`mat.getRegion(rect).getData()` is different than `mat.getRegion(rect).getDataAsArray()`

[piercus]

Scenario

const matC3 = new cv.Mat([
   [[255, 255, 255], [255, 255, 255], [255, 255, 255]],
   [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
], cv.CV_8UC3);
const region = matC3.getRegion(new cv.Rect(0, 0, 2, 2));
const buf = region.getData();
console.log(new Uint8Array(buf)[6], region.getDataAsArray()[1][0][0])

Actual

prints

255, 0

Expected

should print

0, 0

May be related to #402 ?

[piercus]

This problem comes from opencv library

From https://stackoverflow.com/a/12931889

Warning: In this case, miniMat is a subregion of imgray. This means that if you modify the former, you also modify the latter. Use miniMat.copyTo(anotherMat) to avoid this.

So the fix is to do .copyTo() to use a new Matrix

const matC3 = new cv.Mat([
   [[255, 255, 255], [255, 255, 255], [255, 255, 255]],
   [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
], cv.CV_8UC3);

const region = new cv.Mat(2,2,cv.CV_8UC3);
matC3.getRegion(new cv.Rect(0, 0, 2, 2)).copyTo(region);
const buf = region.getData();
console.log(new Uint8Array(buf)[6], region.getDataAsArray()[1][0][0])

Anyway, i think this is misleading and in this specific situation getData() should either :

• return a 3x2 buffer
• throw an error saying "cannot use getData on subRegion, use copyTo function first"
• return a 2x2 buffer corresponding to the subregion

@justadudewhohacks what do you think ?

[justadudewhohacks]

Hmm @piercus, I think I don't quite understand the problem here. Shouldn't new Uint8Array(buf)[6] correspond to region.getDataAsArray()[0][2][0] instead of region.getDataAsArray()[1][0][0]? Assuming that the mat data is stored in the form of [RGBRGBRGB...].

Not quite sure how copyTo would make a difference, since you are not mutating the data?

[piercus]

@justadudewhohacks thanks for quick reply :-)

But region.getDataAsArray()[0][2][0] gives me TypeError: Cannot read property '0' of undefined
The region is 2x2, so there is no region.getDataAsArray()[0][2] here

On one hand region.getDataAsArray() only outputs 2x2 and on the other hand region.getData() outputs the 4 first cells of the 3x2 data, this is not consistent, or should be explained somewhere.

> new Uint8Array(buf)
Uint8Array [ 255, 255, 255, 255, 255, 255, 255, 255, 255, 0, 0, 0 ]
> region.getDataAsArray()
[ [ [ 255, 255, 255 ], [ 255, 255, 255 ] ],
  [ [ 0, 0, 0 ], [ 0, 0, 0 ] ] ]

[justadudewhohacks]

Ahh ok, I see what you mean. I agree that this is kind of confusing, but it also behaves consistently with OpenCV.

We could atleast document this somehow in the readme, I feel like mat.getRegion does cause a lot of misconceptions, if one is not aware of what it's actually doing.

[piercus]

I think getData() should raise an error if mat.data size is not consistent with mat.cols and mat.rows.
Because the output is inconsistent in this case

I'm not sure how we can find the size of mat.data in GetDataWorker without using mat.cols or mat.rows.

It is possible ?

[justadudewhohacks]

Hmm, I don't think it's possible without checking for cols and rows. Is there actually any way to deal with this in OpenCV?