Set partial pixel to be transparent in video

[Sean]

Hi, 

I wrote to customized element (myelement) to set partial pixels of one video
frame to be transparent. My machine is little endian. 


This is myelement implementation, I want to make the upper half of 640x480
(i.e. the upper 640x240 pixels ) invisible by setting the Alpha value to be
0x0

    GstMapInfo info;
    if (!gst_buffer_map(buffer, &info, GST_MAP_READ)) {
      return GST_FLOW_ERROR;
    }
    for (int i  = 0; i < 240; i++) {
      for (int j = 0; j < 640; j++)
      info.data[(i*640+j)*4] = 0x0;
    }
    GST_FLOW_OK;


This is my test command: 
gst_launch -v videotestsrc ! video/x-raw,format=RGBA,width=640, height=480 !
videoconvert ! myelement ! x264enc ! mpegtsmux ! filesink location=test.ts

 I expect the upper half of the test.ts should be transparent. However, the
result is (row,col)=(0,4), (0,8) (0,12)...... become a GREEN pixel. Would
anyone help to comment which part is wrong? 

Thanks a lot.



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