Pixelated result using SVG as label background?

Question

I have points that are labelled with a 2-digit numeric attribute, shown by default with a circle background (white fill, coloured outline of 1 map unit, buffer of 1 map unit beyond the label).

I have been asked to show deleted points (deleted since the last iteration of the map) with a diagonal slash through the number. I am attempting to do this using an SVG as the label background, but the results are pixelated. The width of the stroke element of the SVG also seems to change with zoom level. I am not offered an option for the units of stroke width on the SVG background, just "symbol units".

Are there settings I should check either in the original SVG file or in QGIS that might help with either or both of these?

I am using this SVG: https://commons.wikimedia.org/wiki/File:Cross_within_circle_2.svg - I do not know why only one of the diagonal lines shows up (one or both would be fine). I don't know SVGs well but I am familiar with markup languages in principle and happy to attempt edits if provided with a little guidance.

Alternatively, is there another way that I can have the labels mimic the non-deleted points, just with the addition of a diagonal line?

I have suggested just using strikethrough text but this is deemed not clear enough by the recipient. However, at present it looks like a better alternative if I cannot fix the SVG issue.

(QGIS 3.4.7, Win10)

Answer 1

I'm not sure what's going wrong with your SVG symbol, but it's pretty simple to re-create a similar symbol using simple marker symbol levels. With simple markers, you can control the symbol size and stroke width using millimeters, points, pixels, meters at scale, map units, or inches.

Add three symbol levels:

1. Simple marker - line, 45 degree rotation. Shown below with a 7mm circle, 0.5mm line width.

2. Font marker. Shown below with 4mm size.

   Use this instead of a label if you want the line symbol to cross over (and partially obscure) the label. Select your "label" field from the data-defined control menu next to the character selection (see below).

Otherwise, omit this symbol level and keep the existing point labels. Use data-defined placement settings to control the label placement so they're always placed on top of the point symbol.

3. Default simple marker (circle), transparent fill. Shown below with a 5mm circle in the example.

Note: even though all three of the symbol levels are anchored at the center point, the font marker appears slightly off-center. You can add a small offset to get it properly centered. A -0.5mm Y-offset (not shown) worked well for my example.

Adjust the colors and line widths to meet your needs. Try it with a square, diamond, pentagon or hexagon instead of a circle.

15mm diamond marker, 3mm stroke width

Answer 2

Further testing:

The pixelation is replicable - it is caused when there is a non-zero rotation applied to the SVG label background, or to the label as a whole via Placement. However, on export to PDF the background symbol is sharp.

The resizing behaviour is not replicable. I made a new label from scratch using the same settings and SVG, and it works as expected, with the symbol thickness remaining constant at all zoom levels. I have been through all label options several times and cannot see any setting that is different. The 'problem' label also behaves fine with any other SVG. A mystery!

Apologies for not testing more exhaustively in the first place. As it was my first time using an SVG background I assumed it was my error. Hopefully this helps anyone in future with the apparent pixelation issue, at least.