No output in second RenderTarget when blend state is set - directx

Question

I have a strange issue when using two RenderTargets in SharpDX, using DX11.
I am rendering a set of objects that can be layered, and am using blend modes to achieve partial transparency. Rendering is done to two render targets in a single pass, with the second render target being used as a colour picker - I simply render the object ID (integer) to this second target and retrieve the object ID from the texture under the mouse after rendering.
The issue I am getting is frustrating, as it does not happen on all computers. In fact, it doesn't happen on any of our development machines but has been reported in the wild - typically on machines with integrated Intel (HD) graphics. On these computers, no output is generated in the second render target. We have been able to reproduce the problem on a laptop here, and if we don't set the blend state, then the issue is resolved. Obviously this isn't a fix, since we need the blending.
The texture descriptions for the main render target (0) and the colour picking target look like this:
var desc = new Texture2DDescription
{
BindFlags = BindFlags.RenderTarget | BindFlags.ShaderResource,
Format = Format.B8G8R8A8_UNorm,
Width = width,
Height = height,
MipLevels = 1,
SampleDescription = sampleDesc,
Usage = ResourceUsage.Default,
OptionFlags = RenderTargetOptionFlags,
CpuAccessFlags = CpuAccessFlags.None,
ArraySize = 1
};
var colourPickerDesc = new Texture2DDescription
{
BindFlags = BindFlags.RenderTarget,
Format = Format.R32_SInt,
Width = width,
Height = height,
MipLevels = 1,
SampleDescription = new SampleDescription(1, 0),
Usage = ResourceUsage.Default,
OptionFlags = ResourceOptionFlags.None,
CpuAccessFlags = CpuAccessFlags.None,
ArraySize = 1,
};
The blend state is set like this:
var blendStateDescription = new BlendStateDescription { AlphaToCoverageEnable = false };
blendStateDescription.RenderTarget[0].IsBlendEnabled = true;
blendStateDescription.RenderTarget[0].SourceBlend = BlendOption.SourceAlpha;
blendStateDescription.RenderTarget[0].DestinationBlend = BlendOption.InverseSourceAlpha;
blendStateDescription.RenderTarget[0].BlendOperation = BlendOperation.Add;
blendStateDescription.RenderTarget[0].SourceAlphaBlend = BlendOption.SourceAlpha;
blendStateDescription.RenderTarget[0].DestinationAlphaBlend = BlendOption.InverseSourceAlpha;
blendStateDescription.RenderTarget[0].AlphaBlendOperation = BlendOperation.Add;
blendStateDescription.RenderTarget[0].RenderTargetWriteMask = ColorWriteMaskFlags.All;
_blendState = new BlendState(_device, blendStateDescription);
and is applied at the start of rendering. I have tried explicitly setting IsBlendEnabled to false for RenderTarget[1] but it makes no difference.
Any help on this would be most welcome - ultimately, we may have to resort to making two render passes but that would be annoying.

Answer 1

I have now resolved this issue, although exactly how or why the "fix" works in not entirely clear. Hat-tip to VTT for pointing me to the IndependentBlendEnable flag in the BlendStateDescription. Setting that flag on its own (to true), along with RenderTarget[1].IsBlendEnabled = false, was not enough. What worked in the end was filling a complete set of values for RenderTarget[1], along with the aforementioned flags. Presumably all other values in the second RenderTarget would be ignored, as blend is disabled, but for some reason they need to be populated. As mentioned before, this problem only appears on certain graphics cards so I have no idea if this is the correct behaviour or just a quirk of those cards.