Output of GL_DEPTH_COMPONENT

OpenGL OpenGL: Basic Coding
#1

Hello,
I try to understand the output value of glReadPixels with parameters GL_DEPTH_COMPONENT.
Here a part content of my obj file:

v 47.004219 -60.619961 -34.676708
v 46.93742 -60.368637 -34.942249
v 46.895119 -60.117729 -35.213577
v 46.877819 -59.86705 -35.4883
v 46.885769 -59.61644 -35.763931
v 46.91898 -59.365719 -36.038002
v 49.249428 -63.449627 -32.687481
v 46.97723 -59.114708 -36.308041
v 49.000347 -63.183308 -32.73838
v 47.060051 -58.863247 -36.571617
v 48.755192 -62.918678 -32.813602
v 47.166721 -58.611179 -36.826332
v 48.516319 -62.655716 -32.91267
v 47.296288 -58.358318 -37.06987
v 48.28606 -62.394371 -33.034897
v 47.44759 -58.104519 -37.300022
v 48.066692 -62.13459 -33.17939
v 47.619251 -57.849648 -37.514709
v 47.860363 -61.876278 -33.344997
v 47.809673 -57.593552 -37.711983
v 47.66909 -61.619362 -33.530392
v 48.017082 -57.336121 -37.890038
v 47.49482 -61.363708 -33.734009
v 48.23954 -57.077221 -38.047291
v 47.339272 -61.109211 -33.954151
v 48.474949 -56.81678 -38.18232

My depth buffer output has min/max values ( 0.00011…, 0.00017…). What is the output value unit ? Is it a millimeter value ? Is it a depth value of only visible element ?

If someone can explain me.

#2

Window-space depth values.
Typically 0…1 (with the default glDepthRange( 0, 1 )).
WIth a perspective projection, most values are typically up close to 1.
Though you may be doing something special, like rendering with inverted Z. In which case typically close to 0.

#3

In fact, my questio behind this is how to convert this depth value to real depth value in meter for example?
Is there a way to do this?

#4

Assuming glDepthRange(0,1) (the initial state), convert it to NDC with:

ZNDC = 2*depth-1

Then transform it by the inverse of the projection matrix to get an eye-space coordinate. If necessary, transform that by the inverse of the model-view matrix’ scale factor to get an object-space coordinate.

Your reported depth buffer values only make sense if you’re using either a) an inverted-depth perspective projection (0 at the far plane, 1 at the near plane) or b) an orthographic projection with an excessively large value for the far plane distance.

For a more typical projection, I’d expect to see either a much greater spread of values (for an orthographic projection with sensible near/far distances) or values clustered close to 1.0 (for a perspective projection).

#5

Thank you for the answer. I will try with all that information