Android OpenGL ES – Part 4: Indexed Drawing

1. Visualize interpolation

• Open the main.cpp file.
• Modify the vertex shader source code as follows:

static const GLchar vertexShaderSource[] =
   "#version 310 es\n"
   "layout (location = 0) in vec3 pos;\n"
   "out vec4 vColor;\n"
   "uniform mat4 model;\n"
   "void main()\n"
   "{\n"
   "gl_Position = model * vec4(pos, 1.0);\n"
   "vColor = vec4(clamp(pos, 0.0, 1.0), 1.0);\n"
   "}\n";

• Modify the fragment shader source code as follows:

static const GLchar fragmentShaderSource[] =
   "#version 310 es\n"
   "precision mediump float;\n"
   "in vec4 vColor;\n"
   "out vec4 color;\n"
   "void main()\n"
   "{\n"
   "color = vColor;\n"
   "}\n";

2. Create a rotation animation

• Delete the following global variable declarations:

bool movingRight = true;
float movingOffset = 0.0f;
float movingMaxOffset = 0.5f;
float movingStep = 0.015f;

• Add the following global variable declarations in their place:

/* code */

float currentAngle = 0.0f;
float angleStep = 1.0f;

/* code */

• Delete the following lines of code from the Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnDrawFrame function:

movingRight ? movingOffset += movingStep : movingOffset -= movingStep;
if (abs(movingOffset) >= movingMaxOffset) {
   movingRight = !movingRight;
}

model = glm::translate(model, glm::vec3(movingOffset, 0.0f, 0.0f));

• Still in Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnDrawFrame, make the following changes:

/* code */

glUseProgram(program);

currentAngle += angleStep;
if (currentAngle >= 360.0f) currentAngle -= 360.0f;

glm::mat4 model = glm::mat4(1.0f);
model = glm::rotate(model, glm::radians(movingOffset), glm::vec3(0.0f, 1.0f, 0.0f));

/* code */

3. Convert the triangle into a pyramid

• Declare one more global variable:

/* code */

GLuint program, triangleVAO, triangleVBO, triangleIBO;

/* code */

• Modify the createTriangle function as follows:

void createTriangle() {
   GLuint indices[] = {
      0, 1, 2,
      0, 2, 3,
      0, 3, 4,
      0, 4, 1
};

   GLfloat vertices[] = {
      0.0f, 1.0f, 0.0f,
      -1.0f, -1.0f, 1.0f,
      1.0f, -1.0f, 1.0f,
      1.0f, -1.0f, -1.0f,
      -1.0f, -1.0f, -1.0f
   };

   glGenVertexArrays(1, &triangleVAO);
   glBindVertexArray(triangleVAO);

   glGenBuffers(1, &triangleIBO);
   glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangleIBO);
   glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

   glGenBuffers(1, &triangleVBO);
   glBindBuffer(GL_ARRAY_BUFFER, triangleVBO);
   glBufferData(GL_ARRAY_BUFFER, 15 * sizeof(GL_FLOAT), vertices, GL_STATIC_DRAW);

   glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
   glEnableVertexAttribArray(0);

   glBindBuffer(GL_ARRAY_BUFFER, 0);
   glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
   glBindVertexArray(0);

   return;
}

• Delete the following line of code from the Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnDrawFrame function:

glDrawArrays(GL_TRIANGLES, 0, 3);

• Still in Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnDrawFrame, make the following changes:

/* code */

model = glm::rotate(model, glm::radians(currentAngle), glm::vec3(0.0f, 1.0f, 0.0f));
model = glm::scale(model, glm::vec3(0.4f, 0.4f, 0.4f));
glUniformMatrix4fv(uniformModel, 1, GL_FALSE, glm::value_ptr(model));

glBindVertexArray(triangleVAO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangleIBO);
glDrawElements(GL_TRIANGLES, 12, GL_UNSIGNED_INT, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindVertexArray(0);

glUseProgram(0);

/* code */

4. Enable depth

• Modify the Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnSurfaceCreated function as follows:

/* code */

glClearColor(0.0, 1.0, 0.0, 1.0);

glEnable(GL_DEPTH_TEST);

createProgram();

/* code */

• Modify the Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnDrawFrame function as follows:

/* code */

extern "C" JNIEXPORT void JNICALL Java_dev_anastasioscho_glestriangle_NativeLibrary_nOnDrawFrame(JNIEnv * env, jobject obj) {
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

   glUseProgram(program);

/* code */