What changes were made to the VideoToolbox HEVC encoder in iOS 26?

Because I want to control the grid size and number of HEIC images myself, I decided to perform HEVC encoding manually and then generate the HEIC image. Previously, I used VTCompressionSession to accomplish this task, and the results were satisfactory. It worked perfectly on iOS 16 through iOS 18 — in other words, it was able to generate correct HEVC encoding, and its CMFormatDescription should also have been correct, since I relied on it to generate the decoderConfig; otherwise, the final image would have decoding issues.

However, it can no longer generate a valid HEIC image on a physical device running iOS 26. Interestingly, it still works fine on the iOS 26 simulator — it only fails on real hardware. The abnormal result is that the image becomes completely black, although the image dimensions are still correct.

After my troubleshooting, I suspect that the encoding behavior of VTCompressionSession has been modified on iOS 26, which causes the final hvc1 encoding I pass in to be incorrect.

I created a VTCompressionSession using the following configuration.

var newSession: VTCompressionSession!
var status = VTCompressionSessionCreate(
    allocator: kCFAllocatorDefault,
    width: Int32(frameSize.width),
    height: Int32(frameSize.height),
    codecType: kCMVideoCodecType_HEVC,
    encoderSpecification: nil,
    imageBufferAttributes: nil,
    compressedDataAllocator: nil,
    outputCallback: nil,
    refcon: nil,
    compressionSessionOut: &newSession
)

try check(status, VideoToolboxErrorDomain)

let properties: [CFString: Any] = [
    kVTCompressionPropertyKey_AllowFrameReordering: false,
    kVTCompressionPropertyKey_AllowTemporalCompression: false,
    kVTCompressionPropertyKey_RealTime: false,
    kVTCompressionPropertyKey_MaximizePowerEfficiency: false,
    kVTCompressionPropertyKey_ProfileLevel: profileLevel,
    kVTCompressionPropertyKey_Quality: quality.rawValue,
]

status = VTSessionSetProperties(newSession, propertyDictionary: properties as CFDictionary)

try check(status, VideoToolboxErrorDomain) {
    VTCompressionSessionInvalidate(newSession)
}

Then use the following code to encode each Grid of the image.

let status = VTCompressionSessionEncodeFrame(
    session,
    imageBuffer: buffer,
    presentationTimeStamp: presentationTimeStamp,
    duration: frameDuration,
    frameProperties: nil,
    infoFlagsOut: nil) { [weak self] status, _, sampleBuffer in
        try check(status, VideoToolboxErrorDomain)

        if let sampleBuffer {
            let encodedImage = try self.encodedImage(from: sampleBuffer)

            // handle encodedImage
        }
    }

try check(status, VideoToolboxErrorDomain)

If I try to display this abnormal image in the App, my console outputs the following error, so it can be inferred that the issue probably occurred during decoding.

createImageBlock:3029: *** ERROR: CGImageBlockCreate {0, 0, 2316, 6176} - data is NULL
callDecodeImage:2411: *** ERROR: decodeImageImp failed - NULL _blockArray
createImageBlock:3029: *** ERROR: CGImageBlockCreate {0, 0, 2316, 6176} - data is NULL
callDecodeImage:2411: *** ERROR: decodeImageImp failed - NULL _blockArray
createImageBlock:3029: *** ERROR: CGImageBlockCreate {0, 0, 2316, 6176} - data is NULL
callDecodeImage:2411: *** ERROR: decodeImageImp failed - NULL _blockArray

It needs to be emphasized again that this code used to work fine in the past, and the issue only occurs on an iOS 26 physical device. I noticed that iOS 26 has introduced many new properties, but I’m not sure whether some of these new properties must be set in the new system, and there’s no information about this in the official documentation.