Canvas3D at the moment ignores the alpha flag. If alpha flag is given the created target FBO does have alpha channel, but that is then ignored by the QSGSimpleTextureNode based renderer (QSGSimpleTextureNode uses QSGOpaqueTextureMaterial internally). Allowing users to use the rendered alpha in QtQuick scene composition could be beneficial in some use cases, but it does raise potential issues with anti-aliasing.