Method write_preamble

_plotly_utils/png.py:763–835

(self, outfile)

        return i + 1

    def write_preamble(self, outfile):
        # http://www.w3.org/TR/PNG/#5PNG-file-signature
        outfile.write(signature)

        # http://www.w3.org/TR/PNG/#11IHDR
        write_chunk(
            outfile,
            b"IHDR",
            struct.pack(
                "!2I5B",
                self.width,
                self.height,
                self.bitdepth,
                self.color_type,
                0,
                0,
                self.interlace,
            ),
        )

        # See :chunk:order
        # http://www.w3.org/TR/PNG/#11gAMA
        if self.gamma is not None:
            write_chunk(
                outfile, b"gAMA", struct.pack("!L", int(round(self.gamma * 1e5)))
            )

        # See :chunk:order
        # http://www.w3.org/TR/PNG/#11sBIT
        if self.rescale:
            write_chunk(
                outfile,
                b"sBIT",
                struct.pack("%dB" % self.planes, *[s[0] for s in self.rescale]),
            )

        # :chunk:order: Without a palette (PLTE chunk),
        # ordering is relatively relaxed.
        # With one, gAMA chunk must precede PLTE chunk
        # which must precede tRNS and bKGD.
        # See http://www.w3.org/TR/PNG/#5ChunkOrdering
        if self.palette:
            p, t = make_palette_chunks(self.palette)
            write_chunk(outfile, b"PLTE", p)
            if t:
                # tRNS chunk is optional;
                # Only needed if palette entries have alpha.
                write_chunk(outfile, b"tRNS", t)

        # http://www.w3.org/TR/PNG/#11tRNS
        if self.transparent is not None:
            if self.greyscale:
                fmt = "!1H"
            else:
                fmt = "!3H"
            write_chunk(outfile, b"tRNS", struct.pack(fmt, *self.transparent))

        # http://www.w3.org/TR/PNG/#11bKGD