Idea about how to detect whether a layer is actually showing something in the current view extent

[mapcentia]

Vidi users have requested a visual indication of whether a layer is actually showing something in the current view extent. An idea is to check the bitmap tile of a layer for transparency. If it's 100% transparent a visual indicator in the layer tree will be switch on.
Could this be done using a canvas element?

[sashuk]

@mapcentia Could you please specify if this feature supposed to be working only for raster tile layers?

[mapcentia]

@mapcentia yes

[sashuk]

@mapcentia Leaflet uses canvas to draw tiles, so it can be checked if anything is drawn on it, I will check it!

[sashuk]

Following script answers the question is specific canvas empty?:

$('canvas').each((index, canvas) => {
    let canvasHasData = false;
    let data = canvas.getContext('2d').getImageData(0, 0, canvas.width, canvas.height).data;
    for (let key in data) {
        if (data[key] !== 0) {
            console.log('First not 0 value for ' + index + ' (' + data[key] + '), exiting');
            canvasHasData = true;
            break;
        }
    }    

    console.log('Canvas ' + index + ' has data: ' + canvasHasData);
});

Problems:

• using the getImageData() is pretty slow

Researching further

[sashuk]

@mapcentia Whenever tile layer is loaded, its canvas is checked for data (if there is data, then the green 👁️ icon appears near the layer name). When user moves away (including the buffer), then icon becomes hidden.

The code is in separate branch https://github.com/sashuk/vidi/tree/tile-content or you can test it at https://vidi.alexshumilov.ru/app/aleksandrshumilov/public/#stamenTonerLite/4/60.293/35.8534/public.polygons

However, the solution (1) makes application very slow and (2) buffered data (which can be already hidden from user) is taken into account anyway. For now I have not found how to get the part of canvas which is actually in viewport.

Demo: