Effect of cairo_surface_set_device_scale on already rendered content

Direct Answer

Calling cairo_surface_set_device_scale does not rescale, resample, or modify pixel data that has already been rendered to the surface. The function only updates the device scale factor for subsequent drawing operations performed on that surface. Existing content remains exactly as it was written to the buffer.

Technical Explanation

Cairo treats the device scale as a property of the surface's coordinate transformation. When you invoke cairo_surface_set_device_scale, you are updating the sx and sy factors used to map user-space coordinates to device-space pixels for future operations. Because Cairo surfaces (particularly image and X11 surfaces) operate as raster buffers or command streams, there is no retroactive processing of the pixel grid.

Backend-Specific Implications

• Image Surfaces: The underlying memory buffer is not touched. Old pixels remain; new pixels are drawn at the new scale.
• X11/GPU Surfaces: The framebuffer content is left unchanged. The new scale only affects the next set of drawing commands sent to the window or GPU.
• PDF/SVG/PS: These are recording surfaces. The scale change is emitted as a new operator in the output stream. Content already written to the stream is not rewritten or modified.

Implementation Steps for DPI Adjustments

If your application needs to change the device scale on the fly and ensure the visual output is consistent, follow these steps:

1. Call cairo_surface_set_device_scale to set the new DPI/scale factor.
2. Clear the surface (e.g., using cairo_set_operator(CAIRO_OPERATOR_CLEAR) or filling with a background color).
3. Re-issue all drawing commands to redraw the content at the new scale.
4. Call cairo_surface_flush if you are immediately reading the pixel data from an image surface via cairo_image_surface_get_data to ensure synchronization.

Verification

To verify this behavior in your specific environment, you can use the following scoped test:

// 1. Draw a 10x10 rectangle at (0,0) with scale 1.0
// 2. Capture the pixel data of the surface
// 3. Call cairo_surface_set_device_scale(surface, 2.0, 2.0)
// 4. Draw the same 10x10 rectangle at (0,0)
// 5. Compare pixels: The first 10x10 area should remain unchanged, 
//    while the new drawing should cover a 20x20 area.

Diagnostic Note: To provide a more tailored recommendation, please specify which Cairo backend (e.g., Image, Xlib, PDF) you are using, as this affects how you should handle the final surface flush.