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-- This example demonstrates using the 'get_data' method on an image
-- surface to retrieve the binary data for an image. Typically you
-- would want to do that in order to pass the data to some other library,
-- but in this case we'll write it out in a file format that Cairo doesn't
-- know about, namely PPM, which most image viewers should understand.
local Cairo = require "oocairo"
local IMAGE_WD, IMAGE_HT = 432, 234
-- Write out image data as a PPM file. PPM is the true-colour variant of
-- the PNM class of formats. We're using the binary version of PPM here,
-- which has three bytes per pixel, rather than three ASCII numbers per
-- pixel. This function assumes the data is for an RGB24 or ARGB32 image.
local function write_ppm (filename, width, data, stride)
local fh = assert(io.open(filename, "wb"))
local height = data:len() / stride
fh:write("P6\n", width, " ", height, "\n", "255\n") -- header
for y = 0, height - 1 do
for x = 0, width - 1 do
local offset = y * stride + x * 4
-- Cairo stores pixel components using the native machine's
-- byte order conventions, so to work on all platforms we need
-- to reverse the bytes on little-endian platforms. This is why
-- the 'get_data' method returns an endianness indicator.
local a, r, g, b
if Cairo.BYTE_ORDER == "argb" then -- big endian
a, r, g, b = data:byte(offset + 1, offset + 4)
else -- little endian
b, g, r, a = data:byte(offset + 1, offset + 4)
end
fh:write(string.char(r, g, b))
end
end
fh:close()
end
local surface = Cairo.image_surface_create("rgb24", IMAGE_WD, IMAGE_HT)
local cr = Cairo.context_create(surface)
-- Draw a gradient designed to include lots of different colours (mainly
-- to confirm that the three colour components come out the right way
-- round in the output file).
local grad = Cairo.pattern_create_linear(0, 0, IMAGE_WD, IMAGE_HT)
grad:add_color_stop_rgb(0, 1, 0, 0)
grad:add_color_stop_rgb(.5, 0, 1, 0)
grad:add_color_stop_rgb(1, 0, 0, 1)
cr:set_source(grad)
cr:paint()
write_ppm("get-data.ppm", surface:get_width(), surface:get_data())
-- vi:ts=4 sw=4 expandtab
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