␡
- Code Framework
- Where to Download the Examples
- Hello Triangle Example
- Building and Running the Examples
- Using the OpenGL ES 2.0 Framework
- Creating a Simple Vertex and Fragment Shader
- Compiling and Loading the Shaders
- Creating a Program Object and Linking the Shaders
- Setting the Viewport and Clearing the Color Buffer
- Loading the Geometry and Drawing a Primitive
- Displaying the Back Buffer
This chapter is from the book
This chapter is from the book
This chapter is from the book
This chapter is from the book
Hello Triangle Example
Let's take a look at the full source code for our Hello Triangle example program, which is listed in Example 2-1. For those readers familiar with fixed function desktop OpenGL, you will probably think this is a lot of code just to draw a simple triangle. For those of you not familiar with desktop OpenGL, you will also probably think this is a lot of code just to draw a triangle! Remember though, OpenGL ES 2.0 is fully shader based, which means you can't draw any geometry without having the appropriate shaders loaded and bound. This means there is more setup code required to render than there was in desktop OpenGL using fixed function processing.
Example 2-1. Hello Triangle Example
#include "esUtil.h"
typedef struct
{
// Handle to a program object
GLuint programObject;
} UserData;
///
// Create a shader object, load the shader source, and
// compile the shader.
//
GLuint LoadShader(const char *shaderSrc, GLenum type)
{
GLuint shader;
GLint compiled;
// Create the shader object
shader = glCreateShader(type);
if(shader == 0)
return 0;
// Load the shader source
glShaderSource(shader, 1, &shaderSrc, NULL);
// Compile the shader
glCompileShader(shader);
// Check the compile status
glGetShaderiv(shader, GL_COMPILE_STATUS, &compiled);
if(!compiled)
{
GLint infoLen = 0;
glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &infoLen);
if(infoLen > 1)
{
char* infoLog = malloc(sizeof(char) * infoLen);
glGetShaderInfoLog(shader, infoLen, NULL, infoLog);
esLogMessage("Error compiling shader:\n%s\n", infoLog);
free(infoLog);
}
glDeleteShader(shader);
return 0;
}
return shader;
}
///
// Initialize the shader and program object
//
int Init(ESContext *esContext)
{
UserData *userData = esContext->userData;
GLbyte vShaderStr[] =
"attribute vec4 vPosition; \n"
"void main() \n"
"{ \n"
" gl_Position = vPosition; \n"
"} \n";
GLbyte fShaderStr[] =
"precision mediump float; \n"
"void main() \n"
"{ \n"
" gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); \n"
"} \n";
GLuint vertexShader;
GLuint fragmentShader;
GLuint programObject;
GLint linked;
// Load the vertex/fragment shaders
vertexShader = LoadShader(GL_VERTEX_SHADER, vShaderStr);
fragmentShader = LoadShader(GL_FRAGMENT_SHADER, fShaderStr);
// Create the program object
programObject = glCreateProgram();
if(programObject == 0)
return 0;
glAttachShader(programObject, vertexShader);
glAttachShader(programObject, fragmentShader);
// Bind vPosition to attribute 0
glBindAttribLocation(programObject, 0, "vPosition");
// Link the program
glLinkProgram(programObject);
// Check the link status
glGetProgramiv(programObject, GL_LINK_STATUS, &linked);
if(!linked)
{
GLint infoLen = 0;
glGetProgramiv(programObject, GL_INFO_LOG_LENGTH, &infoLen);
if(infoLen > 1)
{
char* infoLog = malloc(sizeof(char) * infoLen);
glGetProgramInfoLog(programObject, infoLen, NULL, infoLog);
esLogMessage("Error linking program:\n%s\n", infoLog);
free(infoLog);
}
glDeleteProgram(programObject);
return FALSE;
}
// Store the program object
userData->programObject = programObject;
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
return TRUE;
}
///
// Draw a triangle using the shader pair created in Init()
//
void Draw(ESContext *esContext)
{
UserData *userData = esContext->userData;
GLfloat vVertices[] = {0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f};
// Set the viewport
glViewport(0, 0, esContext->width, esContext->height);
// Clear the color buffer
glClear(GL_COLOR_BUFFER_BIT);
// Use the program object
glUseProgram(userData->programObject);
// Load the vertex data
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vVertices);
glEnableVertexAttribArray(0);
glDrawArrays(GL_TRIANGLES, 0, 3);
eglSwapBuffers(esContext->eglDisplay, esContext->eglSurface);
}
int main(int argc, char *argv[])
{
ESContext esContext;
UserData userData;
esInitialize(&esContext);
esContext.userData = &userData;
esCreateWindow(&esContext, "Hello Triangle", 320, 240,
ES_WINDOW_RGB);
if(!Init(&esContext))
return 0;
esRegisterDrawFunc(&esContext, Draw);
esMainLoop(&esContext);
}