[Bf-committers] A contribution

[Dan Brown]

Bill Baxter wrote:

> Dan Brown wrote:
> 
>>
>> The Iriz+zbuf seems to do it like that, 8 channels, 1 each for bits 
>> 0-7, 8-15, 16-23 and 24-31 and 4 channels for RGBA. I wanted to save 
>> floats though, mainly because OpenGL and Direct3D use floats 
>> internally for their depth buffers and it would be more in keeping 
>> with that for people to use. Using tga though means it is easier to 
>> visualise quickly for testing.
> 
> Just to nitpick, all graphics hardware I'm aware of uses a 24-bit (or 
> 16-bit) fixed point depth buffer.  So while OpenGL and Direct3D may do 
> some depth computations internally in floating point, the depth buffer 
> is ultimately not stored as floats on the card.

The hardware does whatever it needs to for speed on that architecture, but the 
software interface is whatever has been defined as. Direct3D uses integers, so I 
was wrong about that, OpenGL accepts ints or floats but uses a float format 
internally. From the developers guide -

"GL_DEPTH_COMPONENT
     Each pixel is a single depth component. Floating-point data is converted 
directly to an internal floating-point format with unspecified precision. Signed 
integer data is mapped linearly to the internal floating-point format such that 
the most postive representable value maps to 1.0, and the most negative 
representable value maps to -1.0. Unsigned integer data is mapped similarly: the 
largest integer value maps to 1.0, and zero maps to 0.0. The resulting 
floating-point depth value is the multiplied by GL_DEPTH_SCALE and added to 
GL_DEPTH_BIAS. The result is clamped to the range [0, 1]."

from <http://www.mevis.de/~uwe/opengl/glDrawPixels.html>

So using a TGA or other non-lossy format would be perfectly acceptable.  But it 
does seem like a workaround, if I go to the bother of adding a 'save 
depth-buffer' button surely it would be better in a direct format, i.e. 32-bit 
ints or floats rather than as 4 planes that have to be spliced back together 
again before it can be used.

Dan

> --bb
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