How does the Transparency Threshold in Display Styles such as Forward-Drawing work in conjunction with Family & Parts?
The Display Style's "Transparency Threshold" option is designed to display elements as transparent when either the Element or Material Transparency value exceeds a certain value; e.g., when they are intended to actually be "transparent".
Most Family & Part definitions have materials assigned, and those materials have a Transparency>Opacity value, where Opacity is the inverse of Transparency. So the higher the Opacity value of the material, the lower the Transparency Threshold of the Display Style must be, and vice versa. This inverse relationship means that the sum of the Threshold and Opacity values must be less than 100 in order for the element to display as transparent. For example, if the Display Style's Transparency Threshold is set to 30, then any element using a part whose assigned material has an Opacity setting of less than 70 should display as transparent in the Dynamic View. Or if the Display Style's Transparency Threshold is set to 50 then an Opacity setting of less than 50 should display as transparent. The same concept applies to materials assigned via level and color.
However, if an element does not have a material assigned either through a Family & Part or by level/color, and it is strictly using the Element Transparency setting, then the Display Style's Transparency Threshold must be less than the Element's Transparency setting in order to see through that element. So in this case, if the Display Style's Transparency Threshold is set to 30 then all elements with an Element Transparency value of greater than 30 should display as transparent, unlike the example above when using Part Rendering.
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