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3618 :: How do I read BMP file header directly to type?

• HonzsSedlomn (orginal) [2018-03-08T14:23:02+01:00] view original

  Hello, I'm trying to load BMP file header to type using FileStream readData, but it doesn't work. I have to read every single variable separately. If I try to load it with readData, there is some weird result, missing data, etc.

  
  type
      BitmapFileHeader* = object
          signature*: array[2, char]
          size*: uint32
          reservedOne*: uint16
          reservedTwo*: uint16
          offset*: uint32
  
  proc loadBitmapFileHeader*(bitmap: Bitmap, file: FileStream): BitmapFileHeader =
      file.setPosition(0)
      
      result.signature[0] = file.readChar()
      result.signature[1] = file.readChar()
      result.size = file.readUint32()
      result.reservedOne = file.readUint16()
      result.reservedTwo = file.readUint16()
      result.offset = file.readUint32()
      
      #TODO: Check if all bytes have been read
      
      return result
  Run

  Is there any way to load it like this?

  
  proc loadBitmapFileHeader*(bitmap: Bitmap, file: FileStream): BitmapFileHeader =
      file.setPosition(0)
      
      let readBytes = file.readData(bitmap.fileHeader.addr, 14)
      
      return result
  Run

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• Araq (orginal) [2018-03-09T08:46:11+01:00] view original

  Certainly, but you need to assign to result:

  proc loadBitmapFileHeader*(bitmap: Bitmap, file: FileStream): BitmapFileHeader =
      static:
          doAssert(cpuEndian == littleEndian)
      discard file.readData(result.addr, 14)
      bitmap.fileHeader = result
  Run

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• Parashurama (orginal) [2018-03-09T09:31:35+01:00] view original

  You should probably also use the packed pragma on your type since depending on platform and compiler there could be some padding between the fields of your struct. (ex: between signature and size )

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• mratsim (orginal) [2018-03-12T10:46:15+01:00] view original

  Also you might want to take into account the endianness of BMP.

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• oswjk (orginal) [2018-03-12T15:08:41+01:00] view original

  If you want to use a library, NESM could be of use. NESM creates the serialization and deserialization procs for you automatically based on your (a bit augmented) types.

  You could use it like this:

  import nesm
  import streams
  
  serializable:
    type
      BitmapFileHeader = object
        set: {endian: littleEndian}
        signature: array[2, byte]
        size: uint32
        reservedOne: uint16
        reservedTwo: uint16
        offset: uint32
  
  let
    fs = newFileStream("image.bmp", fmRead)
    header = deserialize(BitmapFileHeader, stream)
  Run

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