Using normalmap blue channel for greyscale mask issue

anonymous_user_d1c65eb3 · July 18, 2016, 7:27pm

Hey, I’m trying to save some texture memory by using the blue channel of my normalmap as a mask. However, in doing so my normalmap seems to get weird black artifacts that I can’t get rid of.

Does anyone know what could be causing these black lighting errors? I believe I’ve imported the texture and set it up correctly. I could not find the settings for getting the normalmap in the correct range in the texture so I set it up…I think…in the material instead. I’ve tried a dozen different things and sometimes it almost seems to work, but in the end I get these black errors in the normalmap. It looks fine if I just use the normalmap the way it’s “supposed” to be used with the normalmap compression settings and so on.

anonymous_user_d1c65eb3 · July 19, 2016, 3:33pm

Anyone? Is there a better way of doing this? Or a tutorial to be found somewhere?

BoredEngineer · July 20, 2016, 8:21am

Could be compression issue. Overal I’m not sure if it makes sense to do this. When you import 3 channel normal map and set its compression to Normal, ue4 will use BC5 compression which doesn’t store Z component. There is a separate compression for gray scale masks too. Are you doing this to minimize amount of texture samplers?

anonymous_user_c94f194d · July 20, 2016, 6:49pm

This is really counter productive way. Normal maps need to be stored two channel format. Both channels are high quality. If you don’t use compression and sampler mode normal then you get really bad looking artefacts. Better way to save memory is to use smaller textures. Normalmap even with half res texture(1/4 memory) with correct compression settings is better looking than butchered DXT1 texture.

anonymous_user_d1c65eb3 · July 25, 2016, 5:21pm

The idea is to use the blue channel for smoothness or occlusion map and recreate the blue channel for the normal in the shader. To reduce texture sample calls (I won’t need to add one extra texture to the shader) and potentially save some memory. The actual normalmap looks fine when it comes to compression artifacts and such, I don’t see much difference, except for the black artifacts that show up in some areas, I’m thinking I might be doing something wrong in the shader giving the normalmap extreme values in some areas.

Edit/ Ah, this is sort of what I was looking for, going to see if that helps. Perhaps it won’t be worth the quality loss from dxt compression but we’ll see.

Maximum-Dev · July 25, 2016, 7:41pm

Set it up like this:

Deathrey · July 25, 2016, 9:11pm

@anonymous_user_74354b6d
This is happening when sum of your R and G channels squared is more than 1, resulting in attempts to pull out square root from negative number and returning NaN. The root cause of this is essentially the fact, that your normal map was not normalized before export(yet even that can’t gurantee the same issue happening due to compression artifacts). UE4 is handling this for you internally, when you use a normal map sampler. If other sampler is used, you will have to do it yourself. One of the ways to handle it was linked in post above by @Maximum-Dev, and here is another one, virtually the same, but exactly as it is done in the engine shaders:

http://image.prntscr.com/image/3e8cb64474be40df9d4f46e8e8ad7611.png

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[/SPOILER]
As a conclusion, I should mention that you need to turn sRGB off for your texture, and use linear color sampler. Additionally, it would be best to use ALPHA and GREEN channels for R and G normal channels, respectively, retaining blue or red channel for mask.

[MENTION=37019]Jenny Gore[/MENTION] There is a bulk of situations, where DXT compression will be vastly superior to BC, in terms of performance/quality loss trade-off. And no, if done right, you should not see any artifacts, apart from usual compression block squares, but you are right, that this move would be effective in cutting down sampler count and not as good at memory saving.

anonymous_user_c94f194d · July 26, 2016, 12:50pm

There are always artifacts when using this scheme. DXT1 compression is not per channel but per color. So all three channels are compressed with assumption that they do correlate. Each block only store two endpoints as 16-bit rgb color. Then each texel use 2-bit indice to interpolate value between these endpoints. This does mean that normal direction is affecting blue channel and other way around. Which is not correct at all. Even the fact that normal x and y is affecting each other is not correct. DXT1 also only use 5-bit for red and 6-bit for green which is not enough for any smooth surfaces.
Normal map compression aka BC5 use 8-bit uncorrelate endpoints for red and green. It’s also use 3-bit for indices. It’s superior format in every way.

Deathrey · July 26, 2016, 2:48pm

Jenny Gore;570869:

There are always artifacts when using this scheme. DXT1 compression is not per channel but per color. So all three channels are compressed with assumption that they do correlate. Each block only store two endpoints as 16-bit rgb color. Then each texel use 2-bit indice to interpolate value between these endpoints. This does mean that normal direction is affecting blue channel and other way around. Which is not correct at all. Even the fact that normal x and y is affecting each other is not correct. DXT1 also only use 5-bit for red and 6-bit for green which is not enough for any smooth surfaces.
Normal map compression aka BC5 use 8-bit uncorrelate endpoints for red and green. It’s also use 3-bit for indices. It’s superior format in every way.

Understanding BCn Texture Compression Formats – Nathan Reed’s coding blog

I think you are missing the point. It is pretty obvious that BC5 is better for storing normals. That is if you completely ignore where and how those normal maps will be used.

Breakdown for a typical 2k texture setup:


BC1 Normal map , BC1 BaseColor, - 1 additional mask,  ~ 5.5 Mb , 2 samplers

BC3 Normal map , BC1 BaseColor, - 2 additional masks, ~ 8.1 Mb, 2 samplers

BC3 Normal map , BC3 BaseColor - 3 additional masks, ~ 10.9 Mb, 2 samplers

BC5 Normal map , BC1 BaseColor - 0 additional masks, ~ 8.1 Mb, 2 samplers

BC5 Normal map , BC3 BaseColor - 1 additional mask, ~ 10.9 Mb, 2 samplers

BC5 Normal map , BC1 BaseColor, BC1 Masks - 3 additional masks, ~ 10.9 Mb, 3 samplers

Disregarding visual loss, doing what OP intended to saves you ~2.8 Mb on each 2k texture, not even speaking about extra sampler.

Neither all objects in the game have smooth reflective surface nor all of them are meant to occupy whole screen space.
Distant terrain, detail maps,noise masks and anything that requires blending of several texture layers, coupled with need for multiple masks. These are perfect candidates for packing normal map with extra masks.
Additionally in situations where you are:

Memory bound
Bandwidth bound
Fetch bound
Hitting sampler limit

BC5 advantages become non-existent.

anonymous_user_c94f194d · July 26, 2016, 3:58pm

You said “if done right, you should not see any artifacts”. But it can’t be done right because of compression scheme BC1 use. Cross channel talk is real problem. You get far better quality with lower memory consumption if you just use bit lower texture size(quarter size) for normal map and mask. It will use one sampler more but it will be artifact free.

anonymous_user_d1c65eb3 · August 6, 2016, 2:21pm

Hey guys, just wanted to say thank you for the answers and that I got it working. There are some compression artifacts like you said but it might be manageable since my object is pretty gritty and grimy.