Thanks for all the guidance @jcupitt . I've followed your suggestions and seem to have it almost working.

The remaining issue has probably something to do with how gamma is being handled by Cairo (as you suggested it's linear) since my gradients seem to be brighter than expected.

Simply changing the colourspace with vips colourspace output.v output_srgb.v srgb doesn't seem to work as it also changes the format to UCHAR. Can you give me an advice how to proceed from this point?

Additionally - my ultimate goal is to overlay PNG image with rendered SVG (VIPS_FORMAT_FLOAT) gradient. I guess in order to make sure it's smooth I need to:

1. Convert PNG VipsImage to VIPS_FORMAT_FLOAT
2. Make sure to use 16-bit depth while outputting PNG.

Does that make sense?

I'd take the PNG image to scRGB and do the composite there. In python (untested!):

base = pyvips.Image.svgload("gradient.svg", rgb128=True)
overlay = pyvips.Image.pngload("thing.png")
final = base.composite2(overlap, "over", compositing_space="scrgb", x=100, y=200)

Then save to some high bit depth format I suppose.

composite works in srgb space by default (ie. with a gamma), but you can use any other supported colourspace. You probably need to use one of the linear spaces, and scrgb is the simplest. I think xyz would also work.

Thank you @jcupitt for all your guidance. Here's my attempt to introduce the increased color depth support based on your suggestions: #4523 Any feedback will be very appreciated.

Well done! I'll have a look.