Can cairo_surface_set_device_scale double‑scale when CTM already includes scaling?

Direct answer

Yes. When cairo_surface_set_device_scale() is called after the current transformation matrix (CTM) already contains a scaling factor, the two scale factors compose multiplicatively. The overall mapping from logical user coordinates to device pixels becomes CTM_scale × device_scale. If both are set to 2×, the effective pixel density is 4×, which appears as double scaling relative to the intended display scaling.

Composition order

Cairo applies transformations in this sequence: user space → current transformation matrix (CTM) → device space → device scale → pixel space. The device scale is a surface‑level multiplier that sits after the CTM. This means any scaling already present in the CTM will be multiplied by the device scale value you set.

Confirmed facts

1. Device scale is applied after the CTM.
2. Multiplicative effect: if the CTM includes a 2× scale and you set the device scale to 2×, the resulting pixel scaling is 2× × 2× = 4×.
3. Retroactive changes do not rescale existing content: adjusting the device scale after drawing only affects subsequent operations; already‑drawn graphics retain the scale that was in effect when they were rendered.
4. Backend dependence: some backends (PDF, SVG, PS) are resolution‑independent and ignore the device scale for final output, though it still influences the internal coordinate system for any drawing that relies on pixel metrics.

Steps to avoid unintended double scaling

• If the CTM already provides the scaling you need, set the device scale to 1.0 (or omit the call) so the overall effect remains just the CTM scaling.
• If you must use cairo_surface_set_device_scale(), compensate by adjusting the CTM (e.g., remove the extra scale factor) so the product stays at the intended level.
• Set the device scale any drawing commands if you want the scale to apply uniformly to all content on that surface.
• After finishing a scaled drawing block, restore the device scale to its default 1.0, especially if the same surface will later be used for unscaled output.
• Test on your target backend (image surface, PDF, SVG, etc.) to confirm whether the device scale influences the final rendered output, since behavior varies.

Missing diagnostic detail

Which Cairo backend are you using? (e.g., image surface, PDF, SVG, PS) – the device scale’s effect on final output is backend‑specific, and the recommendation to compensate or adjust depends on this choice.