This extension provides a mechanism for applications to enable driver support for render pass transform.

Mobile devices can be rotated and mobile applications need to render properly when a device is held in a landscape or portrait orientation. When the current orientation differs from the device’s native orientation, a rotation is required so that the “up” direction of the rendered scene matches the current orientation.

If the Display Processing Unit (DPU) does not natively support rotation, the Vulkan presentation engine can handle this rotation in a separate composition pass. Alternatively, the application can render frames “pre-rotated” to avoid this extra pass. The latter is preferred to reduce power consumption and achieve the best performance because it avoids tasking the GPU with extra work to perform the copy/rotate operation.

Unlike OpenGL ES, the burden of pre-rotation in Vulkan falls on the application. To implement pre-rotation, applications render into swapchain images matching the device native aspect ratio of the display and “pre-rotate” the rendering content to match the device’s current orientation. The burden is more than adjusting the Model View Projection (MVP) matrix in the vertex shader to account for rotation and aspect ratio. The coordinate systems of scissors, viewports, derivatives and several shader built-ins may need to be adapted to produce the correct result.

It is difficult for some game engines to manage this burden; many chose to simply accept the performance/power overhead of performing rotation in the presentation engine.

This extension allows applications to achieve the performance benefits of pre-rotated rendering by moving much of the above-mentioned burden to the graphics driver. The following is unchanged with this extension:

• Applications create a swapchain matching the native orientation of the display. Applications must also set the VkSwapchainCreateInfoKHR::preTransform equal to the currentTransform as returned by GetPhysicalDeviceSurfaceCapabilitiesKHR.

The following is changed with this extension:

• At CmdBeginRenderPass, the application provides extension struct VkRenderPassTransformBeginInfoQCOM specifying the render pass transform parameters.
• At BeginCommandBuffer for secondary command buffers, the application provides extension struct VkCommandBufferInheritanceRenderPassTransformInfoQCOM specifying the render pass transform parameters.
• The renderArea, viewports, scissors, and fragmentSize are all provided in the current (non-rotated) coordinate system. The implementation will transform those into the native (rotated) coordinate system.
• The implementation is responsible for transforming shader built-ins (FragCoord, PointCoord, SamplePosition, PrimitiveShadingRateKHR, interpolateAt(), dFdx, dFdy, fWidth) into the rotated coordinate system.
• The implementation is responsible for transforming position to the rotated coordinate system.
• If this extension is used with VK_QCOM_tile_properties, then GetFramebufferTilePropertiesQCOM and GetDynamicRenderingTilePropertiesQCOM return tile properties in the rotated coordinate space.

VK_QCOM_render_pass_transform

Name String

VK_QCOM_render_pass_transform

Extension Type

Device extension

Registered Extension Number

283

Revision

3

Extension and Version Dependencies

VK_KHR_swapchain and VK_KHR_surface

Contact

• Jeff Leger jackohound

Other Extension Metadata

Last Modified Date

2022-07-21

Interactions and External Dependencies

• This extension requires VK_KHR_swapchain
• This extension interacts with VK_EXT_fragment_density_map
• This extension interacts with VK_KHR_fragment_shading_rate
• This extension interacts with VK_QCOM_tile_properties

Contributors

• Jeff Leger, Qualcomm Technologies, Inc.
• Brandon Light, Qualcomm Technologies, Inc.
• Matthew Netsch, Qualcomm Technologies, Inc.