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Using Texture as Lerp Alpha Input doesn't work

For some reason, whenever I try using an actual texture channel as the alpha input on my lerp node, it doesn't seem to work. It's a black and white image (and it's got itself in the alpha channel, just for kicks), but no matter which channel I plug into, as soon as I plug that into the alpha pin on the lerp node it just sets the alpha for that node to a constant 0. If I remove it, the alpha gets back to 0.5, which blends both textures halfway, which is not what i want.

This is what it looks like when it's plugged in: alt text

And this is what it looks like when i remove it, and it "properly" blends them: alt text

Things I've tried: Unchecking srgb on that texture, playing with the min-alpha setting of that texture

Product Version: UE 4.10
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asked Feb 06 '16 at 04:56 AM in Rendering

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avatar image Jacky Feb 06 '16 at 04:36 PM

There must be a problem with your alpha channel. It works as expected for me:

alt text

textlerp.jpg (74.5 kB)
avatar image PockyWitch Feb 08 '16 at 12:05 AM

Yeah, it seems to be just this specific texture. Tried saving it in different formats, even saving without alpha and then just using (for example) the red channel.

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1 answer: sort voted first

Reading your post, it's not obvious what you would expect to happen.

If your alpha channel is similar to the image you have for that texture color, it looks like it works correctly.

On the top image you basically tell it to use input A 100% when the alpha is 0 (black if you think it as greyscale) and when the alpha get's bigger value it'll start to use B. Since your B is a bright texture (apparently), it'll just look like your alpha as you essentially tell it to follow the alpha channels intensity.

If you just want to overlay the images, you could try using some of the blend-nodes instead.

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answered Feb 06 '16 at 07:10 AM

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avatar image PockyWitch Feb 08 '16 at 12:04 AM

In this specific case I want to overlay a stripe texture on top of a concrete texture, the alpha is mostly black, with just the stripes being white. I figured using a lerp node would be the easiest way, as I do not need the asphalt to shine through the stripes.

avatar image 0lento Feb 08 '16 at 05:45 AM

Oh, you should then swap A and B from Lerp on your topmost image and it should do what you want as long as your textures alpha is correct. If not, you can just take any channel (R, G or B) from that image and wire it to the Lerp Alpha.

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