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Introduction to OpenGL (Part 4)

Introduction to OpenGL (Part 4). Ref: OpenGL Programming Guide (The Red Book). Part 1 Introduction Geometry Viewing Light & Material Display List. Part 2 Alpha Channel Polygon Offset Part 3 Image Texture Mapping Part 4 Framebuffers Selection & Feedback. Topics. OpenGL.

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Introduction to OpenGL (Part 4)

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  1. Introduction to OpenGL(Part 4) Ref: OpenGL Programming Guide (The Red Book) Fall 2009 revised

  2. Part 1 Introduction Geometry Viewing Light & Material Display List Part 2 Alpha Channel Polygon Offset Part 3 Image Texture Mapping Part 4 Framebuffers Selection & Feedback Topics

  3. OpenGL Framebuffers

  4. color (front/back, left/right, aux) Front/back: animation Left/right: stereo vision Aux: not available on PC (use framebuffer object FBO instead) depth buffer: depth test VC: 24 bits available Value : [0.0,1.0] stencil buffer: restrict drawing to certain portions of the screen VC: 8 bits available accumulation buffer: accumulating composite image Types of Buffers

  5. Using Framebuffers • clearing buffers • clearing individual buffer is expensive • Use glClear with bitwise-ORed masks to clear multiple buffers • selecting color buffers for writing/clearing • glDrawBuffer: useful in FBO (framebuffer object)

  6. Masking Buffers • Before OpenGL writes data into the enabled color, depth, or stencil buffers, a masking operation is applied to the data, as specified with one of the following commands. • A bitwise logical AND is performed with each mask and the corresponding data to be written

  7. Masking Buffers (cont) • void glColorMask(GLboolean red, GLboolean green, GLboolean blue, GLboolean alpha); • void glDepthMask(GLboolean flag); • void glStencilMask(GLuint mask); • If a 1 appears in mask, the corresponding bit in the stencil buffer is written; where a 0 appears, the bit is not written. • The default values of all the GLboolean masks are GL_TRUE, and the default values for the two GLuint masks are all 1's

  8. Only Green Mask TRUE

  9. Per-fragment Operation Sequence • Fragments: the pieces after rasterization, sent to framebuffers if they survive these tests • glScissor • restrict drawing to part of the window • a simpler (and faster) version of stencil test (that does not need stencil buffer) • application: gui panel Dithering and logical op not covered here

  10. While the scissor test is enabled, only pixels that lie within the scissor box can be modified by drawing commands. Including glClear (shown right) Note: validity of current raster position is determined by frustum (world coordinate); while scissor test affects the window coordinate Single-pixel window: glScissor (x,y,1,1); Scissor Test See also stencil_close

  11. Alpha test: accept/reject a fragment based on its alpha value implement transparency use this test to filter opaque objects see-through decal (billboarding): reject the transparent fragments (from ruining the depth buffer) Stencil Test require stencil buffer glStencilFunc, glStencilOp Fragment Operations (cont)

  12. Stenciling

  13. Basic Applications Restrict rendering to limited portions of the screen • “Stenciling” • Decal • Shadow and lightmap

  14. Mimicking Stencil • Compose stencil template • Control template then render • Multi-pass rendering silhouette

  15. Decal Using Stencil Buffers Stencil to resolve z-fighting

  16. Shadow and Lightmap Shadow and light blended with the textured polygon underneath

  17. Technical Details • glStencilFunc (fun, ref, mask) • func Specifies the test function. Eight tokens are valid: GL_NEVER, GL_LESS, GL_LEQUAL, GL_GREATER, GL_GEQUAL, GL_EQUAL, GL_NOTEQUAL, and GL_ALWAYS. • If it's GL_LESS, for example, then the fragment passes if reference value is less than the value in stencil buffer • ref Specifies the reference value for the stencil test. ref is clamped to the range [0, 2n - 1], where n is the number of bitplanes in the stencil buffer. • VC: n = 8 • mask Specifies a mask that is ANDed with both the reference value and the stored stencil value when the test is done

  18. Technical Details (cont) • glStencilOp (fail, zfail, zpass) • fail Specifies the action to take when the stencil test fails. • zfail Specifies stencil action when the stencil test passes, but the depth test fails. • zpass Specifies stencil action when both the stencil test and the depth test pass, or when the stencil test passes and either there is no depth buffer or depth testing is not enabled. • Six symbolic constants are accepted: GL_KEEP, GL_ZERO, GL_REPLACE, GL_INCR, GL_DECR, and GL_INVERT. Set value to ref Bitwise invert

  19. Stenciling • Steps to draw 2 coplanar rectangles: • Make the stencil for yellow one first (by drawing the green polygon) • Draw the yellow one with the stencil • Draw the green one

  20. Stenciling (cont) Stencil buffer Color buffer

  21. Decaling [0] Base: (-5,0)(5,6) [1] R: (-3,1)(2,4) [2] G: (1,2)(3,5) [3] B: (0,0)(4,3) Foreground blocker • Draw everything else (set up the depth buffer) • Draw base; set the blocker id to be 8; rest 0 • Arrange the decal id in increasing order • The decals can be drawn in any order Test the program…

