PHP

用PHP分析GIF动画图片

walkingcoder.gif
Phpclasses有一个生成GIF动态图片的Class，当下来试试，很好用。

gift89a.gif

看了看源代码，只是实现了GIF图片的简单叠加，而如果要想做到分析GIF动态图片，做到将GIF动态图片拆分，甚至修改后重新生成新的GIF动态图片。看来还是要从分析GIF89a规范入手。在了解GIF89a的编码规则以后，还需要了解LZW，因为GIF采用的是这种LZW的无损压缩的算法。
这是我总结的GIF89a规则，希望有时间好好研究一下：

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The Definition of the GIF Format allows for a Data Stream to contain:

  • Header
    Signature: ‘GIF’ => 3 bytes: 0x47 0x49 0x46
    Version: ‘89a’ => 3 bytes: 0x38 0x39 0x61
  • the Logical Screen Descriptor
    Logical Screen Width: unsigned 2 bytes
    Logical Screen Height: unsigned 2 bytes
    Packed Fields:
    Global Color Table Flag 1 Bit
    Color Resolution 3 Bits
    Sort Flag 1 Bit
    Size of Global Color Table 3 Bits
    Background Color Index: 1 byte
    Pixel Aspect Ratio: 1 byte (Aspect Ratio = (Pixel Aspect Ratio + 15) / 64)
  • a Global Color Table
    size: 3 x 2^(Size of Global Color Table+1).
  • Image Descriptor
    Image Separator: 1 byte => 0x2C
    Image Left Position: unsigned 2 bytes
    Image Top Position: unsigned 2 bytes
    Image Width: unsigned 2 bytes
    Image Height: unsigned 2 bytes
    Packed Fields:
    Local Color Table Flag 1 Bit (if 1, follow immediately after this Image Descriptor.)
    Interlace Flag 1 Bit
    Sort Flag 1 Bit
    Reserved 2 Bits
    Size of Local Color Table 3 Bits
  • Table Based Image Data
    LZW Minimum Code Size: 1 byte
    Image Data (Data Sub-blocks)

//////////Animated Control Start

  • Graphic Control Extension.
    Extension Introducer: 1 byte => 0x21
    Graphic Control Label: 1 byte => 0xF9
    Block Size: 1 byte
    Packed Fields:
    Reserved 3 Bits
    Disposal Method 3 Bits (Recommendations: 0)
    User Input Flag 1 Bit (Recommendations: 0)
    Transparent Color Flag 1 Bit
    Delay Time: unsigned 2 bytes
    Transparent Color Index: 1 byte
    Block Terminator: 1 byte
    //////////Animated Control End

  • Comment Extension.
    Extension Introducer: 1 byte => 0x21
    Comment Label: 1 byte => 0xFE
    Comment Data (Data Sub-blocks): using the 7-bit ASCII character set
    (each of size at most 255 bytes and at least 1 byte)
    do not interfere with Control or Data blocks
    Block Terminator: 1 byte => 0x00

  • Plain Text Extension (Skip)
  • Application Extension (Skip)
  • Trailer: 1 byte => 0x3B

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