OpenGL ES 2.0 Making the Hardware Work for You

//
//  OpenGLES_Ch2_1ViewController.m
//  OpenGLES_Ch2_1
//

#import "OpenGLES_Ch2_1ViewController.h"

@implementation OpenGLES_Ch2_1ViewController

@synthesize baseEffect;

/////////////////////////////////////////////////////////////////
// This data type is used to store information for each vertex
typedef struct {
   GLKVector3  positionCoords;
}
SceneVertex;

/////////////////////////////////////////////////////////////////
// Define vertex data for a triangle to use in example
static const SceneVertex vertices[] = 
{
   {{-0.5f, -0.5f, 0.0}}, // lower left corner
   {{ 0.5f, -0.5f, 0.0}}, // lower right corner
   {{-0.5f,  0.5f, 0.0}}, // upper left corner
};


/////////////////////////////////////////////////////////////////
// Called when the view controller's view is loaded
// Perform initialization before the view is asked to draw
- (void)viewDidLoad
{
   [super viewDidLoad];
   
   // Verify the type of view created automatically by the
   // Interface Builder storyboard
   GLKView *view = (GLKView *)self.view;
   NSAssert([view isKindOfClass:[GLKView class]],
      @"View controller's view is not a GLKView");
   
   // Create an OpenGL ES 2.0 context and provide it to the
   // view
   view.context = [[EAGLContext alloc] 
      initWithAPI:kEAGLRenderingAPIOpenGLES2];
   
   // Make the new context current
   [EAGLContext setCurrentContext:view.context];
   
   // Create a base effect that provides standard OpenGL ES 2.0
   // Shading Language programs and set constants to be used for 
   // all subsequent rendering
   self.baseEffect = [[GLKBaseEffect alloc] init];
   self.baseEffect.useConstantColor = GL_TRUE;
   self.baseEffect.constantColor = GLKVector4Make(
      1.0f, // Red
      1.0f, // Green
      1.0f, // Blue
      1.0f);// Alpha
   
   // Set the background color stored in the current context 
   glClearColor(0.0f, 0.0f, 0.0f, 1.0f); // background color
   
   // Generate, bind, and initialize contents of a buffer to be 
   // stored in GPU memory
   glGenBuffers(1,                // STEP 1
      &vertexBufferID);
   glBindBuffer(GL_ARRAY_BUFFER,  // STEP 2
      vertexBufferID); 
   glBufferData(                  // STEP 3
      GL_ARRAY_BUFFER,  // Initialize buffer contents
      sizeof(vertices), // Number of bytes to copy
      vertices,         // Address of bytes to copy
      GL_STATIC_DRAW);  // Hint: cache in GPU memory
}


/////////////////////////////////////////////////////////////////
// GLKView delegate method: Called by the view controller's view
// whenever Cocoa Touch asks the view controller's view to
// draw itself. (In this case, render into a frame buffer that
// shares memory with a Core Animation Layer)
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
   [self.baseEffect prepareToDraw];
   
   // Clear Frame Buffer (erase previous drawing)
   glClear(GL_COLOR_BUFFER_BIT);
   
   // Enable use of positions from bound vertex buffer
   glEnableVertexAttribArray(      // STEP 4
      GLKVertexAttribPosition);
      
   glVertexAttribPointer(          // STEP 5
      GLKVertexAttribPosition, 
      3,                   // three components per vertex
      GL_FLOAT,            // data is floating point
      GL_FALSE,            // no fixed point scaling
      sizeof(SceneVertex), // no gaps in data
      NULL);               // NULL tells GPU to start at 
                           // beginning of bound buffer
                                   
   // Draw triangles using the first three vertices in the 
   // currently bound vertex buffer
   glDrawArrays(GL_TRIANGLES,      // STEP 6
      0,  // Start with first vertex in currently bound buffer
      3); // Use three vertices from currently bound buffer
}


/////////////////////////////////////////////////////////////////
// Called when the view controller's view has been unloaded
// Perform clean-up that is possible when you know the view 
// controller's view won't be asked to draw again soon.
- (void)viewDidUnload
{
   [super viewDidUnload];
   
   // Make the view's context current
   GLKView *view = (GLKView *)self.view;
   [EAGLContext setCurrentContext:view.context];
    
   // Delete buffers that aren't needed when view is unloaded
   if (0 != vertexBufferID)
   {
      glDeleteBuffers (1,          // STEP 7 
                       &vertexBufferID);  
      vertexBufferID = 0;
   }
   
   // Stop using the context created in -viewDidLoad
   ((GLKView *)self.view).context = nil;
   [EAGLContext setCurrentContext:nil];
}

@end

typedef NS_ENUM(NSUInteger, EAGLRenderingAPI)
{
    kEAGLRenderingAPIOpenGLES1 = 1,
    kEAGLRenderingAPIOpenGLES2 = 2,
    kEAGLRenderingAPIOpenGLES3 = 3,
};

- 1、BaseEffect  的属性

- 2、`constantColor` 填充色（设置填充色的前提是`self.baseEffect.useConstantColor = GL_TRUE;`，开启填充色），如果把 Green 置零

- 1、`glClearColor`，设置（view）背景色，定义 -->  `GL_API void           GL_APIENTRY glClearColor (GLfloat red, GLfloat green, GLfloat blue, GLfloat alpha);`修改颜色值观察变化 

- 2、`glGenBuffers`，添加标记，定义`GL_API void  GL_APIENTRY glGenBuffers (GLsizei n, GLuint* buffers);`， GLsizei  `typedef int32_t  GLsizei;`

- 3、`glBindBuffer`，添加绑定，定义`GL_API void  GL_APIENTRY glBindBuffer (GLenum target, GLuint buffer);` GLenum `typedef uint32_t GLenum;`

- 4、`glBufferData`，`定义：GL_API void   GL_APIENTRY glBufferData (GLenum target, GLsizeiptr size, const GLvoid* data, GLenum usage);`

- 5、`vertices`，因为我们是要绘制 三角形，所以有三个坐标点（顶点）：

其中`GLKVector3` 定义 ：

--> 因为 OpenGL ES 的坐标范围为：【-1，1】,三角形在坐标系下的展示为：

- 使能 Buffers `glEnableVertexAttribArray`，函数的定义是：

绘制的选项

- 计算所有点的偏移量 `glVertexAttribPointer` ， 函数定义为 `GL_API void           GL_APIENTRY glVertexAttribPointer (GLuint indx, GLint size, GLenum type, GLboolean normalized, GLsizei stride, const GLvoid* ptr)  __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_3_0);`