如何将此OpenGL指针数学转换为Swift？ [英] How do I convert this OpenGL pointer math to Swift?

问题描述

我正在关注本教程关于OpenGL / GLKit for iOS，但试图在Swift中实现它。在我到达这一部分之前一切正常：

I’m following this tutorial about OpenGL / GLKit for iOS but trying to implement it in Swift. It’s going fine until I get to this part:

- (void)render { 

// 1
self.effect.texture2d0.name = self.textureInfo.name;
self.effect.texture2d0.enabled = YES;

// 2    
[self.effect prepareToDraw];

// 3
glEnableVertexAttribArray(GLKVertexAttribPosition);
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);

//---------------- This is where things break down...
long offset = (long)&_quad;        
glVertexAttribPointer(GLKVertexAttribPosition, 2, GL_FLOAT, GL_FALSE, sizeof(TexturedVertex), (void *) (offset + offsetof(TexturedVertex, geometryVertex)));
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(TexturedVertex), (void *) (offset + offsetof(TexturedVertex, textureVertex)));

// 5    
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

}

@end

我有属性 self.quad 这是一个像这样的Swift结构：

I have a property self.quad that’s a Swift struct like this:

struct TexturedVertex {
let geometryVertex: CGPoint
let textureVertex: CGPoint
}

struct TexturedQuad {

let bottomLeft: TexturedVertex
let bottomRight: TexturedVertex
let topLeft: TexturedVertex
let topRight: TexturedVertex
}

这是一个相当简单的结构，但是我不知道如何将它传递给 glVertexAttribPointer 的最后一个参数。任何帮助都会很棒！

It’s a fairly straightforward structure, but I don’t know how to pass it to the last argument of glVertexAttribPointer. Any help would be stellar!

推荐答案

在Swift中，你可以使用泛型struct UnsafePointer 执行指针算术和转换。 Swift没有 offsetof ，但你可以通过 UnsafePointer 的<$ c $来干净地解决这个问题。 c> geometryVertex 和 textureVertex 元素直接。

In Swift, you can use the generic struct UnsafePointer to perform pointer arithmetic and casting. Swift doesn't have offsetof, but you can work around that cleanly by taking UnsafePointers of the geometryVertex and textureVertex elements directly.

以下代码在iOS中编译操场。我没有测试过它，但我认为它会起作用：

The following code compiles in an iOS playground. I haven't tested it beyond that but I think it'll work:

import OpenGLES
import GLKit

struct TexturedVertex {
    var geometryVertex = GLKVector2()
    var textureVertex = GLKVector2()
}

struct TexturedQuad {
    var bl = TexturedVertex()
    var br = TexturedVertex()
    var tl = TexturedVertex()
    var tr = TexturedVertex()
    init() { }
}

var _quad = TexturedQuad()
withUnsafePointer(&_quad.bl.geometryVertex) { (pointer) -> Void in
    glVertexAttribPointer(GLuint(GLKVertexAttrib.Position.rawValue),
        2, GLenum(GL_FLOAT), GLboolean(GL_FALSE),
        GLsizei(sizeof(TexturedVertex)), pointer)
}
withUnsafePointer(&_quad.bl.textureVertex) { (pointer) -> Void in
    glVertexAttribPointer(GLuint(GLKVertexAttrib.TexCoord0.rawValue),
        2, GLenum(GL_FLOAT), GLboolean(GL_FALSE),
        GLsizei(sizeof(TexturedVertex)), pointer)
}

顺便说一句，使用 CGPoint 你在问题中的表现是危险的，因为 CGFloat 根据你的目标（32位或64位）改变大小，但是 GL_FLOAT 总是表示32位。您关注的教程是在64位iOS出现之前编写的。

By the way, using CGPoint the way you did in your question is dangerous, because CGFloat changes size depending on your target (32-bit or 64-bit), but GL_FLOAT always means 32-bit. The tutorial you're following was written before 64-bit iOS came out.

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