如何調整Videotoolbox的屬性？

Question

我現在使用Videotoolbox來處理h.264編碼。

而且我發現了一個示例代碼，它工作正常：

#define VTB_HEIGHT 480 
    #define VTB_WIDTH 640 

    int bitRate = VTB_WIDTH * VTB_HEIGHT * 3 * 4 * 8; 

    CFNumberRef bitRateRef = CFNumberCreate(kCFAllocatorDefault, 
              kCFNumberSInt32Type, 
              &bitRate); 

    VTSessionSetProperty(encodingSession, 
         kVTCompressionPropertyKey_AverageBitRate, 
         bitRateRef); 

    CFRelease(bitRateRef); 


    int bitRateLimit = bitRate/8; 

    CFNumberRef bitRateLimitRef = CFNumberCreate(kCFAllocatorDefault, 
               kCFNumberSInt32Type, 
               &bitRateLimit); 

    VTSessionSetProperty(encodingSession, 
         kVTCompressionPropertyKey_DataRateLimits, 
         bitRateLimitRef); 

    CFRelease(bitRateLimitRef); 

---

但是這兩條線，我不明白：

int bitRate = VTB_WIDTH * VTB_HEIGHT * 3 * 4 * 8; 

int bitRateLimit = bitRate/8; 

什麼是使用它們的正確方法？

希望有人能告訴我。

謝謝你的時間！

Answer 1

從kvtcompressionpropertykey_dataratelimits文件說：

每個硬性限制由字節的數據大小，以秒爲 持續時間描述...

因此，你需要設置這個屬性2參數（以字節爲單位的數據大小，持續時間以秒爲單位）

int bitRate = VTB_WIDTH * VTB_HEIGHT * 3 * 4 * 8; 
int bitRateLimit = bitRate/8; 

// that's say we set data in byte/second 
CFNumberRef byteNum = CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &bitRateLimit); 
int second = 1; 
CFNumberRef secNum = CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &second); 

// add parameters into a array 
const void* numbers[2] = {byteNum, secNum}; 
CFArrayRef dataRateLimits = CFArrayCreate(NULL, numbers, 2, &kCFTypeArrayCallBacks); 

// then set property with array 
status = VTSessionSetProperty(compressionSession, kVTCompressionPropertyKey_DataRateLimits, arrayValues);