Hi, Thanks a ton for the advice, Robert! Taking an array slice (instead of trying to set up the strides, etc. myself) is a slick way of getting this result indeed.
>> I need to allocate a numpy array that I will then pass to a camera >> driver (via ctypes) so that the driver can fill the array with >> pixels. >> The catch is that the driver requires that rows of pixels start at 4- >> byte boundaries. >> >> The sample C++ code given for allocating memory for this is (pixels >> are unsigned shorts): >> >> // Two bytes for each pixel, then round >> // up to the next multiple of four. >> long width_bytes = ( ( 2 * width_pixels ) + 3 ) & -4; >> long allocated_size = width_bytes * height; >> unsigned char* image_data = new unsigned char[allocated_size]; > Note that the approach above doesn't ensure that the first row is > correctly aligned. It just assumes that the allocator will always > start a new block aligned at 4 bytes (which may be reasonable for the > platforms you are targetting). Hmm, good point. The SDK/example code is pretty flakey, so I bet they just make this assumption. > To solve the initial alignment, you overallocate a 1D array by 3 bytes > and find the offset from the allocated initial address which is > correctly aligned. Sorry -- I haven't had to ever concern myself with alignment before this, so I'm not sure how to go about this step. Do I just look at the raw pointer address and see what offset I need to give it to get it to be divisible by four? > Slice out [:allocated_size] portion of this, > .view() it as uint16, reshape it to > (height,width_pixels+width_pixels%2), then slice out > [:,:width_pixels]. Everything else makes good sense. Thanks again! Zach _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
