VkSubpassDescription

The VkSubpassDescription structure is defined as:

typedef struct VkSubpassDescription {
    VkSubpassDescriptionFlags flags;
    VkPipelineBindPoint pipelineBindPoint;
    uint32_t inputAttachmentCount;
    const VkAttachmentReference* pInputAttachments;
    uint32_t colorAttachmentCount;
    const VkAttachmentReference* pColorAttachments;
    const VkAttachmentReference* pResolveAttachments;
    const VkAttachmentReference* pDepthStencilAttachment;
    uint32_t preserveAttachmentCount;
    const uint32_t* pPreserveAttachments;
} VkSubpassDescription;

• flags is a bitmask of VkSubpassDescriptionFlagBits specifying usage of the subpass.
• pipelineBindPoint is a VkPipelineBindPoint value specifying the pipeline type supported for this subpass.
• inputAttachmentCount is the number of input attachments.
• pInputAttachments is a pointer to an array of VkAttachmentReference structures defining the input attachments for this subpass and their layouts.
• colorAttachmentCount is the number of color attachments.
• pColorAttachments is a pointer to an array of colorAttachmentCount VkAttachmentReference structures defining the color attachments for this subpass and their layouts.
• pResolveAttachments is NULL or a pointer to an array of colorAttachmentCount VkAttachmentReference structures defining the resolve attachments for this subpass and their layouts.
• pDepthStencilAttachment is a pointer to a VkAttachmentReference structure specifying the depth/stencil attachment for this subpass and its layout.
• preserveAttachmentCount is the number of preserved attachments.
• pPreserveAttachments is a pointer to an array of preserveAttachmentCount render pass attachment indices identifying attachments that are not used by this subpass, but whose contents must be preserved throughout the subpass.

Each element of the pInputAttachments array corresponds to an input attachment index in a fragment shader, i.e. if a shader declares an image variable decorated with a InputAttachmentIndex value of X, then it uses the attachment provided in pInputAttachments[X]. Input attachments must also be bound to the pipeline in a descriptor set. If the attachment member of any element of pInputAttachments is VK_ATTACHMENT_UNUSED, the application must not read from the corresponding input attachment index. Fragment shaders can use subpass input variables to access the contents of an input attachment at the fragment’s (x, y, layer) framebuffer coordinates. Input attachments must not be used by any subpasses within a render pass that enables render pass transform.

Each element of the pColorAttachments array corresponds to an output location in the shader, i.e. if the shader declares an output variable decorated with a Location value of X, then it uses the attachment provided in pColorAttachments[X]. If the attachment member of any element of pColorAttachments is VK_ATTACHMENT_UNUSED, or if Color Write Enable has been disabled for the corresponding attachment index, then writes to the corresponding location by a fragment shader are discarded.

If flags does not include VK_SUBPASS_DESCRIPTION_SHADER_RESOLVE_BIT_QCOM, and if pResolveAttachments is not NULL, each of its elements corresponds to a color attachment (the element in pColorAttachments at the same index), and a multisample resolve operation is defined for each attachment unless the resolve attachment index is VK_ATTACHMENT_UNUSED.

Similarly, if flags does not include VK_SUBPASS_DESCRIPTION_SHADER_RESOLVE_BIT_QCOM, and VkSubpassDescriptionDepthStencilResolve::pDepthStencilResolveAttachment is not NULL and does not have the value VK_ATTACHMENT_UNUSED, it corresponds to the depth/stencil attachment in pDepthStencilAttachment, and multisample resolve operation for depth and stencil are defined by VkSubpassDescriptionDepthStencilResolve::depthResolveMode and VkSubpassDescriptionDepthStencilResolve::stencilResolveMode, respectively. If VkSubpassDescriptionDepthStencilResolve::depthResolveMode is VK_RESOLVE_MODE_NONE or the pDepthStencilResolveAttachment does not have a depth aspect, no resolve operation is performed for the depth attachment. If VkSubpassDescriptionDepthStencilResolve::stencilResolveMode is VK_RESOLVE_MODE_NONE or the pDepthStencilResolveAttachment does not have a stencil aspect, no resolve operation is performed for the stencil attachment.

If the image subresource range referenced by the depth/stencil attachment is created with VK_IMAGE_CREATE_SAMPLE_LOCATIONS_COMPATIBLE_DEPTH_BIT_EXT, then the multisample resolve operation uses the sample locations state specified in the sampleLocationsInfo member of the element of the VkRenderPassSampleLocationsBeginInfoEXT::pPostSubpassSampleLocations for the subpass.

If pDepthStencilAttachment is NULL, or if its attachment index is VK_ATTACHMENT_UNUSED, it indicates that no depth/stencil attachment will be used in the subpass.

The contents of an attachment within the render area become undefined: at the start of a subpass S if all of the following conditions are true:

• The attachment is used as a color, depth/stencil, or resolve attachment in any subpass in the render pass.
• There is a subpass S₁ that uses or preserves the attachment, and a subpass dependency from S₁ to S.
• The attachment is not used or preserved in subpass S.

In addition, the contents of an attachment within the render area become undefined: at the start of a subpass S if all of the following conditions are true:

• VK_SUBPASS_DESCRIPTION_SHADER_RESOLVE_BIT_QCOM is set.
• The attachment is used as a color or depth/stencil in the subpass.

Once the contents of an attachment become undefined: in subpass S, they remain undefined: for subpasses in subpass dependency chains starting with subpass S until they are written again. However, they remain valid for subpasses in other subpass dependency chains starting with subpass S₁ if those subpasses use or preserve the attachment.