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@ahocevar This is ready for review as it fixes the immediate bugs.

In ReprojDataTile the pixel ratio was not included in the returned image dimensions (resulting in incomplete images). In some cases inconsistent rounding of fractional values resulted in corrupted images.

In glreproj the pixel ratio was not being included in the stitchTexture sizing, resulting in reduced quality (in the extreme case of half the rows and columns of black and white pixels being removed without interpolation the result was all black or all white).

In common with other hidpi rendering tests a27fba6 only demonstrates that when correctly configured the output is correct in a standard dpi screenshot (i.e. all of the source tile is used and all the reprojected data is exported in the reproj tile image). There are currently no rendering tests which demonstrate the map canvas is genuinely hidpi.

If pixelRatio: 2 is specified in the map options this is the map canvas export without d983fa2 blurred because the reprojection has not considered the tile pixel ratio in the stitch texture

and much sharper with d983fa2 particularly noticeable with the labels in the top right quadrant.

A regular rendering test would not detect the difference in a standard dpi screenshot. A test which scaled the map div might work if it was compliant with the limitations of #14199

const style = document.getElementById('map').style;
style.position = 'relative';
style.left = '-128px';
style.top = '128px';
style.transform = 'scale(2)';