14.4 Video processing part 3: memory management  (Page 2/4)

Now have a look at the main.c file and take note of the memory spaces used. The internal memory is of size 12 * 640. This single memory space is going to be used to store both the input lines from the camera image and also the results of the color conversion, thus explaining its large size. Basically the internal memory is partitioned by us for different buffers. The output data buffer needs only 4*640 bytes thus it's space starts at

int_mem + (8 * cols); //cols = 640

and ends at 12*cols – which gives us 4*cols of space. Though it is useful to partition internal memory in such a way, it is recommended not to. It is very easy to mess up the other data too, so simple, so our solution would have been to create a separate memory space of size 4*cols.

The external memory, though declared here, will not be used in the program, however you may need to allocate some external memory for this project lab assignment.

The input and output buffers and main.c details

Good examples of the external memory use are the input buffer (captured image) and output buffer (to be placed onto the screen). There are a few steps in obtaining these buffers:

• 1. First, we open the capture and display devices in tskMainFunc() using VDIS_open(); VCAP_open();
• 2. If the open calls are successful, we then call the color function to process the video feed using color(VCAP_NTSC, VDIS_640X480X16, numFrames); This specifies:
  • the capture image format – NTSC
  • display image format and size
  • numFrames to run the system for – in our case one day to be passed on to the color function. Please note, we merely specify the formats but do not configure the system to use these formats, yet.
  We then move on to the color(…) function within main.c
• 3. First we declare some useful pointers which we will use for the various images and their components and so forth. The IMAGE structure holds a pointer to the image array (img_data). In addition, it holds integers for the number of image rows (img_rows) and number of image columns (img_cols).(Implementation Details in img_proc.h) Declare more of these structures as needed for any memory spaces you create yourself. Furthermore, “scratch_pad” structures hold information about the location and size of internal and external memories. This is another use of pointers being used to hold the locations of the different memory spaces. (Implementation Details in img_proc.h)We also configure the display and capture formats using VDIS_config(displayMode); VCAP_config(captureMode);
• Following this we enter the loop : for (frameCnt=0; frameCnt<numFrames; frameCnt++) This loop iterates for a set number of frames and processes them one at a time. And the lines following this : input = VCAP_getFrame(SYS_FOREVER); output = (Uint16*)VDIS_toggleBuffs(0); are used to obtain the capture and output frames. After this statement, ‘input’ will hold a pointer to external memory where the captured frame is stored. The ‘input’ pointer holds pointers ‘y1’, ‘c1’ etc to the different color component of the image. These color components are in external memory as well. And ‘output’ will hold a pointer to a buffer in external memory, to which we will write whatever we need to output to the screen. Basically the buffer is the size of the output frame (640 X 480 X 2 bytes/pixel), and we can write what we wish to it. And, the next time togglebufs(0) is called, everything we placed in that buffer will be put on the screen. And a new buffer will be allocated, the pointer ‘output’ will be updated and we can now write to the next frame.The next line out_image.img_data = (unsigned char *) output; updates the pointers we had setup. We then move on to the color_convert(..) routine. We pass the memory pointers we had created so that our color_conv program can process the input frame we obtained.In color_conv, we begin by setting up streams to bring in data and streams to send out data. After that we begin the color-space conversion.