D1126: Alpha Mode Unknown Not Allowed
D2D1_ALPHA_MODE_UNKNOWN is not allowed as a parameter to this API.
The ALPHA_MODE_UNKNOWN specifier is permitted in some contexts, such as a format description for ID2D1RenderTarget::CreateCompatibleRenderTarget. However, similarly to DXGI_FORMAT_UNKNOWN, Direct2D generally requires formats to be fully specified.
While deciding on a format specifier, consider the surface’s treatment of alpha to use an ALPHA_MODE other than ALPHA_MODE_UNKNOWN. Use D2D1_ALPHA_MODE_IGNORE if the surface does not use alpha. See Remarks in D2D1_ALPHA_MODE.