[cairo] Is posible create a png surface from a png buffer read from a zip file?

Lawrence D'Oliveiro ldo at geek-central.gen.nz
Fri Nov 6 13:49:00 PST 2015

On Fri, 6 Nov 2015 15:18:23 +0100, KiwiLib KiwiLib wrote:

> I found the cairo_image_surface_create_from_png_stream
> <http://cairographics.org/manual/cairo-PNG-Support.html#cairo-image-surface-create-from-png-stream>
>  () is hard to manage and not references directly a buffer. It wil be
> easy to implement ?.

This is an example of a programming technique known as a “callback”. It
is very useful for writing general, reusable code that makes minimal
assumptions about what data structures the caller might be
using--instead, you let the caller provide the interface to that.

Here is an example in Python, showing how to decode a PNG stream
contained in a bytes or array buffer. It comes from Qahirah

    def create_from_png_bytes(data) :
        "creates an ImageSurface from a PNG format data sequence. This can be" \
        " of the bytes or bytearray types, or an array.array with \"B\" type code."

        offset = 0

        def read_data(_, data, length) :
            nonlocal offset
            if offset + length <= len(data) :
                libc.memcpy(data, baseadr + offset, length)
                offset += length
                status = CAIRO.STATUS_SUCCESS
            else :
                status = CAIRO.STATUS_READ_ERROR
            #end if
            return \
        #end read_data

    #begin create_from_png_bytes
        if isinstance(data, bytes) or isinstance(data, bytearray) :
            data = array.array("B", data)
        elif not isinstance(data, array.array) or data.typecode != "B" :
            raise TypeError("data is not bytes, bytearray or array of bytes")
        #end if
        baseadr = data.buffer_info()[0]
        return \
            ImageSurface.create_from_png_stream(read_data, None)
    #end create_from_png_bytes

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