API Reference > imodeljs-common > Symbology > GeometryParams > fillTransparency fillTransparency fillTransparency: undefined | number Optional fill color transparency to combine with SubCategoryAppearance.transparency. Transparency values are combined by multiplying the opaqueness. A 50% transparent fill on a 50% transparent sub-category creates a 75% transparent result (1 - ((1 - .5) * (1 - .5)) = 0.75). Value range is [0.0,1.0]. Pass 0.0 for completely opaque, 1.0 for completely transparent, or leave undefined to use GeometryParams.elmTransparency. Defined in core/common/src/GeometryParams.ts Line 103 Last Updated: 12 June, 2024