Perspective

I asked this question on stackoverflow STL passing object

I got to know that we pass objects which in tern call the compare operator in them and compare our values and gives us a result. All good.

Now in this piece of code:

I have a collection where the documents are uniquely identified by a date, and I want to get the n most recent documents. My first thought was to use the date as a document ID, and then my query would sort by ID in descending order. Something like .orderBy(FieldPath.documentId, descending: true).limit(n). This does not work, because it requires an index, which can't be created because __name__ only indexes are not supported.

My next attempt was to use .limitToLast(n) with the default sort, which is documented here.

By default, Cloud Firestore retrieves all documents that satisfy the query in ascending order by document ID

According to that snippet from the docs, .limitToLast(n) should work. However, because I didn't specify a sort, it says I can't limit the results. To fix this, I tried .orderBy(FieldPath.documentId).limitToLast(n), which should be equivalent. This, for some reason, gives me an error saying I need an index. I can't create it for the same reason I couldn't create the previous one, but I don't think I should need to because they must already have an index like that in order to implement the default ordering.

Should I just give up and copy the document ID into the document as a field, so I can sort that way? I know it should be easy from an algorithms perspective to do what I'm trying to do, but I haven't been able to figure out how to do it using the API. Am I missing something?

Edit: I didn't realize this was important, but I'm using the flutterfire firestore library.

I am coding a program in OpenCV where I want to adjust camera position. I would like to know if there is any metric in OpenCV to measure the amount of perspectiveness in two images. How can homography be used to quantify the degree of perspectiveness in two images as follows. The method that comes to my mind is to run edge detection and compare the parallel edge sizes but that method is prone to errors.

I have this 3d rotating text css animation working on chrome desktop, but it won't work on mobile. I tried adding webkit prefixes but nothing will get it to work on iphone. Any obvious reasons why?

https://codepen.io/warpigs666/pen/bGqQbzL

I tried to write a program which takes filepaths as (command line) arguments and returns the first line of each file

I have created an animation of a book with flipping pages. I have used z-index to stack the pages and they are all flipping and displaying in the correct order in the browser UI but inspector is showing something different.

In Firefox, if I right click on page 1 and select inspect it goes to the element for page 4, I also put input elements on the page but on page 1 I can't click and type in it. Some pages do work normally, inspector goes to the right element and I can type in the input but as I flip through the pages, some pages will stop working and others will start working.

This only happens in Firefox, it work exactly as expected in Chrome. Is this a Firefox issue or a problem with my code?

EDIT: https://codesandbox.io/s/my-book-jlrmw?file=/src/components/HelloWorld.vue

Template:

I applied the floodfill function in opencv to extract the foreground from the background but some of the objects in the image were not recognized by the algorithm so I would like to know how I can improve my detections and what modifications are necessary.

I didn't find an answer to this (maybe wrong search), so giving the following simple example:

I have data that were collected from a year but are broken up by months. For my code, I labeled them df1-df12 for each corresponding month. I am trying to group these data using the group_by function to group all the dataframes similarly. When I do the following code- it works fine alone: