我目前正在开发一款 iOS 游戏。现在我想要的是一个用opengl绘制任意发光和半透明椭圆的函数。这有点棘手，因为我使用的是 cocos2d 1.1，它不支持 opengl es 2.0，所以没有着色器。这是我所做的：
首先是绘制挤出线的函数，这是来自http://answers.oreilly.com/topic/1669-how-to-render-anti-aliased-lines-with-textures-in-ios-4/
-(void) drawLineFrom:(CGPoint) start to:(CGPoint) end color:(ccColor4B) color width:(CGFloat) width texture:(CCTexture2D *) texture{
// calculate direction vector
CGPoint e = [self op:end minus:start];
CGFloat length = sqrtf(e.x * e.x + e.y * e.y);
e = ccp(e.x * width / length, e.y * width / length);
CGPoint n = ccp(-e.y, e.x);
CGPoint s = [self op:CGPointZero minus:n];
CGPoint ne = [self op:n plus:e];
CGPoint nw = [self op:n minus:e];
CGPoint sw = [self op:CGPointZero minus:ne];
CGPoint se = [self op:CGPointZero minus:nw];
// init vertices
CGPoint vertices[8];
vertices[0] = [self op:start plus:sw];
vertices[1] = [self op:start plus:nw];
vertices[2] = [self op:start plus:s];
vertices[3] = [self op:start plus:n];
vertices[4] = [self op:end plus:s];
vertices[5] = [self op:end plus:n];
vertices[6] = [self op:end plus:se];
vertices[7] = [self op:end plus:ne];
// init texture coordinates
CGPoint texCoords[8] = {ccp(0, 0) ,ccp(0,1), ccp(0.5, 0), ccp(0.5, 1), ccp(0.5, 0), ccp(0.5, 1), ccp(1, 0), ccp(1, 1)};
// init colors
ccColor4B colors[8];
for (int i = 0; i < 8; i++) {
colors[i] = color;
}
glBindTexture(GL_TEXTURE_2D, texture.name);
glColorPointer(4, GL_UNSIGNED_BYTE, 0, colors);
glTexCoordPointer(2, GL_FLOAT, 0, texCoords);
glVertexPointer(2, GL_FLOAT, 0, vertices);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 8);
}
然后是我的画椭圆函数。基本上它所做的就是将椭圆分割成许多小线并使用上面的画线函数来绘制它们：
-(void) drawEllipse:(float) r focus1:(CGPoint) f1 focus2:(CGPoint) f2 {
// load texture
CCTexture2D *texture = [[CCTextureCache sharedTextureCache] addImage: @"glow.png"];
int segments = 50;
CGFloat width = r / 2;
CGFloat height = sqrtf(powf(r / 2, 2) - powf([Helper distanceFrom:f1 to:f2] / 2, 2));
CGPoint center = CGPointMake((f1.x + f2.x) / 2, (f1.y + f2.y) / 2);
glEnable(GL_TEXTURE_2D);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glTranslatef(center.x, center.y, 0.0);
glRotatef(atanf((f1.y - f2.y) / (f1.x - f2.x)) * 180 / M_PI, 0, 0, 1);
GLfloat vertices[segments * 2];
int count = 0;
for (GLfloat i = 0; i < 360.0f; i += (360.0f / segments)) {
vertices[count++] = (cos(degreesToRadian(i)) * width);
vertices[count++] = (sin(degreesToRadian(i)) * height);
}
int i = 0;
CGFloat lineWidth = 20;
ccColor4B lineColor = ccc4(255, 255, 255, 50);
while (i < segments * 2 - 2) {
[self drawLineFrom:ccp(vertices[i++], vertices[i++]) to:ccp(vertices[i++], vertices[i++]) color:lineColor width:lineWidth texture:texture];
}
[self drawLineFrom:ccp(vertices[segments * 2 - 2], vertices[segments * 2 - 1]) to:ccp(vertices[0], vertices[1]) color:lineColor width:lineWidth texture:texture];
}
我的问题是，使用上述混合模式，我的椭圆看起来像这样：
可以看到混合模式显然不对。由于混合，线条连接在一起的地方看起来不像其他部分。但我不知道应该使用哪种混合模式。
I also tried glBlendEquation(GL_MAX_EXT), and it definitely makes my ellipse look much better:
但问题是，如果我使用 GL_MAX_EXT 颜色的 alpha 值似乎完全被忽略，因此无法使椭圆半透明。有人能帮我找出正确的方法吗？非常感谢！
顺便说一句，这是我的纹理：
None