RAW Formats

RAW Format implies that there is no compression done on the image. The major types of RAW format are RGB, YUV, YIQ. Our eye is more sensitive towards light intensity variation than color variation. So loss on color information will not affect the over all quality of the image. RGB is an end stream format. Information from the image sensor is in RGB format and we need the same format for displaying the image on an end

device. YUV & YIQ formats are developed for Analog TV transmission (NTSC & PAL respectively) and the digital version of YUV, YCbCr is the most common format used for image and video compressions.

Conversion from one format to another is described below:

RGB to YCbCr Conversion

Y = 0.299 R + 0.587 G + 0.11B

Cb = 0.564 (B - Y)

Cr = 0.713 (R - Y)

YCbCr to RGB

R = Y + 1.402 Cr

G = Y – 0.344 Cb – 0.714 C r

B = Y + 1.722 Cb

Y – Luminance Signal

Cb, Cr – Chrominance Signal, Color difference signal

R – Red

G – Green

B – Blue

Need for Compression

Consider an image of resolution 640 × 480. Let us calculate the size of the picture in RAW format. Each of the 10 Color is represented by 8 bits. Then for each pixel it needs 24 bits. Total no of pixels in the image is 640 × 480 = 307200 pixels. Therefore the size of the image turns to 307200 × 3 bytes = 921600 bytes. But an image in compressed format with the same resolution takes only 100 KB.

In the case of RAW video stream of length 1 sec its needs 640 × 480 × 3 × 25 = 23040000 bytes (23 MB) of storage if the frame rate is 25 frames/sec. But it’s known that the VCD format video having a size 700 MB plays for around 80 minutes. In the former case we need 110400 MBs (23 MB × 60 × 80) as storage space for 80 minutes video. Therefore we can achieve a high compression 150: 1 at the cost of computational complexities.