Questions on migration from numpy to xtensor. about xtensor

[argman]

you can assign views to views, like

xt::view(channels, 0) = xt::view(....)

[hassanromhuda]

you can assign views to views, like

xt::view(channels, 0) = xt::view(....)

I can't get something like this to work. See #2748. Any ideas?

[JohanMabille]

channels needs to be an evaluated expression for having this work.

[OleksiiMatiash]

@argman

you can assign views to views, like

xt::view(channels, 0) = xt::view(....)

If I get this correctly, it compiles, but crashes on assignment:

auto channels = xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 });
auto ch0 = xt::view(activeAreaImageR1, xt::range(0, 2));
auto ch01d = xt::reshape_view(ch0, { activeAreaImageHeight * activeAreaImageWidth / 4 });
xt::view(channels, 0) = ch0;

@JohanMabille

channels needs to be an evaluated expression for having this work.

Sorry, I probably didn't get the idea, because this does not work:

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
channels[0] = xt::eval(xt::view(activeAreaImageR1, xt::range(0, 2)));

With any combinations of xt::eval it tells me this:

[JohanMabille]

The first snippet should not build since you still try to assign to a view on an unevaluated expression.

Regading the second one, be carefull that operator[] in xtensor is different from operator[] in numpy, see https://github.com/compiler-research/xeus-cpp/actions/runs/6892192995/job/18748906995?pr=19 for the different ways to access elements in xensor expressions. The following should work:

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
xt::view(channels, 0) = xt::eval(xt::view(activeAreaImageR1, xt::range(0, 2)));

(assuming the shapes are compatible).

[OleksiiMatiash]

The first snippet should not build since you still try to assign to a view on an unevaluated expression.

First variant compiles, but crashes on execution of last line.

see https://github.com/compiler-research/xeus-cpp/actions/runs/6892192995/job/18748906995?pr=19

Wrong link, probably?

This compiles (added reshape), but crashes on execution

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
xt::view(channels, 0) = xt::eval(xt::reshape_view(xt::view(activeAreaImageR1, xt::range(0, 2)), { activeAreaImageHeight * activeAreaImageWidth / 4 }));

[JohanMabille]

Wrong link, probably?

Indeed, sorry for that, here is the correct one: https://xtensor.readthedocs.io/en/latest/expression.html#element-access

This compiles (added reshape), but crashes on execution

Can you print the shapes of both expressions in the last statement? No need for xt::eval on the right hand side.

[OleksiiMatiash]

Can you print the shapes of both expressions in the last statement? No need for xt::eval on the right hand side.

If I got you correctly (sorry if no), this

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
auto reshapedView = xt::reshape_view(xt::view(activeAreaImageR1, xt::range(0, 2)), { activeAreaImageHeight * activeAreaImageWidth / 4 });
auto shape1 = reshapedView.shape();
auto shape2 = xt::view(channels, 0).shape();

gives this:

[JohanMabille]

OK, so the shpaes are compatible, I would try the following:

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
auto image_view = xt::eval(xt::view(activeAreaImageR1, xt::range(0, 2)));
auto reshapedView = xt::reshape_view(image_view, { activeAreaImageHeight * activeAreaImageWidth / 4 });
xt::view(channels, 0)  = reshaped_view;

and the following:

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
auto image_view = xt::eval(xt::view(activeAreaImageR1, xt::range(0, 2)));
auto reshapedView = xt::eval(xt::reshape_view(image_view, { activeAreaImageHeight * activeAreaImageWidth / 4 }));
xt::view(channels, 0)  = reshaped_view;

to see where it crashes. You should also check how activeAreaImageR1 is initialized, and its shape.

[OleksiiMatiash]

This

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
auto image_view = xt::eval(xt::view(activeAreaImageR1, xt::range(0, 2)));
auto reshapedView = xt::reshape_view(image_view, { activeAreaImageHeight * activeAreaImageWidth / 4 });
xt::view(channels, 0)  = reshaped_view;

crashes on the last line:

And this:

auto channels = xt::eval(xt::empty<uint16_t>({ 4, activeAreaImageHeight * activeAreaImageWidth / 4 }));
auto image_view = xt::eval(xt::view(activeAreaImageR1, xt::range(0, 2)));
auto reshapedView = xt::eval(xt::reshape_view(image_view, { activeAreaImageHeight * activeAreaImageWidth / 4 }));
xt::view(channels, 0)  = reshaped_view;

on this line:

auto reshapedView = xt::eval(xt::reshape_view(image_view, { activeAreaImageHeight * activeAreaImageWidth / 4 }));

You should also check how activeAreaImageR1 is initialized, and its shape.

Full code:

uint16_t* sourceBuffer = new uint16_t[sourceFileInfo->metadata->dataSize / sizeof(uint16_t)];

FILE* fileptr;
fileptr = fopen(filePath.toStdString().c_str(), "rb");
if (fileptr == nullptr) return;

fseek(fileptr, sourceFileInfo->metadata->dataOffset, SEEK_SET);
fread(sourceBuffer, sizeof(uint16_t), sourceFileInfo->metadata->dataSize / sizeof(uint16_t), fileptr);
fclose(fileptr);

std::vector shape = { sourceFileInfo->metadata->dataSize / sizeof(uint16_t) };
QList<int> activeArea = sourceFileInfo->metadata->activeArea;
int activeAreaImageHeight = activeArea[2] - activeArea[0];
int activeAreaImageWidth = activeArea[3] - activeArea[1];

auto source = xt::adapt(sourceBuffer, sourceFileInfo->metadata->dataSize / sizeof(uint16_t), xt::no_ownership(), shape);

source.reshape({ sourceFileInfo->metadata->imageHeight, sourceFileInfo->metadata->imageWidth });
auto activeAreaImage = xt::view(source, xt::range(activeArea[0], activeArea[2]), xt::range(activeArea[1], activeArea[3]));

auto activeAreaImageR1 = xt::view(activeAreaImage, xt::all(), 2);
xt::reshape_view(activeAreaImageR1, { -1 });

xt::xarray<uint16_t> activeAreaImageR2 = xt::view(activeAreaImage, xt::range(1, 2));
xt::reshape_view(activeAreaImageR2, { -1 });

and then the code with problems from original question