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视频编解码

问题​

iOS 硬件编解码的原理和使用方式？

答案​

编解码概念​

原始视频帧 (YUV/RGB) → 编码器 (H.264/HEVC) → 压缩数据 → 解码器 → 原始帧

VideoToolbox 硬编码​

import VideoToolbox

class H264Encoder {
    private var session: VTCompressionSession?

    func setup(width: Int32, height: Int32) {
        VTCompressionSessionCreate(
            allocator: kCFAllocatorDefault,
            width: width,
            height: height,
            codecType: kCMVideoCodecType_H264,
            encoderSpecification: nil,
            imageBufferAttributes: nil,
            compressedDataAllocator: nil,
            outputCallback: encoderCallback,
            refcon: Unmanaged.passUnretained(self).toOpaque(),
            compressionSessionOut: &session
        )

        // 设置编码参数
        guard let session = session else { return }
        VTSessionSetProperty(session, key: kVTCompressionPropertyKey_RealTime, value: kCFBooleanTrue)
        VTSessionSetProperty(session, key: kVTCompressionPropertyKey_ProfileLevel,
                           value: kVTProfileLevel_H264_High_AutoLevel)
        VTCompressionSessionPrepareToEncodeFrames(session)
    }

    func encode(sampleBuffer: CMSampleBuffer) {
        guard let session = session,
              let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else { return }
        let pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
        VTCompressionSessionEncodeFrame(session, imageBuffer: pixelBuffer,
                                        presentationTimeStamp: pts, duration: .invalid,
                                        frameProperties: nil, sourceFrameRefcon: nil, infoFlagsOut: nil)
    }
}

编码格式对比​

格式压缩率兼容性iOS 支持
H.264中最广iOS 4+
H.265 (HEVC)高（比 H.264 节省 50%）较新设备iOS 11+

常见面试问题​

Q1: 软编码和硬编码的区别？​

答案：

  • 软编码：用 CPU 运算（如 FFmpeg x264），兼容性好但耗电、发热
  • 硬编码：用 GPU/专用芯片（VideoToolbox），功耗低、速度快，iOS 推荐

相关链接​