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Stenciling Effects

Stenciling Effects. Glenn G. Chappell CHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 481/681 Lecture Notes Monday, February 9, 2004. Review: Details of Jittering [1/2]. Some of the more interesting accumulation-based effects can be accomplished via “jittering”.

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Stenciling Effects

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  1. Stenciling Effects Glenn G. ChappellCHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 481/681 Lecture Notes Monday, February 9, 2004

  2. Review:Details of Jittering [1/2] • Some of the more interesting accumulation-based effects can be accomplished via “jittering”. • Jittering = making small, random-ish movements in a scene, rendering for each movement, and blending the results. • Jittering is useful for making things that look “fuzzy”. • We generally jitter in two dimensions. • Using a table of x & y values is convenient. • The file jitter.h contains a number of such tables. See simplejitter.cpp for an example program that uses them. • For each jitter, we use x and y to generate a transformation. • What sort of transformation? • Where in the pipeline does it go? • Answering these can be the trickiest part of jittering. CS 481/681

  3. Review:Details of Jittering [2/2] • We looked at three effects that can be created using jittering: • Anti-Aliasing • Jitter the entire rendered image. • Transformations are simple translations. • They go after the usual projection (before it, in the code). • Depth-of-Field Effect • Jitter the 3-D scene, leaving a plane fixed. • Transformations are shears, and thus cannot be created using only glTranslate*, glRotate*, glScale*. • They go before the usual projection (after it, in the code). • Soft Shadows • Use some shadowing method. Jitter the light source. • Transformations are translations, perpendicular to the predominant light direction. • They go before (after, in the code) the rest of the model/view transformation(s), for the light-source only. CS 481/681

  4. Review:Introduction to Stenciling [1/3] • Stenciling involves the stencil buffer and the stencil test. • Remember: allocate the buffer, enable the test. • Clear the buffer the same way you clear any buffer. • The two major functions used in stenciling are glStencilFunc and glStencilOp. • glStencilFunc determines what the stencil test does. • glStencilOp determines what happens to the stencil buffer if the stencil test passes or fails. • If the stencil test passes, then you can also have different outcomes based on the depth test. CS 481/681

  5. Review:Introduction to Stenciling [2/3] • glStencilFunc takes three parameters: • A GLenum: what comparison the stencil test will do. • A GLint used as a “reference value” in the stencil test. • It is also used by the GL_REPLACE option of glStencilOp. • A GLuint used as a mask (an “and” mask). • Think: REF COMPARE (buffer pixel & mask) • Examples • Stencil test passes if bit in SB is on:glStencilFunc(GL_EQUAL, 1, 1); • Stencil test passes if bit in SB is off:glStencilFunc(GL_NOTEQUAL, 1, 1); • Test passes if 20 < low 8 bits in SB:glStencilFunc(GL_LESS, 20, 0xff); CS 481/681

  6. Review:Introduction to Stenciling [3/3] • glStencilOp takes three parameters, all GLenum’s: • Operation to perform if stencil test fails. • Op. to perform if stencil test passes and depth test fails. • Op. to perform if stencil test passes and depth test passes. • Examples • Replace the SB value with the reference value (from glStencilFunc):glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE); • Do not modify the SB:glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); • Replace SB value with zero, the reference value, or the bitwise inversion of the current SB value, respectively:glStencilFunc(GL_ZERO, GL_REPLACE, GL_INVERT); • Increment or decrement the SB value, as appropriate:glStencilFunc(GL_DECR, GL_INCR, GL_INCR); CS 481/681

  7. Stenciling Effects:“Ordinary” Stenciling • First we draw a shape in the stencil buffer. • If the shape is fixed, then we only need to redraw it when the viewport changes size. So this code can go in the reshape function. glClearStencil(0); glClear(GL_STENCIL_BUFFER_BIT); glStencilFunc(GL_NEVER, 1, 1); // No "real" drawing. glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE); Draw a shape here. glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); • Then we use the above shape, in the display function. glStencilFunc(GL_EQUAL, 1, 1); Draw something; it will appear only inside the above shape. glStencilFunc(GL_NOTEQUAL, 1, 1); Draw something; it will appear only outside the above shape. CS 481/681