  22. 2 2 2 2 2 2 Foreground blocker 3 3 3 3 3 0 0 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 0 8 8 8 8 8 8 8 0 0 0 8 8 8 8 8 8 8 0 0 StencilOp(fail, zfail, zpass) Color buffer Stencil buffer

  23. 2 2 2 2 2 2 Foreground blocker 3 3 3 3 3 0 0 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 0 8 8 8 8 8 8 8 0 0 0 8 8 8 8 8 8 8 0 0 StencilOp(fail, zfail, zpass) Color buffer Stencil buffer Drawn in different order!

  24. Generating Silhouette Stencil Buffer Debug with GLIntercept

  25. Different line style Hidden line removal Haloed lines Cheap silhouette See also

  26. Shadow matrix to project model onto a flat shadow receiver Blending + stencil buffer techniques involved Stencil Planar Shadow

  27. Reflection Dinosaur is reflected by the planar floor. Easy hack, draw dino twice, second time hasglScalef(1,-1,1) to reflect through the floor Notice right image’s reflection falls off the floor!

  28. Stencil Reflection Clear stencil to zero.Draw floor polygon with stencil set to one.Only draw reflection where stencil is one. Reflection and shadow

  29. [Shadow Volume Using Stencil Buffers] Triangle blockinglight source. Use stencil to tag whetherpixel is inside or outsideof the shadow volume. Two passes: light pixels outsidevolume; no lighting forpixels inside the volume. “Shadow” volumeprojected by trianglefrom the light source.

  30. series of images generated on standard color buffers; accumulated one at a time into the accumulation buffers; result copied back into a color buffer for viewing Accumulation Buffer

  31. scene antialiasing, motion blur, photographic depth of field [softshadow, jittering] analogy: multiple exposures takes longer, but higher quality accum.buffer may have higher precision 16 bits for each RGBA channels scene antialiasing by spatially jittering the image Accumulation Buffer (cont)

  32. Accumulation Buffer APIs • glClearAccum (0,0,0,0); • glGetIntegerv (GL_ACCUM_RED_BITS, &value); • 16 for each RGBA channels • glAccum(GL_ACCUM, 1.f/counts); • glAccum(GL_RETURN, 1.f);

  33. Accumulation Buffer scene antialiasing motion blur depth of field

  34. Ref

  35. FSAA with Multi-Sampling(ref) • glMultiSample

  36. Motion Blur w/o Accum.Buffer Details: scene dynamically render to texture; modulate with a polygon (1,1,1,a)

  37. Scene Anti-aliasing • Can be done in real-time w/o accumulation buffer • By outlining the silhouette edges • See Tom Hall FPS: 10.8 FPS: 9.6

  38. OpenGL Selection and Feedback

  39. Revisit unproject.c A line segment between (x1, y1, zNear) and (x2, y2, zFar) Check which object got hit Complexity problem?! Can do better? How to do Picking? Simple tracking on XZ plane

  40. Selection (Basic Idea) • draw scene into framebuffer • enter selection mode and redraw the scene • the content of framebuffer unchanged until you exit the selection mode • when exit, returns a list of primitives that intersected the viewing volume • as an array of names (integer) and hit records • Picking: a variation of selection • triggered by mouse; mouse defines v.volume

  41. Selection (Steps) • glSelectBuffer (maxselects, selectbuffer) • glRenderMode (GL_SELECT) • glInitNames, glPushName (0) • Something needs to be pushed on the empty stack (otherwise, it cannot be “loaded”) • define viewing volume • Need to save current projection matrix for further rendering … • issue drawing commands • Load a name; then draw something; if the thing drawn is in v.volume, it generates a hit (with the name) • glRenderMode (GL_RENDER) • Return the number of hits • process select buffer for hits

  42. Name Stack • name: GLuint • glInitNames • causes the name stack to be initialized to its default empty state • glPushName/glPopName • as the name implies • glLoadName • replace the top of the stack • need to push something right after InitNames for first load • commands ignored unless in selection mode

  43. Primitive intersecting v.vol causes a hit stack pointer points to the beginning of selection array and updates when hit occurs culled polygons generate no hit composite objects (w/ single name) generate one hit hit records aren’t updated until glRenderMode is called each hit record: number of names on the name stack min and max window-coord z values [0,1] contents of name stack w/ bottommost element first Hit Record

