Set velocity from sample texture 2d node

hello,
I'm trying to set velocity from a series of optical flow textures created externally.
I made the textures to default to 0.5 gray pixel as below so I expected to get 0 velocity by adding (-0.5, -0.5) to the sampled pixel color.


These are my nodes. (I'm only using x and y axis on 2d)


But once I apply this vector to 'Set Velocity' node in the Initialize Particle block, I get this diagonally moving particles. Instead, I expected them to stay still at the center.



If I change the Add node value from -0.5 to -0.2 or something, it looks almost staying at the center, but it doesn't make sense to me. I even try to log the textures' pixel color in the script and it's 0.502. (-0.502 doesn't work either of course.)

I think there must be some very silly mistake in my logic. Any idea?

Best regards,
Y

 

Hello

Your texture is most likely imported as sRGB (used to gamma correct color images) can you check that the sRGB checkbox is not enabled in tour texture asset inspector?

 

Hello, thanks.

Yes, it was imported as sRGB.
I disabled the option but it still doesn't work as I expected.

Attached is one of the optical flow image sequence I'm trying to read in VFX.

Any idea?

Best regards,
Y

 

Maybe it has something to do with texture compression? You can try disabling it.

 

Thanks for the idea.
I tried it, but it still doesn't work.

I ended up sampling the 0.5, 0.5 color pixel at the corner and using it as mid value.
It seems to work, but I still wonder what went wrong with the initial approach.