Open Radar

 
Summary:
The H.264 video compressor on iOS devices (silently) ignores properties set with VTSessionSetProperty. The output bitstream seems unaffected and VTSessionCopyProperty copies a NULL value. No error is returned in either case.

Steps to Reproduce:
The attached project contains two targets. The iOS target demonstrates the problem, while the Mac target demonstrates the expected results. [OpenRadar note I will upload the sample project to github and add a link as soon as I get a chance.]

1. Create a compression session:
VTCompressionSessionRef CompressionSessionCreate(CMVideoCodecType codecType)
{
    NSDictionary* encoderSpecification = nil;
    CFDictionaryRef sourceImageBufferAttributes = NULL;
    CMTimebaseRef timebase;
    OSStatus status = CMTimebaseCreateWithMasterClock(kCFAllocatorDefault, CMClockGetHostTimeClock(), &timebase);
    // CIF: 352x288
    int32_t width = 352;
    int32_t height = 288;
    VTCompressionSessionRef compressionSession;
    void *outputCallbackRefCon = NULL;
    status = VTCompressionSessionCreate(kCFAllocatorDefault,
                                        width, height,
                                        codecType,  (__bridge CFDictionaryRef)encoderSpecification,
                                        sourceImageBufferAttributes, NULL,
                                        CompressionOutputTestCallback, outputCallbackRefCon, &compressionSession);
    NSCAssert(status == noErr, @"Created compression session successfully (%d).", status);
    return compressionSession;
}

2. Attempt to set and read back some supported properties:
void H264CompressionPropertiesTest()
{
    OSStatus status;
    
    CMVideoCodecType codecType = kCMVideoCodecType_H264;
    VTCompressionSessionRef compressionSession = CompressionSessionCreate(codecType);
    
    CFDictionaryRef supportedProperties;
    status = VTSessionCopySupportedPropertyDictionary(compressionSession, &supportedProperties);
    NSCAssert(status == noErr, @"Retrieved supported properties successfully.");
    //NSLog(@"Supported properties: %@", supportedProperties);
    
    CFStringRef propertyKey = kVTCompressionPropertyKey_ProfileLevel;
    status = VTSessionSetProperty(compressionSession, propertyKey, kVTProfileLevel_H264_High_AutoLevel);
    NSCAssert(status == noErr, @"Allegedly set property.");
    
    CFTypeRef value = NULL;
    status = VTSessionCopyProperty(compressionSession, propertyKey, kCFAllocatorDefault, &value);
    NSCAssert(status == noErr, @"Allegedly copied property.");
    
    NSLog(@"H264: %@: %@", propertyKey, value);
    
    propertyKey = kVTCompressionPropertyKey_MaxKeyFrameInterval;
    status = VTSessionSetProperty(compressionSession, propertyKey, (__bridge CFNumberRef)@(1.0));
    NSCAssert(status == noErr, @"Allegedly set property.");
    
    value = NULL;
    status = VTSessionCopyProperty(compressionSession, propertyKey, kCFAllocatorDefault, &value);
    NSCAssert(status == noErr, @"Allegedly copied property.");
    
    NSLog(@"H264: %@: %@", propertyKey, value);
}

Expected Results:
The read back values of the properties should match those set:

H263: MaxKeyFrameInterval: 1
H264: ProfileLevel: H264_High_AutoLevel
H264: MaxKeyFrameInterval: 1

Actual Results:
The read back properties for H.264 are null:

H263: MaxKeyFrameInterval: 1
H264: ProfileLevel: (null)
H264: MaxKeyFrameInterval: (null)

Version:
iOS 8.1.2 (12B440), iOS 8.2 (12D5480a)

Notes:


Configuration:
iPhone 6 iOS 8.1.2 (12B440), iPhone 6 iOS 8.2 (12D5480a)