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I am receiving a video frame in a GstVideoFrame structure of a gstreamer element which is written in C++.

The frame is in YUV420 NV12 format

In this gstreamer element, I am trying to copy y-frame and uv-frame in separate buffers.

According to videolan.org, the YUV420 NV12 data is stored as following in the incoming frame buffer : (info copied from website)

NV12:

Related to I420, NV12 has one luma "luminance" plane Y and one plane with U and V values interleaved.

In NV12, chroma planes (blue and red) are subsampled in both the horizontal and vertical dimensions by a factor of 2.

For a 2×2 group of pixels, you have 4 Y samples and 1 U and 1 V sample.

It can be helpful to think of NV12 as I420 with the U and V planes interleaved.

Here is a graphical representation of NV12. Each letter represents one bit:

    For 1 NV12 pixel: YYYYYYYY UVUV
    For a 2-pixel NV12 frame: YYYYYYYYYYYYYYYY UVUVUVUV
    For a 50-pixel NV12 frame: Y×8×50 (UV)×2×50
    For a n-pixel NV12 frame: Y×8×n (UV)×2×n

but I cant seem to calculate the offset of y-data and uv-data in the buffer.


Update_1 : I have height and width of the frame to calculate the size of y-data and uv-data as: 
y_size = width * height;
uv_size = y_size / 2;

any help or comments regarding this will be appreciated.

thanks

Akkshay
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  • NV12 is coplanar, use `GST_VIDEO_FRAME_PLANE_OFFSET` to get the offset to each plane. – Ext3h Aug 04 '20 at 05:08
  • thanks for the suggestion. wouldn't this just give me an offset to the whole plane containing y-data and uv-data? how to separate this data from the plane? It would be great if you can point me towards some example.. :) – Akkshay Aug 04 '20 at 22:36
  • The other macros tell you the precise data layout within each plane. Together that's enough to copy the data out of the decoded buffer into your own data structure. No example at hand, sorry, but you are already half there. – Ext3h Aug 05 '20 at 03:07
  • @Ext3h Yes, that actually helped. Thanks very much for pointing in correct direction :) – Akkshay Aug 13 '20 at 01:59

1 Answers1

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Thanks to @Ext3h, this is how I was able to separate y-data and uv-data from incoming YUV frame.

y_data = (u8 *)GST_VIDEO_FRAME_PLANE_DATA (in_frame, 0);
uv_data =(u8 *)GST_VIDEO_FRAME_PLANE_DATA (in_frame, 1);
Akkshay
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