site_archiver@lists.apple.com Delivered-To: pro-apps-dev@lists.apple.com If you want to send me an example, I can take a look at it. You can just modify FxShape and send that, if you want. What you’ve written below seems like it should work, but I find it easy to miss things with both Metal and OpenGL. Darrin
On Apr 2, 2020, at 10:03 AM, Bret Battey via Pro-apps-dev <pro-apps-dev@lists.apple.com> wrote:
Thanks for that very fast response on question 1!
So now for question 2:
I am converting a plugin to Metal that used alpha blending to layer and mix OpenGL points (tells you how behind I am).
I am using the FxShape example as my core model to work from. I have adapted the circle draw to take position and RGBA color arguments, have implemented MSAA, and now am trying to get alpha blending of overlapped circles to work. Piecing together examples from online, it seems like it should be (relatively) straightforward. However, I am just getting solid colors, no blends.
So am wondering if there might be something odd about doing this in the Motion context that I am overlooking. I have noted that some errors in online examples have been solved by changing texture types or in one case having to a CAMetalLayer ‘opaque’property to false, but it is not clear to me if/how any of that would apply within an FxPlug.
In FxMTLDeviceCache, I add code to the Shape Pipeline State descriptor to enable blending and set blending options, taking into account that Motion is premultiplied alpha:
pipelineStateDescriptor.colorAttachments[0].blendingEnabled = TRUE; pipelineStateDescriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactorOne; pipelineStateDescriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactorOneMinusSourceAlpha; pipelineStateDescriptor.colorAttachments[0].rgbBlendOperation = MTLBlendOperationAdd; pipelineStateDescriptor.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactorOne; pipelineStateDescriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactorOneMinusSourceAlpha; pipelineStateDescriptor.colorAttachments[0].alphaBlendOperation = MTLBlendOperationAdd;
As far as I understand, that should be enough – that there is no need to alter the fragment shader. All the fragment shader needs to do is return the appropriate color, including its alpha value, and Metal should take care of the blending… Of course, I could have this desperately wrong, as well 😉
Thanks, -=Bret
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