opengl es 从零开始系列9动画基础和关键帧动画(2)

NSTimeInterval timeSinceLastKeyFrame = [NSDate

timeIntervalSinceReferenceDate] - lastKeyframeTime;

if (timeSinceLastKeyFrame > kAnimationDuration) { direction = !direction;

timeSinceLastKeyFrame = timeSinceLastKeyFrame - kAnimationDuration;

lastKeyframeTime = [NSDate timeIntervalSinceReferenceDate]; }

NSTimeInterval percentDone = timeSinceLastKeyFrame / kAnimationDuration; VertexData3D *source, *dest;

if (direction == kAnimationDirectionForward) {

source = (VertexData3D *)Ball1VertexData; dest = (VertexData3D *)Ball2VertexData; } else {

source = (VertexData3D *)Ball2VertexData; dest = (VertexData3D *)Ball1VertexData; }

for (int i = 0; i < kBall1NumberOfVertices; i++) {

GLfloat diffX = dest[i].vertex.x - source[i].vertex.x; GLfloat diffY = dest[i].vertex.y - source[i].vertex.y; GLfloat diffZ = dest[i].vertex.z - source[i].vertex.z; GLfloat diffNormalX = dest[i].normal.x - source[i].normal.x; GLfloat diffNormalY = dest[i].normal.y - source[i].normal.y; GLfloat diffNormalZ = dest[i].normal.z - source[i].normal.z; ballVertexData[i].vertex.x = source[i].vertex.x + (percentDone * diffX);

ballVertexData[i].vertex.y = source[i].vertex.y + (percentDone * diffY);

ballVertexData[i].vertex.z = source[i].vertex.z + (percentDone * diffZ);

ballVertexData[i].normal.x = source[i].normal.x + (percentDone * diffNormalX);

ballVertexData[i].normal.y = source[i].normal.y + (percentDone * diffNormalY);

ballVertexData[i].normal.z = source[i].normal.z + (percentDone * diffNormalZ); }

glEnableClientState(GL_VERTEX_ARRAY);

glEnableClientState(GL_NORMAL_ARRAY); glVertexPointer(3, GL_FLOAT, sizeof(VertexData3D), &Ball2VertexData[0].vertex);

glNormalPointer(GL_FLOAT, sizeof(VertexData3D), &Ball2VertexData[0].normal);

glDrawArrays(GL_TRIANGLES, 0, kBall1NumberOfVertices); glDisableClientState(GL_VERTEX_ARRAY); glDisableClientState(GL_NORMAL_ARRAY); }

首先，有一些初始化设置。我创建了一个静态变量来追踪当前帧是否是最后一帧，这用来判定当前流逝的时间。首先我们初始化当前的时间，然后声明变量来追踪我们的动画是向前还是向后的。

static NSTimeInterval lastKeyframeTime = 0.0; if (lastKeyframeTime == 0.0)

lastKeyframeTime = [NSDate timeIntervalSinceReferenceDate]; static AnimationDirection direction = kAnimationDirectionForward;

然后是一些OpenGL ES一般设置。唯一需要注意的是我把x

轴旋转了-90°。我们知道OpenGL ES使用Y轴向上的坐标体系，同样的我们旋转为Z轴向上。

glClearColor(1.0, 1.0, 1.0, 1.0); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glLoadIdentity();

glTranslatef(0.0f,2.2f,-6.0f);

glRotatef(-90.0, 1.0, 0.0, 0.0); // Blender uses Z-up, not Y-up like OpenGL ES

接下来，声明一个静态数组来存储插值数据：

static VertexData3D

ballVertexData[kBall1NumberOfVertices];

为了简单，我设置了一个颜色并且开启了color materials。我不想使使用texture（纹理）或者materials（材质）使这个例子变得更加混乱。

glColor4f(0.0, 0.3, 1.0, 1.0); glEnable(GL_COLOR_MATERIAL);

现在我计算出上一个帧过去到现在的时间，如果这个时间大于动画时长，改变动画的运动方向。

NSTimeInterval timeSinceLastKeyFrame = [NSDate

timeIntervalSinceReferenceDate] - lastKeyframeTime;

if (timeSinceLastKeyFrame > kAnimationDuration) { direction = !direction;

timeSinceLastKeyFrame = timeSinceLastKeyFrame - kAnimationDuration;

lastKeyframeTime = [NSDate timeIntervalSinceReferenceDate]; }

NSTimeInterval percentDone = timeSinceLastKeyFrame / kAnimationDuration;

为了适应双向动画，我声明了两个指针指向源帧和目的帧的数据，并且根据当前的方向指向适当的数据数组。

VertexData3D *source, *dest;

if (direction == kAnimationDirectionForward) {