How to read a certain level of mipmaps from a RenderTexture? And Blit() performance...

Unity Engine
1

I wrote a shader to collect some statistics and saved them to the alpha channel of the output texture.
Now, I need to get an average value of them, so I tried to read the last level of the RenderTexture (which means a 1x1 Texture) in OnRenderImage(RenderTexture src, RenderTexture dest):

RenderTexture tempBuffer = RenderTexture.GetTemporary(src.width, src.height, 0, src.format, RenderTextureReadWrite.Linear);
tempBuffer.useMipMap = true;
mipTex = new Texture2D(1, 1, src.graphicsFormat, TextureCreationFlags.None);
Graphics.Blit(src, tempBuffer, _Material, 0);
Graphics.CopyTexture(tempBuffer, 0, tempBuffer.mipmapCount - 1, mipTex, 0, 0);
Color c = mipTex.GetPixel(0, 0);
Graphics.Blit(tempBuffer, dest, _Material, 1);

However, I got a negative value of c.a in Unity Editor, which is obviously wrong.

Update:

It seems to work on my Hololens 1, but the fps dropped heavily from ~50 to ~25, and the memory usage increased from 300+MB to 800+MB. It’s confusing, I just read a fullscreen texture and return its colors in pass 1 like this:

fixed4 frag (v2f i) : SV_Target
{
        UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(i);
        fixed4 renderTex = UNITY_SAMPLE_SCREENSPACE_TEXTURE(_MainTex, i.uv);
        return renderTex;
}

How can it be so costly?

Update 2:
It did not work on Hololens. c.a is always 0.804. Besides, if I remove tempBuffer and blit from src to dest twice like this:

Graphics.Blit(src, dest, _Material, 0);
Graphics.Blit(src, dest, _Material, 1);

the fps keeps ~50. I’m extremely confused about this problem…

2

Hi @zyjisdog

You need to use SAMPLE_TEXTURE2D_LOD if you want to sample a certain mipmap level:

SAMPLE_TEXTURE2D_LOD(textureName, samplerName, coord2, lod)

Should work ok if you got a HLSL shader. And on Unity 2019 you have to use SampleLevel if I remember correctly.

3

Dear @Olmi

Thanks anyway, but I need to read the mipmap pixels to CPU.

4

Oh, oops. How did I miss that. Sorry :slight_smile:
Wouldn’t GetPixels work then, I thought it supports reading at specified mip level?

public Color[ ] GetPixels(int x, int y, int blockWidth, int blockHeight, int miplevel);

But you do need to the texture back from GPU first.

And looking at your edit, did you get it working?

5

Did he generated the mip map first from the rendertexture?

6

No, finally it did not work. I found a similar issue at https://discussions.unity.com/t/670476 , but Texture.ReadPixels cannot access to mipmaps.

7

According to the documents of Unity, the mipmap is generated automatically if I set the ‘useMipMap’ flag to True.
I also set ‘autoGenerateMips’ to true to ensure that Unity creates the mipmaps. When debugging, the tempBuffer.mipmapCount is greater than 1. I believe the mipmaps are generated successfully.

8

Seems I must use ReadPixels() according to Using CopyTexture from RenderTexture to Texture2D not working !?!?!? . But ReadPixels() will read all pixels to texture instead of a level of mipmap, which is very expensive.

9

Anything change with this?

10

I struggled with this for a good while before finding a workaround, decided to relay what I did here for posterity. In a roundabout way, Olmi was right!

  • Start with your large render texture, make sure that it has mipmaps enabled (duh).
  • Create a second render texture that is the resolution of your desired mip level; this is what you’ll actually end up running ReadPixels() on!
    - Create a shader/material that does nothing but sample _MainTex at your desired mip level and pipe it back out.
  • Graphics.Blit your first texture into your second texture using your newly made material. This feels like a dirty hack but actually works! ymmv on the performance of this, but for my uses it ran great.
  • ReadPixels on your small texture and enjoy the newfound performance gains.