Maxi Mus Maxi Mus - 6 months ago 14
iOS Question

Replace Part of Pixel Buffer with White Pixels in iOS

I am using the iPhone camera to capture live video and feeding the pixel buffer to a network that does some object recognition. Here is the relevant code: (I won't post the code for setting up the

AVCaptureSession

etc. as this is pretty standard.)

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

OSType sourcePixelFormat = CVPixelBufferGetPixelFormatType( pixelBuffer );
int doReverseChannels;
if ( kCVPixelFormatType_32ARGB == sourcePixelFormat ) {
doReverseChannels = 1;
} else if ( kCVPixelFormatType_32BGRA == sourcePixelFormat ) {
doReverseChannels = 0;
} else {
assert(false);
}

const int sourceRowBytes = (int)CVPixelBufferGetBytesPerRow( pixelBuffer );
const int width = (int)CVPixelBufferGetWidth( pixelBuffer );
const int fullHeight = (int)CVPixelBufferGetHeight( pixelBuffer );
CVPixelBufferLockBaseAddress( pixelBuffer, 0 );
unsigned char* sourceBaseAddr = CVPixelBufferGetBaseAddress( pixelBuffer );
int height;
unsigned char* sourceStartAddr;
if (fullHeight <= width) {
height = fullHeight;
sourceStartAddr = sourceBaseAddr;
} else {
height = width;
const int marginY = ((fullHeight - width) / 2);
sourceStartAddr = (sourceBaseAddr + (marginY * sourceRowBytes));
}
}


The network then takes
sourceStartAddr
,
width
,
height
,
sourceRowBytes
&
doReverseChannels
as inputs.

My question is the following: What would be the simplest and/or most efficient way to replace or delete a part of the image data with all white 'pixels'? Is it possible to directly overwrite e portion of the pixel buffer data and if yes how?

I only have a very rudimentary understanding of how this pixel buffer works, so I apologize if I'm missing something very basic here. The question most closely related to mine I found on Stackoverflow was this one, where a
EAGLContext
is used to add text to a video frame. While this would actually work for my objective which only needs this replacement for single images, I assume this step would kill performance if applied to every video frame, and I would like to find out if there is another method. Any help here would be appreciated.

Answer

Here is an easy way to manipulate a CVPixelBufferRef without using other libraries like Core Graphics or OpenGL:

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

    const int kBytesPerPixel = 4;
    CVPixelBufferLockBaseAddress( pixelBuffer, 0 );
    int bufferWidth = (int)CVPixelBufferGetWidth( pixelBuffer );
    int bufferHeight = (int)CVPixelBufferGetHeight( pixelBuffer );
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow( pixelBuffer );
    uint8_t *baseAddress = CVPixelBufferGetBaseAddress( pixelBuffer );

    for ( int row = 0; row < bufferHeight; row++ )
    {
        uint8_t *pixel = baseAddress + row * bytesPerRow;
        for ( int column = 0; column < bufferWidth; column++ )
        {
            if ((row < 100) && (column < 100) {
                pixel[0] = 255; // BGRA, Blue value
                pixel[1] = 255; // Green value
                pixel[2] = 255; // Red value
            }
            pixel += kBytesPerPixel;
        }
    }

    CVPixelBufferUnlockBaseAddress( pixelBuffer, 0 );

    // Do whatever needs to be done with the pixel buffer
}

This overwrites the top left patch of 100 x 100 pixels in the image with white pixels.

I found this solution in this Apple Developer Example called RosyWriter.

Kind of amazed I didn't get any answers here considering how easy this turned out to be. Hope this helps someone.

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