使用带有alpha组件的glReadPixels（OpenGL）获取UIImage

Question

我尝试使用以下代码从OpenGL保存位图图像（UIImage）：

func savePixels(x: Int, y: Int, w: Int, h: Int) -> UIImage {
    let bitsPerComponent:Int = 8
    let bitsPerPixel:Int = 32
    let byteLength = Int(w * h) * 4;

    let bytes  = UnsafeMutablePointer<GLubyte>.allocate(capacity: byteLength)
    bytes.initialize(to: GLubyte())

    glReadPixels(0, 0, GLsizei(w), GLsizei(h), GLenum(GL_RGBA), GLenum(GL_UNSIGNED_BYTE), bytes)

    for i in 0 ..< Int(w * h) {
        let r = bytes[4 * i + 0]
        let g = bytes[4 * i + 1]
        let b = bytes[4 * i + 2]
        let a = bytes[4 * i + 3]

        bytes[4 * i    ] = b
        bytes[4 * i + 1] = g
        bytes[4 * i + 2] = r
        bytes[4 * i + 3] = a
    }

    let dataProvider = CGDataProvider(dataInfo: nil, data: bytes, size: byteLength, releaseData: { (info, data, size) -> Void in
        //data.deallocate(bytes: size, alignedTo: MemoryLayout<GLubyte>.alignment)
    })

    let colorspace = CGColorSpaceCreateDeviceRGB()
    let bitmapInfo = CGBitmapInfo(rawValue: CGBitmapInfo.byteOrder32Big.rawValue | CGImageAlphaInfo.first.rawValue)
    let aCGImage = CGImage(
        width: Int(w),
        height: Int(h),
        bitsPerComponent: bitsPerComponent,
        bitsPerPixel: bitsPerPixel,
        bytesPerRow: Int(w) * 4,
        space: colorspace,
        bitmapInfo: bitmapInfo,
        provider: dataProvider!,
        decode: nil,
        shouldInterpolate: false,
        intent: .defaultIntent
        )!

    return UIImage.init(cgImage: aCGImage, scale: 1, orientation: .up)
}

，但是我得到的图像带有白色背景，但是我想得到具有透明背景的图像！我该怎么办？请帮助！