14.2 Video processing part 1: introductory exercise  (Page 4/6)

To give you a better understanding of how these streams are created, let's analyze the parameters passed in the first call to dstr_open() which opens an input stream.

External address: in_image->data This is a pointer to the place in external memory serving as the source of our input data (it's the source because the last function parameter is set to DSTR_INPUT). We're going to bring in data from external to internal memory so that we can work on it. This external data represents a frame of camera input. It was captured in the main function using the VCAP_getframe() command.

External size: (rows + num_lines) * cols = (240 + 2) * 640 This is the total size of the input data which we will bring in. We will only be taking two lines at a time from in_image->data, so only 240 rows. The "plus 2" represents two extra rows of input data which represent a buffer of two lines - used when filtering, which is explained later.

Internal address: int_mem This is a pointer to an 8x640 array, pointed to by scratchpad->int_data. This is where we will be putting the data on each call to dstr_get(). We only need part of it, as seen in the next parameter, as space to bring in data.

Internal size: 2 * num_lines * cols = 2 * 2 * 640 The size of space available for data to be input into int_mem from in_image->data. We pull in two lines of the input frame so it num_lines * cols. We have the multiply by 2 as we are using double buffering for bringing in the data. We need double the space in internal memory than the minimum needed, the reason is fully explained in IDK Programmer's Guide.

Number of bytes/line: cols = 640, Number of lines: num_lines = 2 Each time dstr_get_2D() is called, it will return a pointer to 2 new lines of data, 640 bytes in length. We use the function dstr_get_2D(), since we are pulling in two lines of data. If instead we were only bringing in one line, we would use dstr_get() statements.

External memory increment/line: stride*cols = 1*640 The IDM increments the pointer to the external memory by this amount after each dstr_get() call.

Window size: 1 for double buffered single line of data (Look at the three documentation pdfs for a full explanation of double buffering)The need for the window size is not really apparent here. It will become apparent when we do the 3x3 block convolution. Then, the window size will beset to 3 (indicating three lines of buffered data). This tells the IDM to send a pointer to extract 3 lines of data when dstr_get() is called, but only increment the stream's internal pointer by 1 (instead of 3) the next time dstr_get() is called. Thus you will get overlapping sets of 3 lines on each dstr_get() call. This is not a useful parameter when setting up an output stream.

Direction of input: DSTR_INPUT Sets the direction of data flow. If it had been set to DSTR_OUTPUT (as done in the next call to dstr_open()), we would be setting the data to flow from the Internal Address to the External Address.

We then setup our output stream to write data to a location in external memory which we had previously created.