Fixed framerate

[underdoeg]

Is there already any mechanism to set a fixed framerate? in our case we are currently aiming for half of vertical sync, so we target something like 30 FPS to ensure a stable render. (and then think about ramping it up to 60 afterwards...)

[tgfrerer]

It's important in this case to distinguish between display frame rate and render frame rate.

In Vulkan, and by extension Island, choosing the right type of swapchain, and PresentMode is how you can influence frame rates.

In Island, you can select the Presentmode for your window swapchain using setPresentMode, see for example:

island/apps/examples/hello_world/hello_world_app/hello_world_app.cpp

Lines 123 to 136 in 2b718dc

	auto rendererInfo = le::RendererInfoBuilder()
	.addSwapchain()
	.setFormatHint( le::Format::eB8G8R8A8Unorm )
	.setWidthHint( 1920 )
	.setHeightHint( 1080 )
	.setImagecountHint( 3 )
	.asWindowSwapchain()
	.setWindow( app->window )
	.setPresentmode( le::Presentmode::eFifo )
	.end()
	.end()
	.build();
	app->renderer.setup( rendererInfo );

The types of PresentMode available are enumerated here:

island/modules/le_renderer/private/le_renderer_types.h

Lines 1025 to 1033 in 2b718dc

	enum class Presentmode : uint32_t {
	eDefault = 0,
	eImmediate,
	eMailbox,
	eFifo,
	eFifoRelaxed,
	eSharedDemandRefresh,
	eSharedContinuousRefresh,
	};

Note that Vulkan only requires drivers to implement FIFO mode (which is the default vsynced behaviour) and therefore the fallback in case a selected mode is not available on a particular system.

How each PresentMode is supposed to work in detail is described best in the vulkan spec