Transcript No Slide Title

Using the Stencil Buffer
Advanced D3D Programming
Sim Dietrich
[email protected]
Overview
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What is a Stencil Buffer?
Direct3D Stencil Pipeline
Stencil Comparison Function
Stencil Operations
Example - Masking a Car Interior
Example - Depth Complexity
Example - Shadow / Light Volumes
What is a Stencil Buffer?
• Stencil bits are stored with the Z Buffer
• Typically consists of 1 or 8 bits
• Stencil allows apps to ‘tag’ various regions
of the frame buffer
• Can be used for shadow/light volumes,
masking, reflections and measuring depth
complexity
Stencil Pipeline
&
STENCIL_REF
STENCIL_FAIL
STENCIL_MASK &
Fail
Stencil Buffer
Stencil Write Mask
Stencil Test
Pixel Rejected
Pass
STENCIL_ZFAIL
Fail
Depth Test
Pixel Rejected
& &
Pass
Pixel Accepted
&
STENCIL_PASS
Stencil Comparison Function
• D3DCMPFUNC ( Just like Depth Test )
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D3DCMP_NEVER
D3DCMP_LESS
D3DCMP_EQUAL
D3DCMP_LESSEQUAL
D3DCMP_GREATER
D3DCMP_NOTEQUAL
D3DCMP_GREATEREQUAL
D3DCMP_ALWAYS
– D3DSTENCIL_WRITEMASK
– Limits updates to bits of the stencil buffer
Stencil Operations
• Set for each of three cases
– STENCIL_FAIL ( Failed Stencil Test, pixel is
dropped )
– STENCIL_ZFAIL ( Passed Stencil, Failed Z )
– STENCIL_PASS ( Passed Both Stencil and Z )
• D3DSTENCILOP_KEEP
– Leave current stencil value alone
Stencil Operations (cont.)
• D3DSTENCILOP_ZERO
– Set Stencil to Zero
• D3DSTENCILOP_REPLACE
– Set to Stencil Reference Value
• D3DSTENCILOP_INCRSAT
– Add one to Stencil, clamp to maximum value
• D3DSTENCILOP_DECRSAT
– Minus one from Stencil, clamp to Zero
Stencil Operations (cont.)
• D3DSTENCILOP_INVERT
– If bit is 1, make it 0
– If bit is 0, make it 1
• D3DSTENCILOP_INCR
– Add 1 to stencil, wrap to 0
• D3DSTENCILOP_DECR
– Minus 1 from stencil, wrap to maximum value
• Maximum Value is 2(# of Stencil Bits) - 1
Example - Masking a Car
Interior
• Imagine a in-car view of a driving game
• Why re-draw the car’s interior each frame?
• Stencil can be used to Mask it
• Clear Stencil Buffer to 0 when Z buffer is cleared
• Set Stencil Compare Function D3DCMP_ALWAYS
• Set Stencil Operation for STENCIL_PASS to
D3DSTENCILOP_INCRSAT
• Set Stencil Write Mask to 0xFF
• Draw car interior, thereby setting stencil >= 1 for
each pixel it covers
Masking a Car Interior (cont.)
– Now draw the rest of the scene
• Set Stencil Reference Value to Zero
• Set Stencil Compare Function:D3DCMP_EQUAL
• Set Stencil Operation for STENCIL_PASS ( for
passing both Z and Stencil Tests ) to
D3DSTENCILOP_KEEP
– This tells the card to only draw where it sees a
Zero in the Stencil Buffer
– The car interior was drawn with stencil of 1 or
higher, so it will not be overwritten.
Measuring Depth Complexity
• Depth Complexity is the average # of times
a pixel is redrawn in one scene
• Determines the maximum performance
limit at various resolutions
• Maximum Frame Rate <=
( Fill Rate ) / ( Depth Complexity * Resolution )
• Stencil can be used to count each time a
pixel is drawn, thereby measuring Depth
Complexity of any scene
Measuring Depth Complexity
– Clear Stencil Buffer to 0 when Z buffer is
cleared
– Set Stencil Compare Function to
D3DCMP_ALWAYS
– Set Stencil Operation for STENCIL_PASS and
STENCIL_ZFAIL to
D3DSTENCILOP_INCRSAT
– Set Stencil Write Mask to 0xFF
– Draw scene, thereby setting stencil >= 1 for
each pixel drawn
Measuring Depth Complexity
• Stencil Buffer now holds a count of how
many times each pixel was drawn
• Lock and read back the stencil buffer
• Sum all stencil buffer values and divide by
resolution of the viewport
• That number is the scene’s depth
complexity
• Set the Z Fail operation to INCRSAT to
count Z rejected pixels
Shadow and Light Volumes
• Many shadow algorithms project on to the
floor or ground plane
• They don’t handle objects that are inside
another object’s shadow
• To handle this case, apps have to test
vertices against the shadowed area
• Stencil and depth buffer can perform perpixel intersection calculations instead
Shadow And Light Volumes
• Shadow volumes are best used when the
shadow-casting object is simple, and the
shadowed geometry is complex
• Think of the volume of light that is blocked
by an object from the light direction
• Find the silhouette of our shadowing object
– Use the lowest detail level model if possible
• Create a volume from the silhouette’s
projection from the light direction
Shadow Volume Example
Shadow and Light Volumes
• We want the stencil bit for the shadow
volume to be ‘1’ for each pixel that should
be darkened, and Zero for all others
• We can use the depth buffer to perform an
intersection operation, and record the result
in the stencil buffer
• We draw the back and front faces of the
shadow volume into the stencil buffer only
Shadow and Light Volumes
• We want to mark all pixels that are inside
the shadow volume
– In front of the back faces of the shadow volume
– In back of the front faces of the shadow volume
• So, a pixel inside the shadow volume
should cause the the first depth test to fail,
and the second one to pass. Other cases will
pass or fail or twice.
