Draw Material To Render Target node has inverted alpha

Development Rendering
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1

I was trying the new Draw Material To Render Target node in 4.13 Preview 1 and it seems that its default behaviour produces inverted alpha in the render target.

Here is my test setup using a blank C++ project:

Import a PNG with alpha as a texture

Right click the texture asset and make a material. Set the material to Translucent Unlit. Wire it up with emissive/opacity:

101626-basic_mat.png

Make a pass through material to draw your Render texture to a static mesh. Again make it Translucent Unlit.

101627-passthrough.png

Create the following BP. Add a static mesh (I used starter cube) and assign its material to the passthrough mat. Next the BP creates a canvas RT and draws the alpha texture material to it. Set the RT to the texture param of the passthrough mat.

101630-blueprint.png
101630-blueprint.png1223Ă—593 128 KB

Place the actor in the world and I get the following result.

101628-outcome.png
101628-outcome.png943Ă—668 496 KB

If I add a 1-x node from the InTex texture parameter to the material opacity output for the passthrough material I get the expected results:

101629-ue413.png
101629-ue413.png990Ă—772 663 KB

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2

Hey astonish,

I was able to reproduce this issue on my end and have gone ahead and entered a bug report for this issue, UE-34314. You can track this report on our new Known Issues tracker here.

Let me know if you have further questions or need additional assistance.

Cheers,

3

You may want to add to the issue that this isn’t limited to the new DrawMaterial function - the alpha is also inverted now on translucent objects being rendered by a SceneCapture2D actor (but I’m guessing it’s the same cause).

In the below screenshot I am using a SceneCapture2D off-screen to capture some falling leaves (so that I can use the rendered falling leaves texture in UMG to “wipe away” my loading screen). Note the inverted alpha (it works as expected in 4.11):

102228-inverted_alpha.png
102228-inverted_alpha.png1272Ă—713 658 KB

(Hoping this can be fixed for the full 4.13 release)

4

Yep, I do some custom rendering of a material to UCanvasRenderTarget2D’s in C++ for our project. In 4.11 transparency was straight up broken and I had to render the alpha to a separate RT, in 4.12 I can do it to a single target, but the alpha is inverted as it is in 4.13. If you invert the alpha in the source material the color channels come out black in 4.12

5

tldr: this is a limitation of alpha blending hardware

In order to accumulate a bunch of translucent layers and composite them onto a background later, you have to accumulate two values:

NewColor = Color0 * Opacity0 + Color1 * Opacity1 + ...
BackgroundAmount = (1 - Opacity0) * (1 - Opacity1) * ...

BackgroundAmount is what we’re storing in the alpha channel when you do DrawMaterialToRenderTarget or a SceneCapture. To get ForegroundAmount from that (what’s typically called Opacity), you do 1 - BackgroundAmount. With the way fixed function alpha blending hardware works, we can only accumulate BackgroundAmount, not ForegroundAmount.

6

Thanks for the explanation DanielW. I haven’t had any trouble working with the feature as is, but having code to loop through a TArray to val.A = 255 - val.A; made me shiver a bit on large buffers so I was hoping to gain a bit of perf.

7

I’ve un-one-minused my material so it’s fixed on PC - but on mobile ES2 now it is backwards (so it’s inverted on one platform and not the other) - I can get around this with a platform switch, but they should probably be brought to parity?

8

In 4.17 my workaround is:

  • Set draw material to AlphaComposite blend mode;
  • Use standard Alpha (not inverted) on draw material;
  • Set the passthrough material to use inverted alpha output from RenderTexture (OneMinus node);
  • Clear RenderTexture to (0, 0, 0, 1) before drawing.
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