png_loader.jl

run(`wget http://julialang.org/images/logo.png`)

# Open png file for reading
fp = ccall(:fopen, Ptr{Uint8}, (Ptr{Uint8}, Ptr{Uint8}), "logo.png", "r")

# Read first 8 bytes
initial_bytes = Array(Uint8, 8)
ccall(:fread, Ptr{Uint8}, (Ptr{Uint8}, Uint8, Uint8, Ptr{Uint8}), initial_bytes, 1, 8, fp)

# Check that we actually have a png file
@assert ccall((:png_sig_cmp, "libpng"), Bool, (Array{Uint8}, Int, Int), initial_bytes, 0, 8)

png_ptr = ccall((:png_create_read_struct, "libpng"), Ptr{Uint8}, (Ptr{Uint8}, Ptr{Uint8}, Ptr{Uint8}, Ptr{Uint8}), "1.5.13", C_NULL, C_NULL, C_NULL)

info_ptr = ccall((:png_create_info_struct, "libpng"), Ptr{Uint8}, (Ptr{Uint8}, ), png_ptr)

ccall((:png_init_io, "libpng"), Void, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, fp)

# Alert libpng that we've already read the first 8 bytes of the file
ccall((:png_set_sig_bytes, "libpng"), Void, (Ptr{Uint8}, Uint8), png_ptr, 8);

ccall((:png_read_info, "libpng"), Void, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

height = ccall((:png_get_image_height, "libpng"), Int, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

width = ccall((:png_get_image_width, "libpng"), Int, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

bit_depth = ccall((:png_get_bit_depth, "libpng"), Int, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

channels = ccall((:png_get_channels, "libpng"), Int, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

# color_type = ccall((:png_get_color_type, "libpng"), String, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

rowbytes = ccall((:png_get_rowbytes, "libpng"), Int, (Ptr{Uint8}, Ptr{Uint8}), png_ptr, info_ptr)

pixel_type = (bit_depth == 8 ) ? Uint8 : Uint16;

rows = [ Array(pixel_type, rowbytes) for h in 1:height ]

for row in rows
  ccall((:png_read_row, "libpng"), Void, (Ptr{Uint8}, Ptr{Uint8}, Ptr{Uint8}), png_ptr, row, C_NULL)
end

#so far so good, but I'd like to replace the for loop above with the following. Unfortunately, it segfaults.
# ccall((:png_read_image, "libpng"), Void, (Ptr{Uint8}, Array{Ptr{Uint8}}), png_ptr, rows)