Setting Up the Stencil Pipeline
• Here is the process :
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Clear the Stencil Buffer to 0 with Depth clear
Render entire lighted scene as usual
Set the Stencil Mask for the light’s bit (ie 0x01)
Set the Stencil Write Mask for the light’s bit (
ie 0x01 )
• We can have a separate bit for each light that casts
shadows, up to the number of stencil bits, unless we
wish perform multiple stencil passes for more lights
Setting Up the Stencil Pipeline
– Set the Stencil Comparison Function to
D3DCMP_ALWAYS
– Set the Stencil Z Fail to
D3DSTENCILOP_INVERT
– Set the Stencil Pass Function to
D3DSTENCILOP_KEEP
– The Stencil Fail function is a don’t care,
because we set the stencil test to always pass
– Set the Stencil Reference value to 0xFF
Shadow and Light Volumes
• Set cull mode to D3DCULL_NONE
• We need the both back and front faces of
the shadow volume to be ‘drawn’ into the
stencil buffer
• We don’t want them in the frame buffer, so
set the alpha blend mode to
– D3DRENDERSTATE SRCBLEND =
D3DBLEND_ZERO
– D3DRENDERSTATE DESTBLEND =
D3DBLEND_ONE
Shadow And Light Volumes
• Turn off Z writes
– D3DRENDERSTATE_ZWRITEENABLE =
FALSE
• ‘Draw’ the Shadow volume
• The stencil buffer will now contain a 1 at
the appropriate bit for each pixel in the
scene that lies within the shadow volume
Scene before Shadow Volume
Depth = .6
Stencil = 0
+ Z Axis
Depth = .5
Stencil = 0
Light Vector
Occluding Object
Depth = .4
Stencil = 0
+ X Axis
After Front Face of Volume
Depth = .6
Stencil = 0
+ Z Axis
Depth = .5
Stencil = 0
Light Vector
Occluding Object
Depth = .4
Stencil = 1
Front Face of Shadow Volume
+ X Axis
After Back Face of Volume
Depth = .6
Stencil = 0
+ Z Axis
Depth = .5
Stencil = 1
Light Vector
Occluding Object
Depth = .4
Stencil = 0
Back Face of Shadow Volume
+ X Axis
Shadow And Light Volumes
• Now, we need to apply an effect to these
pixels...
• Set up the stencil tests to only affect pixels
that correspond to the appropriate stencil bit
for our light
• Set the Stencil Comparison Function to
D3DCMP_EQUAL
– Set the Stencil Mask value to match the light’s
bit ( ie 0x01 )
Shadow and Light Volumes
– Set the Stencil Write Mask to match the light’s
bit ( ie 0x01 )
– Set the Alpha Blend function for our effect
• Make the rectangle out of the light color and use
– ONE, ONE for a Volumetric Additive effect
– ZERO, SRCALPHA for a Darkening Shadow
– ONE, ZERO for a color replacement effect
• Draw the rectangle and the effect is composited into
the scene
Handling Multiple Lights
• We can use as many lights as the stencil
buffer has bits, ie 1 or 8.
• We could handle more with multiple stencil
passes, by changing which bits refer to
which lights
• We need to do one blend operation per light
• We can do the all volumes for one light by
using a big enough rectangle
Handling Complex Volumes
• If your geometry produces a selfintersecting volume, INVERT won’t work
• Instead, you can render the volume’s back
faces with ZFAIL set to INCRSAT, and the
volume’s front faces with DECRSAT
• Pixels in the volume will have non-zero
stencil bits
• But, multiple lights require more passes
Caveats
• When clipping your volume, make sure it
remains closed - You need an even number
of intersections with the shadow volume to
avoid being darkened
• If the viewer is inside a shadow volume,
there will be only one intersection with that
volume, thus reversing what is in or out of
shadow, unless you cap your volume with a
polygon at the near clipping plane
Caveats
• We can draw the front or back faces of the
volume instead of the screen-aligned
rectangle, but it is likely to be more
expensive, and really should be sorted for
some blending effects
• The shadow volume is capped at the view
frustum, which means that shadows can be
cast onto more than one wall if not clipped
to the first wall.
Other Approaches
• One fundamental problem with the
‘darkening’ approach presented here is that
it is a purely pixel-space process. You have
lost all information about materials, fog, etc.
The advantage is that you render the scene
only once.
• An alternative is to re-render the stencilmarked pixels within the shadow volume
with the light turned off for more accuracy
Other Approaches
• A more accurate way to simulate objects in
shadow is to avoid lighting them with the
blocked lights to begin with, rather than just
darkening them after the fact
– Render flat-shaded scene into depth buffer only
– Careful with alpha-tested transparent texures
– Render shadow volume into stencil
– Render textured scene with the light ON to
everywhere but shadowed area
– Render scene with light OFF to shadowed area
Other Approaches
• For a smoother shadow edge on the floor or
walls, connect a scaled-up version of the
shadow volume to the original shadow
volume. Set the Alpha on the original
vertices to 0xff and the Alpha on the scaled
up version to 0 and connect them.
• Note that this only works for surfaces that
you cap the volume to ( like walls or a floor
), not for intermediate objects in the shadow
volume
Questions
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Sim Dietrich
[email protected]
www.nvidia.com
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