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给定一个UIImage/NSImage或NSData实例，如何以编程方式找出PNG 是 8 位还是 24 位？如果它有一个阿尔法通道？

是否有任何 Cocoa/Cocoa Touch API 对此有帮助？

为避免重复，这里是对该问题的非程序化答案，这是一种确定图像是否为 PNG 的方法。

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As a person who programmed for a long time on a J2ME platform, I know the PNG format very well. If you have the raw data as a NSData instance, looking up the information is very easy since the PNG format is very straightforward.

See PNG Specification

The PNG file starts with a signature and then it contains a sequence of chunks. You are interested only in the first chunk, IHDR.

So, the code should be along the lines of:

  1. Skip the first 8 bytes (signature)
  2. Skip 4 bytes (IHDR chunk length)
  3. Skip 4 bytes (IHDR chunk type = "IHDR")
  4. Read IHDR

Width: 4 bytes
Height: 4 bytes
Bit depth: 1 byte
Color type: 1 byte
Compression method: 1 byte
Filter method: 1 byte
Interlace method: 1 byte

For alpha, you should also check if there is a tRNS (transparency) chunk in the file. To find a chunk, you can go with the following algorithm:

  1. Read chunk length (4 bytes)
  2. Read chunk type (4 bytes)
  3. Check chunk type whether it is the type we are looking for
  4. Skip chunk length bytes
  5. Skip 4 bytes of CRC
  6. Repeat

EDIT:

To find info about a UIImage instance, get its CGImage and use one of the CGImageGet... functions. It should be noted that All integer values in a PNG file are read in Big-endian format.

于 2013-04-18T11:46:48.023 回答