Get RGB values from AVPicture and change to grey-scale in FFMPEG - c++

Question

The main motive of my code is to change the RGB values from the AVPicture in FFMPEG.
I have been able to get the image data "data[0]" by following the article : http://blog.tomaka17.com/2012/03/libavcodeclibavformat-tutorial/
I would like to know that how can I access the 3 bytes of pic.data[0] which is in RGB format. I have been trying to access the pic.data[i][j] via for-loop in 2D matrix fashion but jth element>3.
Any guidance in this regard will be helpful.
Code is here :
AVPicture pic;
avpicture_alloc(&pic, PIX_FMT_RGB24, mpAVFrameInput->width,mpAVFrameInput->height);
auto ctxt = sws_getContext(mpAVFrameInput->width,mpAVFrameInput->height,static_cast<PixelFormat>(mpAVFrameInput->format),
mpAVFrameInput->width, mpAVFrameInput->height, PIX_FMT_RGB24, SWS_BILINEAR, nullptr, nullptr, nullptr);
if (ctxt == nullptr)
throw std::runtime_error("Error while calling sws_getContext");
sws_scale(ctxt, mpAVFrameInput->data, mpAVFrameInput->linesize, 0, mpAVFrameInput->height, pic.data,
pic.linesize);
for (int i = 0; i < (mpAVFrameInput->height-1); i++) {
for (int j = 0; j < (mpAVFrameInput->width-1); j++) {
printf("\n value: %d",pic.data[0][j]);
}
}
Pseudo code which is in my mind is :
For each pixel in image {
Red = pic.data[i][j].pixel.RED;
Green = pic.data[i][j].pixel.GREEN;
Blue = pic.data[i][j].pixel.BLUE;
GRAY = (Red+Green+Blue)/3;
Red = GRAY;
Green = GRAY;
Blue = GRAY;
Save Frame;}
I am quite new to FFMPEG therefore any guidance and help will be highly appreciable.
Many Thanks

Answer 1

First extract the row data row-by-row of each frame; iterate the loop keeping in view the frame's height.
Here's the sample:
int FrameHeight = FrameInput->height;
int FrameWidth = FrameInput->width;
for(int Counter=0; Counter<FrameHeight; Counter++)
{
int RowSize = FrameWidth*sizeof(uint8_t)*3;
uint8_t* RowData = (uint8_t*) malloc(RowSize);
memset(RowData, 0, RowSize);
memcpy(RowData, AVFrameInput->data[0]+Counter*AVFrameInput->linesize[0], RowSize);
for(int k=0;k<AVFrameInput->linesize[0];++k)
{
if(RowData[k]> 200)
{
RowData[k] = RowData[k]/3;
}
else
{
if(RowData[k] > 150)
{
RowData[k] = RowData[k]/3;
}
else
{
RowData[k] = RowData[k]/3;
}
}
}
memcpy(AVFrameInput->data[0]+Counter*AVFrameInput->linesize[0], RowData, RowSize);
}