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Set the virtual display size in xvfb-run, not with DPI. Pass Xvfb a screen definition in the form WIDTHxHEIGHTxDEPTH:
xvfb-run --server-args="-screen 0 1920x1080x24" wkhtmltopdf --use-xserver input.html output.pdf
xvfb-run --server-args="-screen 0 1920x1080x24" wkhtmltoimage --width 1920 --height 1080 --disable-smart-width input.html output.pngThe Xvfb geometry controls the virtual X display. wkhtmltoimage then applies its own image viewport settings, while wkhtmltopdf uses the X display when its build requires one.
How the resolution settings fit together
There are several independent layers, which is why changing one flag often appears to do nothing.
| Layer | Setting | What it controls |
|---|---|---|
| Xvfb virtual display | --server-args="-screen 0 WIDTHxHEIGHTxDEPTH" |
The virtual screen width, height and color depth. |
| wkhtmltopdf | --use-xserver |
Tells builds that need X to use the X server supplied by xvfb-run. |
| wkhtmltoimage | --width, --height |
The image renderer’s screen-width and screen-height guides. |
| wkhtmltoimage | --disable-smart-width |
Keeps the requested width strict instead of allowing smart-width adjustment. |
| wkhtmltopdf | --dpi |
Output DPI-related behavior; it does not change X11 screen geometry. |
| Library/API callers | screenWidth, smartWidth |
Image settings corresponding to renderer viewport behavior, separate from Xvfb geometry. |
Think of Xvfb as the desktop-sized canvas and wkhtmltoimage as the program deciding how much of that canvas to use. A larger virtual display does not, by itself, force every renderer output to have the same pixel dimensions.
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Set the screen resolution for wkhtmltopdf
Use this pattern when generating a PDF from an HTML file or URL:
xvfb-run --server-args="-screen 0 1920x1080x24" wkhtmltopdf --use-xserver input.html output.pdf- Choose the geometry. Replace
1920x1080x24with the width, height and depth your layout needs. The final value,24, is the color depth; it is not a second height or a DPI value. - Pass it through
--server-args. The option belongs toxvfb-run. Its quoted value is forwarded to Xvfb. - Keep wkhtmltopdf options after the Xvfb wrapper. Put
--use-xserverand the input/output arguments on the wkhtmltopdf command. - Verify the resulting PDF. If the page still wraps unexpectedly, inspect the PDF’s page-size and layout options separately; changing X11 geometry is not a substitute for PDF page configuration.
For a URL, replace input.html with the URL, for example:
xvfb-run --server-args="-screen 0 1366x768x24" wkhtmltopdf --use-xserver https://example.com report.pdfThe exact need for --use-xserver depends on the wkhtmltopdf build. Include it when that build requires an X server; it makes the dependency explicit and works with the X display created by xvfb-run.
Set the screen resolution for wkhtmltoimage
wkhtmltoimage has renderer-level width and height controls, so set both the Xvfb display and the image viewport when you need predictable output:
xvfb-run --server-args="-screen 0 1920x1080x24" wkhtmltoimage --width 1920 --height 1080 --disable-smart-width input.html output.pngHere, Xvfb creates a 1920 by 1080 display at 24-bit depth. wkhtmltoimage receives the same width and height as its screen guide, and --disable-smart-width prevents smart-width behavior from widening the requested image.
Capture a fixed-width image
If the required contract is an exact width, keep --width and --disable-smart-width together:
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xvfb-run --server-args="-screen 0 1200x900x24" wkhtmltoimage --width 1200 --height 900 --disable-smart-width https://example.com page.pngUse a virtual height that is at least as large as the viewport you expect the page to occupy. For a full-page capture, the renderer’s page behavior determines how the document extends beyond the initial viewport; the Xvfb width still needs to accommodate the intended layout.
Why an image may still look like 800 by 600
That result usually means only one layer was changed. Check all three values:
- The
-screen 0 WIDTHxHEIGHTxDEPTHargument passed to Xvfb. - The wkhtmltoimage
--widthand--heightvalues. - Whether smart-width behavior was disabled when strict width was required.
Changing --dpi will not resize the X11 display. On X11-based systems, wkhtmltopdf documentation states that its DPI option has no effect on X11 geometry.
