I have rendering issues or crashes, how do I solve it?

Take a look at the troubleshooting page in the C++ manual. You are also welcome to join the RmlUi’s Gitter channel for chatting with other users, or write a post describing your situation in the main repository.

How do I set up custom cursors?

You display custom cursors using your OS cursor facilities. See here for details on responding to the cursor property through the system interface.

Is RmlUi thread safe?

No guarantees are provided in terms of thread safety. Any access into the RmlUi API from multiple threads need to be synchronized such as by using a mutex.

Can I change decorators from script?

It is possible to set decorators by inline style. However, for performance reasons it is highly recommended to change the element’s class instead to affect which decorators are applied to it.

How do I achieve high DPI support?

RmlUi has extensive support for making a scalable user interface. In order to get sizes and lengths scaled properly, the dp length unit should be used extensively in RCSS. This unit can be scaled relative to px units by setting the dp-ratio using the function Context::SetDensityIndependentPixelRatio.

In addition, RmlUi provides the following features which should make high DPI graphics a breeze:

  • Media queries support the resolution feature to toggle styles and sprites based on the dp-ratio.
  • Sprite sheets can specify their desired scaling by using the resolution property.
  • Sprites can be overrided by later @spritesheet rules, making it easy to define high DPI versions of sprites.
  • Decorators and <img> elements automatically update when the dp-ratio changes and new sprites are selected.
  • Sprites in decorators and <img> elements scale according to the source scaling and targeted dp-ratio.

Clients are themselves responsible for querying the platform for the desired scaling ratio, and then setting the dp-ratio on the RmlUi context. Take a look at RmlUi’s included backends for how this is implemented there for different platforms with support for high DPI.

Can I implement hot reloading of documents?

For sure, this is one of the great advantages of having UI document declarations separate from your main application logic.

It is always possible to do a full reload of the document.

Rml::ElementDocument* my_document = context->LoadDocument("main_menu.rml");
// ...
// Later, after RML source was changed
my_document = context->LoadDocument("main_menu.rml");

This could eg. automatically be done any time .rml documents are saved to have your changes be reflected instantly. Users are themselves responsible to implement this automation, look into how to do this on your platform. Note that style sheets and templates are automatically cached, you may want to clear these first by calling Rml::Factory::ClearStyleSheetCache() and Rml::Factory::ClearTemplateCache(), respectively.

Now, clearly any state or programmatically changed elements will be reset during this operation. Occasionally, it is desirable to keep the current state such as when only working on the style of the document. Then, users may call the following.


This only reloads the style sheet(s) applied to the current document, while keeping the document structure and its state intact. This also updates styles declared in the header, but notably not styles declared inline using the element attribute style. You might want to call this automatically for visible documents any time .rcss files are changed. This call automatically clears any cache first.

Finally, the following function may be helpful while editing textures,


which is defined in <RmlUi/Core/Core.h>. This call forces the library to reload all textures in use.

How do I bind to events from C++/script?

If you are using data bindings, you can use the data-event view together with a callback function in C++.

Alternatively, use the Element::AddEventListener function, passing in the Rml::EventId or the name of the event you want to bind to (without the “on” prefix), the listener object to attach, and whether you want to bind in the capture phase or not, as in the following example.

class MyListener : public Rml::EventListener {
	void ProcessEvent(Rml::Event& event) {
		printf("Processing event %s", event.GetType().c_str());

void main() {
	/* ... */
	auto my_listener = std::make_unique<MyListener>();
	element = document->GetElementById("my_button");
	element->AddEventListener(Rml::EventId::Click, my_listener.get(), false);
	/* ... */

See the documentation on event listeners for details.

Finally, it is also possible to respond to inline events such as

<button onclick="game.start()">Start Game</button>

See the documentation on inline events for details.