  44. void selectObjects(void) { GLuint selectBuf[512]; GLint hits; glSelectBuffer (512, selectBuf); (void) glRenderMode (GL_SELECT); glInitNames(); glPushName(0); glPushMatrix (); glMatrixMode (GL_PROJECTION); glLoadIdentity (); glOrtho (0.0, 5.0, 0.0, 5.0, 0.0, 10.0); glMatrixMode (GL_MODELVIEW); glLoadIdentity (); glLoadName(1); drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -5.0); glLoadName(2); drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0, -5.0); glLoadName(3); drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, 0.0); drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0); glPopMatrix (); hits = glRenderMode (GL_RENDER); processHits (hits, selectBuf); } void display(void) { glClearColor (0.0, 0.0, 0.0, 0.0); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); drawScene (); selectObjects (); glFlush(); } void drawScene (void) { glMatrixMode (GL_PROJECTION); glLoadIdentity (); gluPerspective (40.0, 4.0/3.0, 1.0, 100.0); glMatrixMode (GL_MODELVIEW); glLoadIdentity (); gluLookAt (7.5, 7.5, 12.5, 2.5, 2.5, -5.0, 0.0, 1.0, 0.0); glColor3f (0.0, 1.0, 0.0); /* green */ drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -5.0); glColor3f (1.0, 0.0, 0.0); /* red */ drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0, -5.0); glColor3f (1.0, 1.0, 0.0); /* yellow */ drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, 0.0); drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0); drawViewVolume (0.0, 5.0, 0.0, 5.0, 0.0, 10.0); } void processHits (GLint hits, GLuint buffer[]) { unsigned int i, j; GLuint names, *ptr; printf ("hits = %d\n", hits); ptr = (GLuint *) buffer; for (i = 0; i < hits; i++) { /* for each hit */ names = *ptr; printf (" number of names for hit = %d\n", names); ptr++; printf(" z1 is %g;", (float) *ptr/0x7fffffff); ptr++; printf(" z2 is %g\n", (float) *ptr/0x7fffffff); ptr++; printf (" the name is "); for (j = 0; j < names; j++) { /* for each name */ printf ("%d ", *ptr); ptr++; } printf ("\n"); } }

  45. Process Hits • When returning to RENDER mode, • All hit records got recorded in a GLuint array as follows [# of names, zmin, zmax, names…] • Names for an object need not be a single integer • [object type, object number, object part,…]

  46. About the depth value (refs: 1,2) • The depth is taken from the Z buffer (where it lies in the range [0,1]). • It gets multiplied by 2^32 -1 (0xffff ffff)and is rounded to the nearest integer. • Note that the depths you get are not linearly proportional to the distance to the viewpoint due to the nonlinear nature of the z buffer (See Opengl-2.ppt, p.44)

  47. Picking restrict drawing to a small region of the viewport (near the cursor) mouse click to initiate the selection mode Issues: interpreting mouse (x,y)! hierarchical pick: name stack manipulation 3D pick (picking and depth values)

  48. creates a projection matrix that can be used to restrict drawing to a small region of the viewport gluPickMatrix Often called from mouse callback; need to convert glut to OpenGL mouse coordinates

  49. void drawSquares(GLenum mode) { GLuint i, j; for (i = 0; i < 3; i++) { if (mode == GL_SELECT) glLoadName (i); for (j = 0; j < 3; j ++) { if (mode == GL_SELECT) glPushName (j); glColor3f ((GLfloat) i/3.0, (GLfloat) j/3.0, (GLfloat) board[i][j]/3.0); glRecti (i, j, i+1, j+1); if (mode == GL_SELECT) glPopName (); } } } void processHits (GLint hits, GLuint buffer[]) { unsigned int i, j; GLuint ii, jj, names, *ptr; printf ("hits = %d\n", hits); ptr = (GLuint *) buffer; for (i = 0; i < hits; i++) { /* for each hit */ names = *ptr; printf (" number of names for this hit = %d\n", names); ptr++; printf(" z1 is %g;", (float) *ptr/0x7fffffff); ptr++; printf(" z2 is %g\n", (float) *ptr/0x7fffffff); ptr++; printf (" names are "); for (j = 0; j < names; j++) { /* for each name */ printf ("%d ", *ptr); if (j == 0) /* set row and column */ ii = *ptr; else if (j == 1) jj = *ptr; ptr++; } printf ("\n"); board[ii][jj] = (board[ii][jj] + 1) % 3; } } void pickSquares(int button, int state, int x, int y) { GLuint selectBuf[512]; GLint hits; GLint viewport[4]; if (button != GLUT_LEFT_BUTTON || state != GLUT_DOWN) return; glGetIntegerv (GL_VIEWPORT, viewport); glSelectBuffer (512, selectBuf); (void) glRenderMode (GL_SELECT); glInitNames(); glPushName(0); glMatrixMode (GL_PROJECTION); glPushMatrix (); glLoadIdentity (); gluPickMatrix ((GLdouble) x, (GLdouble) (viewport[3] - y), 5.0, 5.0, viewport); gluOrtho2D (0.0, 3.0, 0.0, 3.0); drawSquares (GL_SELECT); glMatrixMode (GL_PROJECTION); glPopMatrix (); glFlush (); hits = glRenderMode (GL_RENDER); processHits (hits, selectBuf); glutPostRedisplay(); }

  50. Example: picking spheres and cubes

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