Outline Image formats and basic operations Image representation

Outline • Image formats and basic operations – Image representation – Basic operations 12/25/2021 Visual Perception Modeling 1

Four Stages of Visual Processing • • Image-based stage Surface-based stage Object-based stage Category-based stage 12/25/2021 Visual Perception Modeling 2

Problems in Computer Vision • Classification – Edge detection • Segmentation • Recognition • Motion analysis 12/25/2021 Visual Perception Modeling 3

Digital Image Formation • How to acquire a digital image – Digital cameras – The continuous spatial space is digitized into small cells – The color of a cell is all the photons fall into the cell within a very short period of time – Charge Coupled Device - CCD cameras • An electronic memory which is able to be charged by light 12/25/2021 Visual Perception Modeling 4

Image Representation • Mathematically, an image can be represented by a 2 -D matrix – Each entry (i, j) represents the value at the corresponding location, which is called a pixel – The value of a pixel can have different types, depending on the image types • • 12/25/2021 Unsigned char Int Float A vector (Color image, for example) Visual Perception Modeling 5

Image Sequence Representation • Mathematically, an image sequence is represented as a 3 -D matrix – At any time, it is a 2 -D matrix – A commonly used format is animated gif 12/25/2021 Visual Perception Modeling 6

Image as a File • Image formats – TIFF – GIF – JPEG – PBM 12/25/2021 Visual Perception Modeling 7

Image as a File – cont. • Image file consists of two parts – Header • Type of the image • Size of the image • Color map – Data • Uncompressed • Compressed 12/25/2021 Visual Perception Modeling 8

PBM Format • Portable Bitmap Utilities – Header • A two-letter magic number that identifies the format type • File parameters – Height, width, maximum value of a pixel of the image – Data • In ASCII or binary, depending on the format type 12/25/2021 Visual Perception Modeling 9

Portable Gray Map • Binary PGM format – P 5 – Width – Height – Max – Pixels as binary values 12/25/2021 Visual Perception Modeling 10

Portable Pixel Map • Binary PPM format – P 6 – Width – Height – Max 12/25/2021 Visual Perception Modeling 11

Programs to Read/Write Images • These functions are based on my program – type-def. h defines the structures – imageio. c defines the functions to read and write images of different types – util. c defines functions to dynamically allocate memory needed for an image 12/25/2021 Visual Perception Modeling 12

Image Sequence • There are many different kinds of formats for movie or video sequence – avi – mpeg • An easy way to generate a video sequence is to concatenate images together – Animated gif is such a format 12/25/2021 Visual Perception Modeling 13

Visualization of Images • xv – A utility for X-window system – Can display and convert many different kinds of image formats • Image. Magick package – convert – display – animate – available at http: //www. simplesystems. org/Image. Magick/ 12/25/2021 Visual Perception Modeling 14