QRhiRenderPassDescriptor Class | Qt GUI | Qt 6.12.0
Description: Render pass resource.
Keywords: class

- Qt 6.12
- Qt GUI
- C++ Classes
- QtGuiPrivate
- QRhiRenderPassDescriptor
- Qt 6.12.0 Reference Documentation

# QRhiRenderPassDescriptor Class

Render pass resource. More...

Header: | `#include <rhi/qrhi.h>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS GuiPrivate)` `target_link_libraries(mytarget PRIVATE Qt6::GuiPrivate)`
qmake: | `QT += gui-private`
Since: | Qt 6.6
Inherits: | QRhiResource

- List of all members, including inherited members

## Public Functions

virtual bool | isCompatible(const QRhiRenderPassDescriptor *other) const = 0
virtual const QRhiNativeHandles * | nativeHandles()
virtual QRhiRenderPassDescriptor * | newCompatibleRenderPassDescriptor() const = 0
virtual QVector<quint32> | serializedFormat() const = 0

## Reimplemented Public Functions

virtual QRhiResource::Type | resourceType() const override

## Detailed Description

A render pass, if such a concept exists in the underlying graphics API, is a collection of attachments (color, depth, stencil) and describes how those attachments are used.

Note: This is a RHI API with limited compatibility guarantees, see QRhi for details.

## Member Function Documentation

### `[pure virtual]` bool QRhiRenderPassDescriptor::isCompatible(const QRhiRenderPassDescriptor *other) const

Returns true if the other QRhiRenderPassDescriptor is compatible with this one, meaning `this` and other can be used interchangeably in QRhiGraphicsPipeline::setRenderPassDescriptor().

The concept of the compatibility of renderpass descriptors is similar to the layout compatibility of QRhiShaderResourceBindings instances. They allow better reuse of QRhiGraphicsPipeline instances: for example, a QRhiGraphicsPipeline instance cache is expected to use these functions to look for a matching pipeline, instead of just comparing pointers, thus allowing a different QRhiRenderPassDescriptor and QRhiShaderResourceBindings to be used in combination with the pipeline, as long as they are compatible.

The exact details of compatibility depend on the underlying graphics API. Two renderpass descriptors created from the same QRhiTextureRenderTarget are always compatible.

Similarly to QRhiShaderResourceBindings, compatibility can also be tested without having two existing objects available. Extracting the opaque blob by calling serializedFormat() allows testing for compatibility by comparing the returned vector to another QRhiRenderPassDescriptor's serializedFormat(). This has benefits in certain situations, because it allows testing the compatibility of a QRhiRenderPassDescriptor with a QRhiGraphicsPipeline even when the QRhiRenderPassDescriptor the pipeline was originally built with is no longer available (but the data returned from its serializedFormat() still is).

See also newCompatibleRenderPassDescriptor() and serializedFormat().

### `[virtual]` const QRhiNativeHandles *QRhiRenderPassDescriptor::nativeHandles()

Returns a pointer to a backend-specific QRhiNativeHandles subclass, such as QRhiVulkanRenderPassNativeHandles. The returned value is `nullptr` when exposing the underlying native resources is not supported by the backend.

See also QRhiVulkanRenderPassNativeHandles.

### `[pure virtual]` QRhiRenderPassDescriptor *QRhiRenderPassDescriptor::newCompatibleRenderPassDescriptor() const

Returns a new QRhiRenderPassDescriptor that is compatible with this one.

This function allows cloning a QRhiRenderPassDescriptor. The returned object is ready to be used, and the ownership is transferred to the caller. Cloning a QRhiRenderPassDescriptor object can become useful in situations where the object is stored in data structures related to graphics pipelines (in order to allow creating new pipelines which in turn requires a renderpass descriptor object), and the lifetime of the renderpass descriptor created from a render target may be shorter than the pipelines. (for example, because the engine manages and destroys renderpasses together with the textures and render targets it was created from) In such a situation, it can be beneficial to store a cloned version in the data structures, and thus transferring ownership as well.

See also isCompatible().

### `[override virtual]` QRhiResource::Type QRhiRenderPassDescriptor::resourceType() const

Reimplements: QRhiResource::resourceType() const.

Returns the resource type.

### `[pure virtual]` QVector<quint32> QRhiRenderPassDescriptor::serializedFormat() const

Returns a vector of integers containing an opaque blob describing the data relevant for compatibility.

Given two QRhiRenderPassDescriptor objects `rp1` and `rp2`, if the data returned from this function is identical, then `rp1->isCompatible(rp2)`, and vice versa hold true as well.

Note: The returned data is meant to be used for storing in memory and comparisons during the lifetime of the QRhi the object belongs to. It is not meant for storing on disk, reusing between processes, or using with multiple QRhi instances with potentially different backends.

Note: Calling this function is expected to be a cheap operation since the backends are not supposed to calculate the data in this function, but rather return an already calculated series of data.

When creating reusable components as part of a library, where graphics pipelines are created and maintained while targeting a QRhiRenderTarget (be it a swapchain or a texture) managed by the client of the library, the components must be able to deal with a changing QRhiRenderPassDescriptor. For example, because the render target changes and so invalidates the previously QRhiRenderPassDescriptor (with regards to the new render target at least) due to having a potentially different color format and attachments now. Or because variable rate shading is taken into use dynamically. A simple pattern that helps dealing with this is performing the following check on every frame, to recognize the case when the pipeline needs to be associated with a new QRhiRenderPassDescriptor, because something is different about the render target now, compared to earlier frames:

     QRhiRenderPassDescriptor *rp = m_renderTarget->renderPassDescriptor();
     if (m_pipeline && rp->serializedFormat() != m_renderPassFormat) {
         m_pipeline->setRenderPassDescriptor(rp);
         m_renderPassFormat = rp->serializedFormat();
         m_pipeline->create();
     }
     // remember to store m_renderPassFormat also when creating m_pipeline the first time

See also isCompatible().