On Thu, 2007年01月11日 at 09:56 -0600, John Hunter wrote:
> >>>>> "Steve" == Steve Chaplin <ste...@ya...> writes:
> Steve> This is the official definition from the manual:
> Steve> CAIRO_FORMAT_ARGB32 each pixel is a 32-bit quantity, with
> Steve> alpha in the upper 8 bits, then red, then green, then
> Steve> blue. The 32-bit quantities are stored
> Steve> native-endian. Pre-multiplied alpha is used. (That is, 50%
> Steve> transparent red is 0x80800000, not 0x80ff0000.)
>
> Steve> What I think this means is: cairo ARGB32 is stored not as 4
> Steve> 8-bit quantities, but as one 32-bit int. On big-endian
> Steve> systems the byte order is ARGB, as you would expect, but on
> Steve> little-endian systems (like a PC) the byte order is BGRA.
>
> Steve> I imagine the reason is that its easier/faster to
> Steve> read/write one 32-bit int than it is to read/write four
> Steve> bytes.
>
> OK, I see the source of my confusion. argb determines the order but
> it doesn't determine whether the most significant bit is first or
> last....
>
> I added a method buffer_bgra32 to the image backend. I'm not sure
> this is the right way to deal with the endianness bit it's easy and
> will probably work. I'll leave it to you guys to fix the cairo
> backend to selectively call the right method and test it across
> platforms, or propose a better solution if you don't like this one...
Thanks, I used the new buffer_bgra32 and now examples/image_demo.py
looks correct (except that sometimes it looks like the pixels right at
the edge of the image might be corrupted).
The backend_cairo.py code does look a little strange, it calls
rows, cols, str_ = im.buffer_bgra32()
and then
X = numx.fromstring (str_, numx.UInt8)
which is used merely to convert the (readonly) string returned from
buffer_bgra32 into a read-write buffer for cairo. If buffer_bgra32
returned a buffer (as its name suggests!) instead of a string this would
not be needed, and it would save copying the image around in memory.
I propose this new version of buffer_bgra32 (and buffer_argb32). I
tested it with cairo and it seems to work OK.
Py::Object
Image::buffer_bgra32(const Py::Tuple& args) {
//"Return the image object as bgra32";
_VERBOSE("RendererAgg::buffer_bgra32");
args.verify_length(0);
int row_len = colsOut * 4;
PyObject* py_buffer = PyBuffer_New(row_len * rowsOut);
if (py_buffer ==NULL)
throw Py::MemoryError("RendererAgg::buffer_bgra32 could not allocate
memory");
unsigned char* buf;
int buffer_len;
int ret = PyObject_AsWriteBuffer(py_buffer, (void **)&buf,
&buffer_len);
if (ret !=0)
throw Py::MemoryError("RendererAgg::buffer_bgra32 could not allocate
memory");
agg::rendering_buffer rtmp;
rtmp.attach(buf, colsOut, rowsOut, row_len);
agg::color_conv(&rtmp, rbufOut, agg::color_conv_rgba32_to_bgra32());
PyObject* o = Py_BuildValue("llN", rowsOut, colsOut, py_buffer);
return Py::asObject(o);
}
Steve
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