PNG Alpha channel V's Seperate Alpha map.

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When I set a .png to the iray base colour, and 'reuse' it in the cutout capacity, you need to click the image and go to the image editor and set the 'Greyscale From:' setting to alpha, setting the 'Image Gamma:' has no effect, but the default is 0.0.

If I then export the png alpha channel to a seperate image using photoshop, and load that into the cutout capacity instead, I get a different result. By clicking the image editor it seems that the 'Image Gamma:' setting is default to 1.0. Also changing the 'Image Gamma:' has no effect until you change the 'Greyscale From:' setting, and back again (or not).

The only way to get the seperate alpha channel image to match the png alpha is to change the 'Image Gamma:' to 0.0 (and changing the 'Greyscale From:' so it accepts the change).

My question is, which one is the correct effect? It completly changes my hair renders if I choose a gamma of 0.0 to match the grayscale from alpha setting.

It seems odd that I can't change the gamma on the alpha channel, it makes it seem that ALL images are too bright because in setting the default to 1.0 ALL images have their luminosity increased by default.

 

Bejaymac

The image editor changes the texture and not the surface channel the texture is plugged into, so basically to do what you want you will need two PNG, one for Diffuse and the other to drive Opacity, which is probably  why so many of them still use JPEG for Diffuse and Opacity.