P2: Important Custom Material only uses two properties to control alpha behaviour but for correct lighting we need 4 -> at least to achieve the same behaviour as it is done in principled material.
PrincipledMaterial uses (qssglayerrenderdata.cpp):
case QSSGRenderDefaultMaterial::MaterialBlendMode::Screen: targetBlend->srcColor = QRhiGraphicsPipeline::SrcAlpha; targetBlend->dstColor = QRhiGraphicsPipeline::One; targetBlend->srcAlpha = QRhiGraphicsPipeline::One; targetBlend->dstAlpha = QRhiGraphicsPipeline::One; break; case QSSGRenderDefaultMaterial::MaterialBlendMode::Multiply: targetBlend->srcColor = QRhiGraphicsPipeline::DstColor; targetBlend->dstColor = QRhiGraphicsPipeline::Zero; targetBlend->srcAlpha = QRhiGraphicsPipeline::One; targetBlend->dstAlpha = QRhiGraphicsPipeline::One; break; default: // Use SourceOver for everything else targetBlend->srcColor = QRhiGraphicsPipeline::SrcAlpha; targetBlend->dstColor = QRhiGraphicsPipeline::OneMinusSrcAlpha; targetBlend->srcAlpha = QRhiGraphicsPipeline::One; targetBlend->dstAlpha = QRhiGraphicsPipeline::OneMinusSrcAlpha; break; }
But with Custom Material the two properties are assigned both to src and dst:
qssgrhicustommaterialsystem.cpp:
blend.srcColor = material.m_srcBlend; blend.srcAlpha = material.m_srcBlend; blend.dstColor = material.m_dstBlend; blend.dstAlpha = material.m_dstBlend;