Access pixel values in the current view

[ivan-katkov]

It might be useful to provide a method for retrieving pixel values of the displayed HiPS layer for the current view at a given zoom level.

One usecase, particularly valuable for FITS HiPS maps of physical parameters, is generating histograms to assist with dynamic image cuts. This could be easily implemented if healpix values were accessible in the layer objects.

[skybber]

It would be great to have this option available. It was possible in version 2, and we're currently using it in our application: CzSky - Comet K23A030 . I'm now trying to port this functionality to version 3 and are looking for a way to do it.

[tboch]

Hi @skybber . Can you specify what functions/API were available in v2, and no longer in v3? Thanks.

[skybber]

I'm using some "internals" from v2:

aladin.view.imageCanvas
https://github.com/skybber/czsky/blob/198ae49d11274ff7637d3a8fd762498965774ec8/app/static/js/fchart.js#L441

hook on redrawFinised event:
https://github.com/skybber/czsky/blob/198ae49d11274ff7637d3a8fd762498965774ec8/app/static/js/fchart.js#L235C9-L235C41

gotoRaDec:
https://github.com/skybber/czsky/blob/198ae49d11274ff7637d3a8fd762498965774ec8/app/static/js/fchart.js#L898

setZoom:
https://github.com/skybber/czsky/blob/198ae49d11274ff7637d3a8fd762498965774ec8/app/static/js/fchart.js#L904

access to aladin.getBaseImageLayer().getColorMap()
https://github.com/skybber/czsky/blob/198ae49d11274ff7637d3a8fd762498965774ec8/app/static/js/fchart.js#L228

aladin.view.imageSurvey.flipX (my extension to mirror x/y)
https://github.com/skybber/czsky/blob/198ae49d11274ff7637d3a8fd762498965774ec8/app/static/js/fchart.js#L226

[skybber]

I've found, that nearly all API from v2 is working on v3, there are alternatives for missing ones :

aladin.setZoom() -> aladin.view.setZoom()

pixels from canvas are accessible using :

ctx.drawImage(aladin.view.imageCanvas, 0, 0)

[tboch]

aladin.setZoom() is now aladin.setFoV()

As for access to pixels, we will propose something a bit less messy than direct access to aladin.view.imageCanvas which might not be available in future versions.

[skybber]

I've also noticed that aladin.gotoRaDec(..) does not instantly update the imageCanvas. Is there any mechanism to force an update immediately after gotoRaDec() ?