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bmp_reader.rb
# coding: binary
class BMP
class Reader
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)
read_dib_header(file)
read_pixels(file)
end
end
def [](x,y)
@pixels[y][x]
end
attr_reader :width, :height
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
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
end
end
bmp = BMP::Reader.new("example1.bmp")
p bmp.width #=> 2
p bmp.height #=> 2
p bmp[0,0] #=> "ff0000"
p bmp[1,0] #=> "00ff00"
p bmp[0,1] #=> "0000ff"
p bmp[1,1] #=> "ffffff"
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