Acquisition & Save

Images Format

Matrix Formats

A matrix format is characterized by recording data in table shape. The pixels are ordered with coordinates, an ordinate and an abscysse associated with a color value.

Bitmap (BMP)

The format is the easiest. The compression of this format is rarely used. Indeed, it is only available for 8 and 4-bit BMP. The algorithm used is the Run-length encoding (RLE).

• Header of file.
  • 2 bytes, use of the file
  • 4 bytes, size of the file
  • 4 bytes, identification of the software which created it
  • 4 bytes, origine address of image datas
• Header of image
  • 4 bytes, size of this header
  • 4 bytes, number of horizontal pixels
  • 4 bytes, number of vertical pixels
  • 2 bytes, number of plan (one plan for one image)
  • 2 bytes, number of bit to code the color
  • 4 bytes, compression used (1 RLE 8 bits, 2 RLE 4 bits et 3 bitfields)
  • 4 bytes, size of the image in bytes
  • 4 bytes, number of pixel by metter in horizontal
  • 4 bytes, number of pixel by metter in vertical
  • 4 bytes, colors of the pallet
  • 4 bytes, number of major colors
• The color table (pallet)
  • 1 byte, red
  • 1 byte, green
  • 1 byte, blue
  • 1 byte, reserved
• Image It is coded from the bottom of the picture, from left to right, to the top. If a byte code several pixels, the left bits correspond to the pixels to the left.

We can see below 4 Bitmap images with from left to rigth 24bits, 8bits, 4 bits et 1 bit.

Thoses images have a size of 256 by 256 pixels. The size of files are : 260Ko, 68ko, 36Ko et 12Ko.
http://www.fileformat.info/format/bmp/egff.htm

Graphics Interchange Format (GIF)

For this format, up to 256 different colors can be chosen. In other words, a pixel is coded on one byte. This allows a very good compression without loss by the algorithm Lempel-Ziv-Welch (LZW). This format is very popular for icons and logos. In addition the GIF89a revision enables the creation of sequence succession of animated images.
http://www.fileformat.info/format/gif/egff.htm

Tagged Image File Format(TIFF)

The TIFF format can save large bitmaps of several GBs and without loss of quality. The colors can be coded from 1 to 64 bits for a wide range of color space: RGB, YCbCr, CMYK, CIE Lab. A wide choice of compression is also available: PackBits, LZW, CCITT, JPEG. Furthermore, additional information can be added as the contrast or brightness.
http://www.fileformat.info/format/tiff/egff.htm

Portable Network Graphics (PNG)

Alternative format of the GIF format, it is free. No Royalties shall be paid by software publishers who using it. It allows you to store images to gray level upto 16 bits precision or 48 bits for color. The constitution of the file is made of chunks. All segments are made in the same way.

• 4 bytes, size of chunks
• 4 bytes, kind of chunks
• x bytes, datas
• 4 bytes, an error correcting code

The chunks can be mixed but the header chunk IHDR must be put first and IEND end chunk at the end. There are many segments (the first 4 are required):

• IHDR Image header
• PLTE Pallet
• IDAT Image data
• IEND Image trailer
• BKGD Background color
• CHRM Primary chromaticities and white point
• GAMA Image gamma
• HIST Image histogram
• PHYs Physical pixel dimensions
• SBIT Significant bits
• TEXt Textual data
• TIME Image last-modification time
• TRNS Transparency
• ZTXt Compressed textual data

http://www.fileformat.info/format/PNG/egff.htm

Joint Photographic Experts Group (JPEG)

This format is actually a standard decoding. The method of encoding is left free to industrialists and academics, the important is that the resulting file can be decoded by the JPEG standard. Two types of compression can be used for this format, lossy compression (irreversible) or we see a loss of information but in return the file. jpg is much smaller. The other type is a lossless (reversible), the compression ratio is worse than before.
Look at this link for more information.
http://www.fileformat.info/format/jpeg/egff.htm

Vectorial Formats

.

encapsulated post-script (EPS)

.
http://www.fileformat.info/format/eps/egff.htm

Drawing eXchange Format (DXF)

.
http://www.fileformat.info/format/dxf/egff.htm

Copyright © 2010-2014, all rights reserved, contact: operationpixel@free.fr