Your image gos from RGB 0|0|0 to RGB 97|97|97, with the specific region you have highlighted going from RGB 19|19|19 to RGB 22|22|22 in increments of 1.

In other words, what you are seeing as banding is actually consecutive greyscale colours in increments of 1, but at the darker end of the range.

Measuring 140 next to 141 in an image program is not representative of the colour range you are using; you need to measure darker colours, in the 10s, 20s or 30s, to be able to do an equal comparison.

What you're seeing is just colour banding, caused by the fact that (and irrespective of how much precision you use internally) the final destination framebuffer is an 8|8|8|8 RGBX or RGBA format.

This is nothing to do with OpenGL and everything to do with your hardware.

Your image goes from RGB 0|0|0 to RGB 97|97|97, with the specific region you have highlighted going from RGB 19|19|19 to RGB 22|22|22 in increments of 1.

In other words, what you are seeing as banding is actually consecutive greyscale colours in increments of 1, but at the darker end of the range.

Measuring 140 next to 141 in an image program is not representative of the colour range you are using; you need to measure darker colours, in the 10s, 20s or 30s, to be able to do an equal comparison.

What you're seeing is just colour banding, caused by the fact that (and irrespective of how much precision you use internally) the final destination framebuffer is an 8|8|8|8 RGBX or RGBA format.

This is nothing to do with OpenGL and everything to do with your hardware.