14.3 Video processing part 1: introductory exercise  (Page 3/6)

In this lab, you will modify a component to implement the flipping and inverting algorithm. And you will perform some simple auto-contrasting as well as work with color.

In addition, the version of Code Composer that the IDK uses is different from the one you have used previously. The IDK uses Code Composer Studio v2.1. It is similar to the other version, but the process of loading code is slightly different.

Code description

Overview and i/o

The next few sections describe the code used. First please copy the files needed by following the instructions in the "Part 1" section of this document. This will help you easily follow the next few parts.

The program flow for image processing applications may be a bit different from your previous experiences in C programming. In most C programs, the main function is where program execution starts and ends. In this real-time application, the main function serves only to setup initializations for the cache, the CSL, and the DMA (memory access) channel. When it exits, the main task, tskMainFunc(), will execute automatically, starting the DSP/BIOS. It will loop continuously calling functions to operate on new frames and this is where our image processing application begins.

The tskMainFunc(), in main.c, opens the handles to the board for image capture (VCAP_open()) and to the display (VCAP_open()) and calls the grayscale function. Here, several data structures are instantiated that are defined in the file img_proc.h. The IMAGE structures will point to the data that is captured by the FPGA and the data that will be output to the display. The SCRATCH_PAD structure points to our internal and external memory buffers used for temporary storage during processing. LPF_PARAMS is used to store filter coefficients for the low pass filter.

The call to img_proc() takes us to the file img_proc.c. First, several variables are declared and defined. The variable quadrant will denote on which quadrant of the screen we currently want output; out_ptr will point to the current output spot in the output image; and pitch refers to the byte offset (distance) between two lines. This function is the high level control for our image-processing algorithm. See algorithm flow.

The first function called is the pre_scale_image function in the file pre_scale_image.c. The purpose of this function is to take the 640x480 image and scale it down to a quarter of its size by first downsampling the input rows by two and then averaging every two pixels horizontally. The internal and external memory spaces, pointers to which are in the scratch pad, are used for this task. The vertical downsampling occurs when every other line is read into the internal memory from the input image. Within internal memory, we will operate on two lines of data (640 columns/line) at a time, averaging every two pixels (horizontal neighbors) and producing two lines of output (320 columns/line) that are stored in the external memory.

To accomplish this, we will need to take advantage of the IDM by initializing the input and output streams. At the start of the function, two instantiations of a new structure dstr_t are declared. You can view the structure contents of dstr_t on p. 2-11 of the IDK Programmer's Guide. These structures are stream "objects". They give us access to the data when using the dstr_open() command. In this case dstr_i is an input stream as specified in the really long command dstr_open(). Thus after opening this stream we can use the get_data command to get data one line at a time. Streams and memory usage are described in greater detail in the second project lab. This data flow for the pre-scale is shown in data flow.