Changes between Version 16 and Version 17 of tutorial

Timestamp:

Jun 28, 2013, 9:20:20 AM (12 years ago)

Author:

leon

Comment:

Add pressure and temperature shaders

tutorial

@@ -132 +132 @@
 }}}
  How can we add color to vertices? See [http://www.opengl.org/sdk/docs/man2/xhtml/glColorPointer.xml glColorPointer] and [http://www.opengl.org/sdk/docs/man2/xhtml/glEnableClientState.xml glEnableClientState].
- 6. Change background to {{{glClearColor(0.9,1,1,1.0);}}}
+ 6. Change background to {{{glClearColor(0.9,1,1,1.0);}}} and suggest initial window in `main()`
+  {{{
+  #!c
+  glutInitWindowSize(512, 512);
+  glutInitWindowPosition((glutGet(GLUT_SCREEN_WIDTH)-512)/2,
+                         (glutGet(GLUT_SCREEN_HEIGHT)-512)/2);
+  }}}
  7. Add [http://www.opengl.org/resources/libraries/glut/spec3/node49.html keyboard event]
   to quit the program when pressing ESCape key with keycode 27 by adding callback function
@@ -143 +149 @@
    }
   }}}
-  and registering event within `main()` by `glutKeyboardFunc(keyboard);`
-  
-  
+  and registering event within `main()` by `glutKeyboardFunc(keyboard);`. Some prefer `key == 'q'`, though.
 
 
@@ -151 +155 @@
 [[Image(OpenGL-pipeline.svg, 320px, right, title=OpenGL pipeline)]]
 We extend previous exercise with example that introduces OpenGL 3.x techniques:
- * OpenGL Shading Language where simple vertex and fragment shader are required.
- * Vertex Aray Objects (VAOs) stored in GPU
+ * OpenGL Shading Language (GLSL 1.2) where simple vertex and fragment shader are required.
+ * Vertex Aray Objects (VAOs) and vertex buffers (VBOs) stored in GPU.
 Create {{{triangle.c}}} and update {{{Makefile}}} with new target 
 {{{
@@ -257 +261 @@
       glGetShaderiv( v, GL_INFO_LOG_LENGTH, &len );
       GLchar* log = malloc(sizeof(GLchar)*(len+1));
-      printf("Shader compilation failed: %s\n", log);
+      printf("Vertex Shader compilation failed: %s\n", log);
       free(log);
     }
@@ -277 +281 @@
    }}}
    Create some error to verify if it works.
-  For general (core) OpenGL errors we can use the following utility at suspicious places.
-  {{{
-  #!c
-     GLenum errCode;
-     if ((errCode = glGetError()) != GL_NO_ERROR) {
-        const GLubyte *errString = gluErrorString(errCode);
-        fprintf (stderr, "OpenGL Error: %s\n", errString);
-     }
-  }}}
- 2. Introduce vertex temperature with additional array
-  {{{
-  #!c
-  GLfloat vertex_temperature[] = {0, 0.5, 1, 0.7, 0.2, 0.9};
-  }}}
-  and replace shaders with
+  For general (core) OpenGL errors we can use the following `glcheck()` utility at suspicious places.
+  {{{
+  #!c
+  #define glcheck() {GLenum s; if ((s=glGetError()) != GL_NO_ERROR) \
+                  fprintf (stderr, "OpenGL Error: %s at line %d\n", \
+                           gluErrorString(s),  __LINE__);}
+  }}}
+ 2. Introduce vertex temperature with additional array and buffer at the end off `init()`
+  [[Image(temperature.png,right)]]
+  {{{
+  #!c
+  GLfloat vertex_temperature[] = {0, 1, 0.2, 0.1, 0.5, 0.9};
+  glBindBuffer(GL_ARRAY_BUFFER, Buffers[TempBuffer]);
+  glBufferData(GL_ARRAY_BUFFER, sizeof(vertex_temperature),
+               vertex_temperature, GL_STATIC_DRAW);
+  glVertexAttribPointer(tPosition, 1, GL_FLOAT, GL_FALSE, 0, NULL);
+  glEnableVertexAttribArray(tPosition);//toggle this
+  }}}
+  and adding corresponding IDs to `VAOs` and buffers. Replace shaders with
   {{{
   #!c
 static const GLchar * vertex_shader[] = {
-"attribute float VertexTemp;" // receive this custom attribute along with vertex position
-"varying float Temperature;"  // communicate between the vertex and the fragment shader
-"void main() { gl_position  = gl_ModelViewProjectionMatrix * gl_Vertex; }"
+  ""
+  "attribute float temperature;" // custom variable along with vertex position
+  "varying float  t;" // communicate between the vertex and the fragment shader
+  "void main()"
+  "{"
+  "  t = temperature;"
+  "  gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;"
+  "}"
 };
 static const GLchar * fragment_shader[] = {
-"vec3 Cool = vec3(0, 0, 1);" // Red
-"vec3 Hot  = vec3(1, 0, 1);" // Blue
-"void main()
-"{"
-"  vec3 color = mix(Cool, Hot, Temperature);" // use the built-in mix() function
-"  gl_FragColor = vec4(color, 1.0);"          // append alpha channel
-"}"
-}; 
-  }}}
+  "vec3 Cool = vec3(0, 0, 1);" // Red
+  "vec3 Hot  = vec3(1, 0, 0);" // Blue
+  "varying float t;" // Interpolated by fragment
+  "void main()"
+  "{"
+  "  vec3 color = mix(Cool, Hot, t);"  // use the built-in mix() function
+  "  gl_FragColor = vec4(color, 1.0);" // append alpha channel
+  "}"
+};
+  }}}
+  What happens if we don't enable temperature vertex array? Confer [attachment:temperature.c] attached if having troubles.
+ 3. Add additional custom vertex array for the pressure. Change the temperature array to have values in Celsius for water boiling range [0-100]°C. Pressure should be in the range of [0-1] MPa. Scaling to color range [0-1] should be done in shaders. Toggle between them with the keyboard event by using the keys `'p'` and `'t`' that {{{glEnableVertexAttribArray()}}} and {{{glDisableVertexAttribArray()}}} corresponding vertex attribute arrays.
+