How to directly rotate CVImageBuffer image in IOS 4 without converting to UIImage?

Question

I am using OpenCV 2.2 on the iPhone to detect faces. I\'m using the IOS 4\'s AVCaptureSession to get access to the camera stream, as seen in the code that follows.

Answer 1

I know this is quite old question, but I've been solving similar problem recently and maybe someone can find my solution useful.

I needed to extract raw image data from image buffer of YCbCr format delivered by iPhone camera (got from [AVCaptureVideoDataOutput.availableVideoCVPixelFormatTypes firstObject]), dropping information such as headers, meta information etc to pass it to further processing.

Also, I needed to extract only small area in the center of captured video frame, so some cropping was needed.

My conditions allowed capturing video only in either landscape orientation, but when a device is positioned in landscape left orientation, image is delivered turned upside down, so I needed to flip it in both axis. In case the image is flipped, my idea was to copy data from the source image buffer in reverse order and reverse bytes in each row of read data to flip image in both axis. That idea really works, and as I needed to copy data from source buffer anyway, it seems there's not much performance penalty if reading from the start or the end (Of course, bigger image = longer processing, but I deal with really small numbers).

I'd like to know what others think about this solution and of course some hints how to improve the code:

/// Lock pixel buffer
CVPixelBufferLockBaseAddress(imageBuffer, 0);

/// Address where image buffer starts
uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer);

/// Read image parameters
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);

/// See whether image is flipped upside down
BOOL isFlipped = (_previewLayer.connection.videoOrientation == AVCaptureVideoOrientationLandscapeLeft);

/// Calculate cropping frame. Crop to scanAreaSize (defined as CGSize constant elsewhere) from the center of an image
CGRect cropFrame = CGRectZero;
cropFrame.size = scanAreaSize;
cropFrame.origin.x = (width / 2.0f) - (scanAreaSize.width / 2.0f);
cropFrame.origin.y = (height / 2.0f) - (scanAreaSize.height / 2.0f);

/// Update proportions to cropped size
width = (size_t)cropFrame.size.width;
height = (size_t)cropFrame.size.height;

/// Allocate memory for output image data. W*H for Y component, W*H/2 for CbCr component
size_t bytes = width * height + (width * height / 2);

uint8_t *outputDataBaseAddress = (uint8_t *)malloc(bytes);

if(outputDataBaseAddress == NULL) {

    /// Memory allocation failed, unlock buffer and give up
    CVPixelBufferUnlockBaseAddress(imageBuffer, 0);

    return NULL;
}

/// Get parameters of YCbCr pixel format
CVPlanarPixelBufferInfo_YCbCrBiPlanar *bufferInfo = (CVPlanarPixelBufferInfo_YCbCrBiPlanar *)baseAddress;

NSUInteger bytesPerRowY = EndianU32_BtoN(bufferInfo->componentInfoY.rowBytes);
NSUInteger offsetY = EndianU32_BtoN(bufferInfo->componentInfoY.offset);

NSUInteger bytesPerRowCbCr = EndianU32_BtoN(bufferInfo->componentInfoCbCr.rowBytes);
NSUInteger offsetCbCr = EndianU32_BtoN(bufferInfo->componentInfoCbCr.offset);

/// Copy image data only, skipping headers and metadata. Create single buffer which will contain Y component data
/// followed by CbCr component data.

/// Process Y component
/// Pointer to the source buffer
uint8_t *src;

/// Pointer to the destination buffer
uint8_t *destAddress;

/// Calculate crop rect offset. Crop offset is number of rows (y * bytesPerRow) + x offset.
/// If image is flipped, then read buffer from the end to flip image vertically. End address is height-1!
int flipOffset = (isFlipped) ? (int)((height - 1) * bytesPerRowY) : 0;

int cropOffset = (int)((cropFrame.origin.y * bytesPerRowY) + flipOffset + cropFrame.origin.x);

/// Set source pointer to Y component buffer start address plus crop rect offset
src = baseAddress + offsetY + cropOffset;

for(int y = 0; y < height; y++) {

    /// Copy one row of pixel data from source into the output buffer.
    destAddress = (outputDataBaseAddress + y * width);

    memcpy(destAddress, src, width);

    if(isFlipped) {

        /// Reverse bytes in row to flip image horizontally
        [self reverseBytes:destAddress bytesSize:(int)width];

        /// Move one row up
        src -= bytesPerRowY;
    }
    else {

        /// Move to the next row
        src += bytesPerRowY;
    }
}

/// Calculate crop offset for CbCr component
flipOffset = (isFlipped) ? (int)(((height - 1) / 2) * bytesPerRowCbCr) : 0;
cropOffset = (int)((cropFrame.origin.y * bytesPerRowCbCr) + flipOffset + cropFrame.origin.x);

/// Set source pointer to the CbCr component offset + crop offset
src = (baseAddress + offsetCbCr + cropOffset);

for(int y = 0; y < (height / 2); y++) {

    /// Copy one row of pixel data from source into the output buffer.
    destAddress = (outputDataBaseAddress + (width * height) + y * width);

    memcpy(destAddress, src, width);

    if(isFlipped) {

        /// Reverse bytes in row to flip image horizontally
        [self reverseBytes:destAddress bytesSize:(int)width];

        /// Move one row up
        src -= bytesPerRowCbCr;
    }
    else {

        src += bytesPerRowCbCr;
    }
}

/// Unlock pixel buffer
CVPixelBufferUnlockBaseAddress(imageBuffer, 0);

/// Continue with image data in outputDataBaseAddress;