  8. Stenciling Effects:Reverse Painter’s Algorithm • Recall that the R.P.A. can be used, with a method to order polygons front-to-back, to do HSR. • To order polygons, you might use a BSP tree. • To use the R.P.A., we draw front-to-back, and we make sure that no pixel color is set more than once. • We can do the latter using stenciling. glClearStencil(0); glClear(GL_STENCIL_BUFFER_BIT | GL_COLOR_BUFFER_BIT); // No depth buffer! glStencilFunc(GL_NOTEQUAL, 1, 1); glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE); Draw scene front-to-back. CS 481/681

  9. Stenciling Effects:Capping • Here is an implementation of “capping” (see Red p. 446). • Say we are drawing a number of closed objects. We wish to make sure that the inside of these is never visible, even if the near clipping plane slices one of them. glClearStencil(0); glClear(GL_STENCIL_BUFFER_BIT | GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glStencilFunc(GL_ALWAYS, 1, 1); glStencilOp(GL_INVERT, GL_INVERT, GL_INVERT); Draw scene. glStencilFunc(GL_EQUAL, 1, 1); Draw rectangle covering entire viewport, in “capping” color. CS 481/681

  10. Stenciling Effects:Odd Things to Do • Draw each pixel at most 5 times: glClearStencil(0); glClear(GL_STENCIL_BUFFER_BIT); glStencilFunc(GL_GREATER, 5, 0xff); glStencilOp(GL_KEEP, GL_INCR, GL_INCR); • Draw each pixel successfully only on every other attempt: glClearStencil(0); glClear(GL_STENCIL_BUFFER_BIT); glStencilFunc(GL_EQUAL, 0, 1); glStencilOp(GL_INVERT, GL_INVERT, GL_INVERT); CS 481/681

  11. Shadow Volumes:Introduction • In CS 381 we covered two shadowing techniques: • Shadowing via Projection • The shadow is the shadow-caster drawn using the proper projection in the model/view matrix (and probably in a different color, too). • Shadowing via Textures • Draw the shadow-caster from the point of view of the light source. Grab the resulting image for use as a texture. • Using the texture transformation, draw the shadow on the shadow-receiver. • Now we look at a third technique: shadow volumes. • This technique uses stenciling. CS 481/681

  12. Shadow Volumes:The Idea [1/2] • Consider a number of polygons that cast non-overlapping shadows. • The “non-overlapping” part is important, unfortunately. • These polygons may be part of a single surface. • For each polygon, the region in its shadow is called its shadow volume. • If we can draw the portions of polygons that lie inside shadow volumes into the stencil buffer, then we can use stenciling to draw the shadowed scene. • Right? • So, how do we do this? CS 481/681

  13. Shadow Volumes:The Idea [2/2] • Think about the boundaries of shadow volumes. • A point is in shadow precisely when we must look through an odd number of these boundaries to see it. • Assuming the viewer is not in shadow. If the viewer is in shadow, change “odd” to “even”. • Zero is even (and not odd)! • Do you see why it isimportant that shadowvolumes do not overlap? CS 481/681

  14. Shadow Volumes:The Technique • First, draw the scene in the depth buffer: • Clear the depth buffer. • Set up the stencil test so that it always passes. • Draw the scene in the depth buffer, with the usual depth test. • Next the magic. Draw shadow-volume boundaries, inverting the SB value if the depth test passes. • Clear the SB. • Set up stenciling: • The depth buffer is never changed. • The depth test is as usual. • The stencil test always passes (this is already set). • If the stencil and depth tests pass, then invert the SB value. Otherwise, leave it alone. • Draw the shadow-volume boundaries. • Now the non-zero portions of the SB are those in which we see something in shadow, and we can draw the final scene: • Fix the stencil buffer so it cannot be changed, and the depth buffer so it can. • Clear the color and depth buffers. • Draw scene with only ambient lighting, restricted to non-zero in SB. • Draw scene with full lighting, restricted to zero in SB. CS 481/681

  15. Note • Due to the Discovery Lab becoming available, we will stop the buffers/tests/blending unit here. • Next we will look at dealing with transformations in ways that are helpful in VR. CS 481/681

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