Copy a CVPixelBuffer on any iOS device - ios

Question

I'm having a great deal of difficulty coming up with code that reliably copies a CVPixelBuffer on any iOS device. My first attempt worked fine until I tried it on an iPad Pro:
extension CVPixelBuffer {
func deepcopy() -> CVPixelBuffer? {
let width = CVPixelBufferGetWidth(self)
let height = CVPixelBufferGetHeight(self)
let format = CVPixelBufferGetPixelFormatType(self)
var pixelBufferCopyOptional:CVPixelBuffer?
CVPixelBufferCreate(nil, width, height, format, nil, &pixelBufferCopyOptional)
if let pixelBufferCopy = pixelBufferCopyOptional {
CVPixelBufferLockBaseAddress(self, kCVPixelBufferLock_ReadOnly)
CVPixelBufferLockBaseAddress(pixelBufferCopy, 0)
let baseAddress = CVPixelBufferGetBaseAddress(self)
let dataSize = CVPixelBufferGetDataSize(self)
let target = CVPixelBufferGetBaseAddress(pixelBufferCopy)
memcpy(target, baseAddress, dataSize)
CVPixelBufferUnlockBaseAddress(pixelBufferCopy, 0)
CVPixelBufferUnlockBaseAddress(self, kCVPixelBufferLock_ReadOnly)
}
return pixelBufferCopyOptional
}
}
The above crashes on an iPad Pro because CVPixelBufferGetDataSize(self) is slightly larger than CVPixelBufferGetDataSize(pixelBufferCopy), so the memcpy writes to unallocated memory.
So I gave up with that and tried this:
func copy() -> CVPixelBuffer?
{
precondition(CFGetTypeID(self) == CVPixelBufferGetTypeID(), "copy() cannot be called on a non-CVPixelBuffer")
var _copy: CVPixelBuffer?
CVPixelBufferCreate(
nil,
CVPixelBufferGetWidth(self),
CVPixelBufferGetHeight(self),
CVPixelBufferGetPixelFormatType(self),
CVBufferGetAttachments(self, .shouldPropagate),
&_copy)
guard let copy = _copy else { return nil }
CVPixelBufferLockBaseAddress(self, .readOnly)
CVPixelBufferLockBaseAddress(copy, [])
defer
{
CVPixelBufferUnlockBaseAddress(copy, [])
CVPixelBufferUnlockBaseAddress(self, .readOnly)
}
for plane in 0 ..< CVPixelBufferGetPlaneCount(self)
{
let dest = CVPixelBufferGetBaseAddressOfPlane(copy, plane)
let source = CVPixelBufferGetBaseAddressOfPlane(self, plane)
let height = CVPixelBufferGetHeightOfPlane(self, plane)
let bytesPerRow = CVPixelBufferGetBytesPerRowOfPlane(self, plane)
memcpy(dest, source, height * bytesPerRow)
}
return copy
}
That works on both my test devices, but it's just reached actual customers and it turns out it crashes on the iPad 6 (and only that device so far). It's an EXC_BAD_ACCESS on the call to memcpy() again.
Seems crazy that there isn't a simple API call for this given how hard it seems to be to make it work reliably. Or am I make it harder than it needs to be? Thanks for any advice!

