Download GL Stenciling Algorithms for Rendering Object Silhouettes and more Exams Computer Science in PDF only on Docsity!
** save the depth buffer and turn off the color buffer */ glReadPixels(0, 0, winWidth, winHeight, GL_DEPTH_COMPONENT, GL_FLOAT, depthSave); glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
/* ** clear the stencil buffer for this object / glEnable(GL_STENCIL_TEST); glStencilFunc(GL_ALWAYS, 0, 0); / clear stencil for this object */ glStencilOp(GL_ZERO, GL_ZERO, GL_ZERO);
/* ** set the polygon offset a tiny bit */ glEnable(GL_POLYGON_OFFSET_FILL); glPolygonOffset(0.5, 1.0);
drawMyObject();
glDisable(GL_POLYGON_OFFSET_FILL);
/* ** set up for stenciling, write a ’1’ whereever there is ** a line */ glStencilFunc(GL_ALWAYS, 1, 1); glStencilOp(GL_ZERO, GL_ZERO, GL_REPLACE);
/* ** disable writes to the Z−buffer test ** test for less than or equal to */ glDepthFunc(GL_LEQUAL); glDepthMask(GL_FALSE);
** render all polygons in outline mode without updating the ** color buffer or the depth buffer, ** this sets the stencil bit where for all the non−hidden ** lines in the object */ glPolygonMode(GL_FRONT_AND_BACK, GL_LINE); drawMyObject();
/* ** now the stencil only has the bit set for the non−hidden lines ** Turn on the color buffer ** restore the depth buffer ** write only were the stencil bit is set ** now the depth buffer and color buffer are current **/
glDepthMask(GL_TRUE); glDepthFunc(GL_ALWAYS); glDisable(GL_STENCIL_TEST);
** now the stencil only has the bit set for the non−hidden lines ** Turn on the color buffer ** restore the depth buffer ** write only were the stencil bit is set ** now the depth buffer and color buffer are current **/
glDepthMask(GL_TRUE); glDepthFunc(GL_ALWAYS); glDisable(GL_STENCIL_TEST); pushOrthoView(0, 1, 0, 1, 0, 1); glRasterPos2f(0, 0); glDrawPixels(winWidth, winHeight, GL_DEPTH_COMPONENT, GL_FLOAT, depthSave); popView(); glEnable(GL_STENCIL_TEST);
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE); glStencilFunc(GL_EQUAL, 1, 1); glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE); glColor3f(1,1,1); drawMyObject();
/* ** Turn off stenciling, enable writes to the color buffer, then ** render all polygons again in outline mode using foreground color */ glDisable(GL_STENCIL_TEST); glPolygonMode(GL_FRONT_AND_BACK, GL_FILL); glDepthFunc(GL_LESS);
Stenciling
Alg 1:
- Draw the object 4 times, offsetting in X & Y (screen−space) each time
- increment the stencil buffer each time
stencil values meaning 0 no object 1 outline of object 2 silhouette 3 silhouette 4 interior
- test for cases 2 & 3, draw only these in line mode
Alg 2.
- offset the polygon
- disable color buffer
- draw the objects (fills the depth buffer)
- disable offset
- make no further changes to the depth buffer (mask out)
- toggle stencil bits
- cull the back faces
- draw objects in line mode
- set stencil test = 1, zeroing out the stencil buffer
- enable color buffers
- disable the Z buffer
- draw objects in line mode with stencil test
- reset state
** render silhouette edges using the second algorithm
else if (mode == SILHOUETTE_ALG2) {
** render the offset depth image
glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(1,1);
glColorMask(0,0,0,0);
DrawObjects();
glDisable(GL_POLYGON_OFFSET_FILL);
** make no further changes to the depth image
glDepthMask(0);
** cull all facets of one (arbitrary) orientation. render the
** remaining facets in outline mode, toggling the stencil bit
** at each pixel.
glEnable(GL_STENCIL_TEST);
glStencilFunc(GL_ALWAYS, 0, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_INVERT);
glEnable(GL_CULL_FACE);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
DrawObjects();
** color all pixels in the framebuffer with stencil value 1
glStencilFunc(GL_EQUAL, 1, 1);
glStencilOp(GL_ZERO, GL_ZERO, GL_ZERO);
glColorMask(1,1,1,1);
glColor3f(1,1,1);
glDisable(GL_DEPTH_TEST);
DrawObjects();
** return state to default values
glDisable(GL_STENCIL_TEST);
glDisable(GL_CULL_FACE);
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glDepthMask(1);
glEnable(GL_DEPTH_TEST); /* XXX */
