罗大柚OpenGL ES教程系列LessonOne（Part 2）：绘制一个五角星
五角星的各顶点坐标如下图所示：
五角星一共有10个顶点,各顶点的坐标在上图中已经标注，下面给出各顶点坐标计算公式：
#define PI3.1415926
double L1=0.2*cos(36*PI/180);
doubleL2=0.2*sin(36*PI/180);
doubleL3=0.2*cos(72*PI/180);
doubleL4=0.2*sin(72*PI/180);
doubleL5=L2*tan(72*PI/180);
doubleL6=L2/cos(72*PI/180);
doubleL7=L6*sin(54*PI/180);
double L8=L6*cos(54*PI/180)+0.2;
我们通过上面各点的位置关系，得到下面所示的顶点数据，然后我们采用索引的方式来绘制这个五角星，
// Thisdata type is used to store information for each vertex
typedef struct {
GLKVector3 positionCoords;
}Vertex;
//eachvertex coordinates and indexes of the pentacle
static constVertexvertices[] =
{
{-0.307768, -0.423607, 0.0},
{-0.190211, -0.061803, 0.0},
{-0.497980, 0.161803, 0.0},
{-0.117557, 0.161803, 0.0},
{0.0, 0.523607, 0.0},
{0.117557, 0.161803, 0.0},
{0.497980, 0.161803, 0.0},
{0.190211, -0.061803, 0.0},
{0.307768, -0.423607, 0.0},
{0.0, -0.2, 0.0}
};
const GLubyte Indices[]= {
9,0,1,
1,2,3,
3,4,5,
5,6,7,
7,8,9,
9,1,5,
5,7,9,
1,3,5
};
接下来就是写ViewDidLoad方法了：
/////////////////////////////////////////////////////////////////
//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 createdautomatically by the
// Interface Builder storyboard
GLKView *view = (GLKView *)self.view;
NSAssert([view isKindOfClass:[GLKView class]],
@"View controller'sview is not a GLKView");
// Create an OpenGL ES 2.0 context andprovide it to the
// view
view.context = [[EAGLContextalloc]
initWithAPI:kEAGLRenderingAPIOpenGLES2];
// Make the new context current
[EAGLContext setCurrentContext:view.context];
// Create a base effect that providesstandard OpenGL ES 2.0
// Shading Language programs and setconstants to be used for
// all subsequent rendering
self.baseEffect = [[GLKBaseEffectalloc] init];
self.baseEffect.useConstantColor =GL_TRUE;
self.baseEffect.constantColor =GLKVector4Make(
1.0f, // Red
0.0f, // Green
0.0f, // Blue
1.0f);// Alpha
// Set the background color stored in the currentcontext
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);//background color
// setup VBOs
glGenBuffers(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); //
//索引
glGenBuffers(1, &indexBufferID);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,indexBufferID);
glBufferData(GL_ELEMENT_ARRAY_BUFFER,sizeof(Indices),Indices, GL_STATIC_DRAW);
}
GLKit提供了绘制OpenGL图形的代理方法:
#pragma mask Delegate Method
- (void)glkView:(GLKView *)viewdrawInRect:(CGRect)rect
{
[self.baseEffectprepareToDraw];
// Clear Frame Buffer (erase previous drawing)
glClear(GL_COLOR_BUFFER_BIT);
// Enable use of positions from bound vertexbuffer
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(Vertex),// nogaps in data
NULL); // NULL tells GPU to start at
// beginning of bound buffer
//利用index来绘制五角星
glDrawElements(GL_TRIANGLES,sizeof(Indices)/sizeof(Indices[0]),
GL_UNSIGNED_BYTE, 0);
}
最后写一下ViewDidUnload方法，我们的任务就完成了：
/////////////////////////////////////////////////////////////////
// Called when the view controller's view hasbeen unloaded
// Perform clean-up that is possible when youknow the view
// controller's view won't be asked to drawagain 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 whenview 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];
}
很简单很方便吧，这可都是GLKBaseEffect的好处，我们不需要写shader也能创建各种图形，下一篇将介绍如何在GLKit框架下绘制一个立方体，我们会使用VBO， VAO以及VAO索引三种方式来分别绘制，如果你对VA， VBO， VAO都不懂，那你就要去拜访你亲戚度娘或者谷哥了。同样的，我们附上本教程源码的下载地址：
http://download.csdn.net/detail/luozhonglan/6985799
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祝大家打码愉快
--罗大柚
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