14.4 Video processing part 2: grayscale and color  (Page 6/3)

Introduction

The purpose of this project lab is to introduce how to further manipulate data acquired in grayscale mode and then expand this to the realm of color. This lab is meant as a follow-up to “Video Processing Part 1: Introductory Exercise,”. This lab will implement a grayscale auto-contrast and color image manipulation.

You will complete an introductory exercise to demonstrate your familiarity with the IDK programming environment. You will then complete an introductory exercise in how to use color; and modify a C skeleton to apply simple color masks to video input from the camera.

After this lab, you should be able to effectively and efficiently manipulate grayscale images, as well as modify color images.

You may want to refer to the following TI manuals:

• IDK User's Guide
• IDK Video Device Drivers User's Guide
• IDK Programmer's Guide . Section 2 is very, very important in this lab.

Prelab

Having familiarized yourself with grayscale images in the previous project lab, the first part of the prelab will require you to code a function similar to the flip_invert function you have already designed, while the second part of the prelab will introduce how to use and access color images.

Grayscale

In this part of the prelab exercise, you will develop an algorithm to find the maximum and minimum values of a grayscale input image. Create a function that will process one row of the image at a time and find the overall minimum and maximum intensities in the image.

auto_contrast_find_extrema(in_data, min, max, col)

Color

The NTSC camera acquires images in the color format YCbCr, where Y represents luminosity, Cb the blue component, and Cr the red component. Each image must be converted to 16-bit RGB for output on a standard color computer monitor. The function “ycbcr422pl_to_rgb565” performs this conversion. Knowing how this function converts each pixel to RGB is relatively unimportant, however, knowing the packed (5:6:5) RBG format is essential.

Before we ignore the ycbcr422pl_to_rgb565 function completely, it is useful to look at how it operates. Find the run time of the function by examining the file “ycbcr422pl_to_rgb565.c” and note that it must convert an even number of pixels at a time. If it were possible to have this function process the whole color image at in one function call, how many clock cycles would the function take? Since we are limited in the number of rows we can modify at a time, how many clock cycles should it take to process the whole image one row at a time? To demonstrate the overhead needed for this function, note how many clock cycles the function would take if it converted the whole image two pixels at a time.

Since each color is not individually addressable in the packed RGB format (e.g. bits representing red and blue are stored in the same byte), being able to modify different bits of each byte is necessary. To help clarify what bits are being set/cleared/toggled, numbers can be represented in hex format. For example, the integer 58 can be represented by “00111010” in binary or by “3A” in hex. In C, hex numbers are indicated with the prefix “0x.”