The strides don't describe the size of the image, they are the spacing in
memory of where you want the values to be placed upon being read (or taken from
in order to write). There is no invalid set of strides, because the caller
might want them to end up anywhere in memory.
Or am I misunderstanding?
For a fully "contiguous" memory buffer where you intend for every plane,
scanline, pixel, and channel immediately follows the previous one, then in our
example the strides would be xstride=3, ystride=30, zstride=300. (Though for a
2D image, the zstride is not used.)
Here's an example of where you might have a stride range that is wildly outside
this: Let's say that you have 40 of these 10 x 10 x 3 x uint8 image files and
you are trying to read them in and assemble them into a single RGBA mosaic
image of 16x5 x 4 x uint8 (the additional channel is alpha, which you will
separately fill in as 1.0 [or 255 uint8] because it's not in your RGB files).
Here's a cartoon to illustrate this:
+-----------------------------------------+
| | | | | | | | | | | | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | | | | | | | | | | | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | | | | | | | | |X| | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | | | | | | | | | | | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | | | | | | | | | | | | | | | | | | | | |
+-----------------------------------------+
Each of my little grid cells is a 10x10 image. But that 10x10 image denoted by
the "X" needs to be placed in memory in the right portion of the 16x10 x 5x10
mosaic. So what are the strides we use for the read? Well, the xstride is 4
because we're making room for an alpha channel that wasn't present in the file,
the ystride is 640 (= 10*16*4), because each scanline of the little 10x10 image
that you read needs to be placed on the proper scanline of the 160x50 mosaic
you are assembling in memory.
-- lg
P.S. Woo-hoo for making a Rust wrapper. I think that's a totally great thing.
> On Oct 16, 2020, at 10:46 PM, Scott Wilson <[email protected]> wrote:
>
> I'm experimenting with a Rust wrapper for OIIO, and had some questions about
> the stride.
>
> Let's say I have an image that is 10x10 pixels, and 3 channels, and 1 byte
> per channel. What strides would be invalid for that image? I'm guessing that
> anything between -10 * 10 * 3 * 1 to 10 * 10 * 3 * 1 and the AutoStride would
> be valid, and everything else may try to access memory that isn't
> initialized. Is this assumption correct, or am I missing something?
>
> Thanks!
> _______________________________________________
> Oiio-dev mailing list
> [email protected]
> http://lists.openimageio.org/listinfo.cgi/oiio-dev-openimageio.org
--
Larry Gritz
[email protected]
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