Understand the -screen 0 WIDTHxHEIGHTxDEPTH syntax
The Xvfb manual defines -screen screennum WxHxD. In the common form -screen 0 1920x1080x24:
0is the screen number.1920is the screen width in pixels.1080is the screen height in pixels.24is the color depth.
Do not write spaces inside the geometry token. The complete server argument is normally quoted so the shell passes it as one value:
--server-args="-screen 0 1920x1080x24"xvfb-run starts Xvfb, runs the wrapped command against that display, and handles the temporary display setup. The resolution is therefore selected before wkhtmltopdf or wkhtmltoimage starts.
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Distribution defaults and reproducible builds
The default Xvfb geometry is package-specific. Debian testing documents a default of 1280x1024x24, while older Ubuntu and Debian manual pages document other defaults. A command that omits --server-args can consequently produce different wrapping or image dimensions after a package or distribution change.
For repeatable CI and production output, always specify the geometry explicitly. Record the width, height and depth alongside the wkhtmltopdf or wkhtmltoimage command, and use the same values in local debugging and automation. If you intentionally depend on a distribution default, document the package version and distribution because that default is not universal.
Library and API settings
If you call the renderer through a library instead of the command line, look for the image settings named screenWidth and smartWidth. They control renderer behavior and are separate from the Xvfb screen geometry.
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- Set
screenWidthto the intended pixel width when the library exposes that setting. - Turn the library’s
smartWidthswitch off when the requested width must remain strict. - Still configure Xvfb with
-screen 0 WIDTHxHEIGHTxDEPTHwhen the library depends on an X display.
Matching the values across both layers avoids a narrow virtual screen constraining a wider renderer request, while keeping the distinction clear when diagnosing output-size problems.
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Troubleshooting checklist
The command says it cannot open a display
Confirm that the command is wrapped by xvfb-run and that the Xvfb package is installed. Keep the wkhtml command inside the wrapper; running wkhtmltopdf separately will not inherit the temporary display.
The page is clipped or wraps at the wrong breakpoint
Increase the Xvfb width and set the same intended width with wkhtmltoimage’s --width. If exact width matters, add --disable-smart-width. For wkhtmltopdf, inspect PDF page and margin settings separately because Xvfb geometry does not define PDF paper size.
Changing DPI had no visible effect
That is expected for X11 geometry. Replace the DPI change with an explicit --server-args="-screen 0 WIDTHxHEIGHTxDEPTH" value and, for images, set --width and --height.
Output differs between machines
Compare the installed Xvfb and wkhtml packages and look for an omitted geometry argument. Distribution defaults vary, so make the screen definition explicit in every automated command.
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The requested image width is not respected
Check that the command is using wkhtmltoimage rather than wkhtmltopdf, that --width is present, and that --disable-smart-width follows the renderer options. Also verify that the Xvfb width is not smaller than the requested viewport.
Practical configuration checklist
- Choose width, height and depth deliberately.
- Pass them through
xvfb-run --server-args. - Use
--use-xserverfor wkhtmltopdf builds that require X. - Set wkhtmltoimage
--widthand--heightwhen producing images. - Use
--disable-smart-widthfor a strict image width. - Do not treat
--dpias an Xvfb resolution switch. - Record the explicit geometry so CI and local runs use the same display.
FAQ
Where should --server-args appear when the renderer has many options?
Put it on xvfb-run before the wkhtml command. Renderer-specific flags such as --use-xserver, --width and --disable-smart-width belong after the wrapper command.
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Is the depth value another resolution dimension?
No. In WIDTHxHEIGHTxDEPTH, the third value is color depth. Width and height are the two pixel dimensions.
Do I need both Xvfb geometry and renderer dimensions?
For wkhtmltoimage, use both when you need predictable viewport output. Xvfb defines the available virtual display; wkhtmltoimage’s width, height and smart-width settings define how the image renderer uses it.
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Can I use a different screen number than 0?
Yes, Xvfb’s syntax names the screen explicitly, but the standard xvfb-run examples and the commands here use screen 0. Keep the number consistent with the server arguments your wrapper starts.
Does a larger Xvfb display automatically create a larger PDF?
No. Xvfb supplies the virtual X geometry; wkhtmltopdf’s PDF page, margin and layout settings remain separate.
What should I change first when a responsive page selects the wrong breakpoint?
Set an explicit Xvfb width, then align wkhtmltoimage’s width with it and disable smart-width if the image width must be exact.
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