Today's Topics

• Bitmap Image Graphic Overview
• PCX Files
• BMP Files
• Programming Techniques

Bitmap Image Graphic: Minimal Components

• Header
  • Size (Horizonal by Vertical Pixels)
  • Number Bits/pixel
  • Palette Entries (Except for True-color)
  • Version (Makes life difficult)
  • Compression Technique

• Data
  • Raw Pixels (early .BMP files)
  • Run-Length encoding (.PCX files)
  • LZW encoding (.GIF files)
  • Transform encoding (.JPG files)

PCX Files

• 128-Byte header

• Palette Storage
  • 3 Bytes/Pixel (R/G/B) [24-bits total]
    • 16-Color palettes (3*16=48 bytes stored in header
    • 256-Color palettes (3*256=768 bytes) stored in trailer

• Run-Length encoded data storage
  • 11000000b = C0: Indicates Duplicate pixels
  • 00111111b = 3F AND i: Indicates Length
  • Following byte indicates palette entry

• PCX_HEADER STRUC MANUFACTURER DB 10 ; A ZSoft PCX File VERSION DB ? ENCODING DB 1 BITS_PER_PIXEL DB ? ; 1,2,4,8,24 (8 for ECE291 images) XMIN DW ? YMIN DW ? XMAX DW ? ; Window YMAX DW ? HORZ_DPI DW ? ; Horizontal/Vertical Dots per inch VERT_DPI DW ? PALETTE DB 48 DUP (?) ; For Palettes with =< 16 colors RESERVED DB ? COLOR_PLANES DB ? BYTES_PER_LINE DW ? PALETTE_TYPE DW ? HSCR_SIZE DW ? ; only supported by VSCR_SIZE DW ? ; PBrush IV or higher FILLER DB 54 DUP (?) PCX_HEADER ENDS PCX_DATA STRUC IMAGE_DATA DB FileSize DB ? ; RLE-Encoded Data (line-by-line) PCX_DATA ENDS PCX_TRAILER STRUC PALETTE_MARK DB 12 ; Indicates Trailer Palette PALETTE_256C DB 256 DUP('RGB') ; 768 Bytes of Palette Data PCX_TRAILER ENDS

• Sample PCX File (horizon.pcx)
  
  • Created with Corel PhotoPaint
  • Uses vga291.cpl Color Palette that has 16 standard colors and 16 shades of gray.

• Sample PCX File (letters.pcx)
  
  • Created with Corel PhotoPaint
  • Extends vga291.cpl Color Palette to include 64 shades of red, green, and blue.

• References
  • ZSoft PCX File Format Technical Reference Manual
    • Documents all modes
    • Includes C source code
  • Chapter 11 of ECE291 Lab manual (Full Screen only)

BMP Files

• BMPHEADERTYPE STRUC ; --- BitMapFileHeader --- ; 14 bytes BFType DB 'BM' ; File Type BFSize DD ? ; File Size (in Bytes) BFReserved1 DW 0 ; Reserved BFReserved2 DW 0 ; " BFOffBits DD ? ; Offset to start of image data ; --- BitMapInfoHeader --- ; 40 bytes BISize DD ? ; Size of BitMapInfoHeader (28h = 40d) BIWidth DD ? ; # Pixel Rows BIHeight DD ? ; # Pixel Columns BIPlanes DW 1 ; BIBitCount DW ? ; Log2(palette size) = 4 for 16-color image BICompression DD 0 ; RGB = 0 = Uncompressed BISizeImage DD ? ; Size of Image (Bytes) BIXPelsPerMeter DD ? ; BIYPelsPerMeter DD ? ; BIColorsUsed DD 0 ; 0=All BIColorsImportant DD 0 ; ; --- Color Table --- ; 64 Bytes RGBQuad DB 16 dup ( 4 dup(?) ) ; Blue, Green, Red, Unused BMPHEADERTYPE ENDS

• Example: 32x32 pixel, uncompressed, 16-color BMP
  • Each nibble (4 bits) stores 1 pixel
  • Header size = 118 bytes (as shown above)
  • Image size = (1/2 bytes/pixel) * (32*32 pixels) = 512 bytes
  • Total file size = 118+512=630 bytes
  • Pixels stored left-to-right, from bottom to top of image.
  • Each pixel color represented by (Blue,Green,Red,Unused) bytes in palette
  • Image file: myimage.bmp

Programming Techniques

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