PIXEL_ARRAY_OFFSET = 54

BITS_PER_PIXEL = 24

DIB_HEADER_SIZE = 40

def initialize(bmp_filename)

File.open(bmp_filename, "rb") do |file|

read_bmp_header(file) # does some validations

read_dib_header(file) # sets @width, @height

read_pixels(file) # populates the @pixels array

end

end

attr_reader :width, :height

def [](x,y)

@pixels[y][x]

end

def read_bmp_header(file)

header = file.read(14)

magic_number, file_size, reserved1,

reserved2, array_location = header.unpack("A2Vv2V")

fail "Not a bitmap file!" unless magic_number == "BM"

unless file.size == file_size

fail "Corrupted bitmap: File size is not as expected"

end

unless array_location == PIXEL_ARRAY_OFFSET

fail "Unsupported bitmap: pixel array does not start where expected"

end

end

def read_dib_header(file)

header = file.read(40)

header_size, width, height, planes, bits_per_pixel,

compression_method, image_size, hres,

vres, n_colors, i_colors = header.unpack("Vl<2v2V2l<2V2")

unless header_size == DIB_HEADER_SIZE

fail "Corrupted bitmap: DIB header does not match expected size"

end

unless planes == 1

fail "Corrupted bitmap: Expected 1 plane, got #{planes}"

end

unless bits_per_pixel == BITS_PER_PIXEL

fail "#{bits_per_pixel} bits per pixel bitmaps are not supported"

end

unless compression_method == 0

fail "Bitmap compression not supported"

end

unless image_size PIXEL_ARRAY_OFFSET == file.size

fail "Corrupted bitmap: pixel array size isn't as expected"

end

@width, @height = width, height

end

def read_pixels(file)

@pixels = Array.new(@height) { Array.new(@width) }

(@height-1).downto(0) do |y|

0.upto(@width - 1) do |x|

@pixels[y][x] = file.read(3).unpack("H6").first

end

advance_to_next_row(file)

end

end

def advance_to_next_row(file)

padding_bytes = @width % 4

return if padding_bytes == 0

file.pos = padding_bytes

end

end