On 07/17/2012 06:55 PM, Dag Sverre Seljebotn wrote:
On 07/17/2012 05:38 PM, Christian Heimes wrote:
Hello,
I'm the author of https://bitbucket.org/tiran/smc.freeimage , a Cython
wrapper of the FreeImage and LCMS libraries. FreeImage supports a broad
variety of image formats and pixel formats from standard RGB up to RGBAF
and complex numbers.
Now I like to add support for buffer interface to my code to support
zero-copy access of pixel data from NumPy. The wiki describes several
ways to access objects through the new and old Python buffer interface.
However I'm unable to find an example how to *implement* the buffer
interface.
Read PEP 3118. Then implement __getbuffer__ and __releasebuffer__ in
your cdef class (don't know if it's documented but you can see example
in tests/run/buffer.pyx).
Memory alignment makes the buffer interface more complicated, too.
FreeImage stores the raw pixels non-contiguously. The lines are memory
aligned (usually 32bit). For example a standard RGB image with 1 byte
per pixel and size of 2*3 pixels has a memory layout of
BGRBGRxxBGRBGRxxBGRBGRxx
on a little-endian machine. 'xx' are the two alignment bytes.
This is easily supported; above you would let
ndim = 2
strides = [8, 1]
shape = [2, 3]
Sorry; shape = [3, 2] to match your example.
Dag
itemsize = 1
The alignment bytes are skipped simply because shape[0] * itemsize <
strides[0].
Dag
Here is an excerpt from the FreeImage manual on how to access the raw
pixel data. The above example has width: 2, height: 3, pitch: 8 and
bpp: 3.
unsigned width = FreeImage_GetWidth(dib); // 2
unsigned height = FreeImage_GetHeight(dib); // 3
unsigned pitch = FreeImage_GetPitch(dib); // 8
unsigned bpp = FreeImage_GetBPP(dib); // 3
BYTE *bits = (BYTE*)FreeImage_GetBits(dib);
for(y = 0; y< height; y++) {
BYTE *pixel = (BYTE*)bits;
for(x = 0; x< width; x++) {
pixel[FI_RGBA_RED] = 128;
pixel[FI_RGBA_GREEN] = 128;
pixel[FI_RGBA_BLUE] = 128;
pixel += bpp;
}
// next line
bits += pitch;
}
Please help,
Christian
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