set the Hardness to 100% (full brush hardness will avoid blurred middle values). Paint the half bottom of the Metallic map as dielectric.
In the 3D Preview, you'll notice that in the painted area the mirror-like characteristics of silver are gone and it now behaves more like plastic or painted metal. This happens because more light is now being diffused and less light reflected from the painted area.
Now return to the Base Color editor. Switch on PBR Validate at the bottom: red is now overlaid on top of the areas we marked as dielectric in the Metallic map. This means that Base Color value for those pixels is too bright and needs to be darkened.
Select the Paint tool and choose its Paint operation. Notice that four numbers appear in the Color settings: r g b (v), with the last one (v) being the value or brightness of the color. Click Color and choose a gray (or a color) whose value/brightness is less than 240.
Now paint over the area that appears in red: it should turn to green meaning the dielectric areas are now valid.
Where did this 240 value came from? There are two types of standard PBR Validate ranges, a tolerant and a strict range. Which one is in use can be selected in the Preferences window.
But how does one remember the values in these ranges? Leave the mouse over the PBR Validate switch at the bottom: shortly after a tooltip appears with the valid brightness ranges. Double check that the color value/brightness you just selected is within the valid range for dielectrics.
Let's now paint a color too dark for a metallic area, which is invalid.
In the Paint tool's Color picker choose a color with value/brightness below 153 and paint in the middle half of the Base Color map. As you paint a red overlay appears above the new strokes meaning the value/brightness of the color is invalid for a metallic area.
To fix this, pick a brighter color (check the valid range in the PBR Validate switch tooltip) and paint over. You can paint with the PBR Validate overlay enabled or not.
When no more red areas are visible in the PBR Validate overlay, this means that the Base Color and Metallic maps are valid and can be used without worries.