Answer 1

This questions and answer combo is solid gold. Let me add value with a slight refactor and some control flow to account for CVPixelBuffers that do not have planes.
public extension CVPixelBuffer {
func copy() throws -> CVPixelBuffer {
precondition(CFGetTypeID(self) == CVPixelBufferGetTypeID(), "copy() cannot be called on a non-CVPixelBuffer")
var _copy: CVPixelBuffer?
let width = CVPixelBufferGetWidth(self)
let height = CVPixelBufferGetHeight(self)
let formatType = CVPixelBufferGetPixelFormatType(self)
let attachments = CVBufferGetAttachments(self, .shouldPropagate)
CVPixelBufferCreate(nil, width, height, formatType, attachments, &_copy)
guard let copy = _copy else {
throw PixelBufferCopyError.allocationFailed
}
CVPixelBufferLockBaseAddress(self, .readOnly)
CVPixelBufferLockBaseAddress(copy, [])
defer {
CVPixelBufferUnlockBaseAddress(copy, [])
CVPixelBufferUnlockBaseAddress(self, .readOnly)
}
let pixelBufferPlaneCount: Int = CVPixelBufferGetPlaneCount(self)
if pixelBufferPlaneCount == 0 {
let dest = CVPixelBufferGetBaseAddress(copy)
let source = CVPixelBufferGetBaseAddress(self)
let height = CVPixelBufferGetHeight(self)
let bytesPerRowSrc = CVPixelBufferGetBytesPerRow(self)
let bytesPerRowDest = CVPixelBufferGetBytesPerRow(copy)
if bytesPerRowSrc == bytesPerRowDest {
memcpy(dest, source, height * bytesPerRowSrc)
}else {
var startOfRowSrc = source
var startOfRowDest = dest
for _ in 0..<height {
memcpy(startOfRowDest, startOfRowSrc, min(bytesPerRowSrc, bytesPerRowDest))
startOfRowSrc = startOfRowSrc?.advanced(by: bytesPerRowSrc)
startOfRowDest = startOfRowDest?.advanced(by: bytesPerRowDest)
}
}
}else {
for plane in 0 ..< pixelBufferPlaneCount {
let dest = CVPixelBufferGetBaseAddressOfPlane(copy, plane)
let source = CVPixelBufferGetBaseAddressOfPlane(self, plane)
let height = CVPixelBufferGetHeightOfPlane(self, plane)
let bytesPerRowSrc = CVPixelBufferGetBytesPerRowOfPlane(self, plane)
let bytesPerRowDest = CVPixelBufferGetBytesPerRowOfPlane(copy, plane)
if bytesPerRowSrc == bytesPerRowDest {
memcpy(dest, source, height * bytesPerRowSrc)
}else {
var startOfRowSrc = source
var startOfRowDest = dest
for _ in 0..<height {
memcpy(startOfRowDest, startOfRowSrc, min(bytesPerRowSrc, bytesPerRowDest))
startOfRowSrc = startOfRowSrc?.advanced(by: bytesPerRowSrc)
startOfRowDest = startOfRowDest?.advanced(by: bytesPerRowDest)
}
}
}
}
return copy
}
}
Valid with Swift 5. To provide a little more background... There are many formats that AVCaptureVideoDataOutput .videoSettings property can take. Not all of them have planes especially ones that ML Models might need.

Answer 2

The second implementation looks quite solid. The only problem I can imagine is that a plane in the new pixel buffer is allocated with a different stride length (bytes per row). The stride length is based on width × (bytes per pixel) and then rounded up in an unspecified way to achieve optimal memory access.
So check if:
CVPixelBufferGetBytesPerRowOfPlane(self, plane) == CVPixelBufferGetBytesPerRowOfPlane(copy, plane
If not, copy the pixel plane row by row:
for plane in 0 ..< CVPixelBufferGetPlaneCount(self)
{
let dest = CVPixelBufferGetBaseAddressOfPlane(copy, plane)
let source = CVPixelBufferGetBaseAddressOfPlane(self, plane)
let height = CVPixelBufferGetHeightOfPlane(self, plane)
let bytesPerRowSrc = CVPixelBufferGetBytesPerRowOfPlane(self, plane)
let bytesPerRowDest = CVPixelBufferGetBytesPerRowOfPlane(copy, plane)
if bytesPerRowSrc == bytesPerRowDest {
memcpy(dest, source, height * bytesPerRowSrc)
} else {
var startOfRowSrc = source
var startOfRowDest = dest
for _ in 0..<height {
memcpy(startOfRowDest, startOfRowSrc, min(bytesPerRowSrc, bytesPerRowDest))
startOfRowSrc += bytesPerRowSrc
startOfRowDest += bytesPerRowDest
}
}
}