## 25.9. Stencil Test

The stencil test conditionally disables coverage of a sample based on the outcome of a comparison between the stencil value in the depth/stencil attachment at location $(x_f,y_f)$ (for the appropriate sample) and a reference value. The stencil test also updates the value in the stencil attachment, depending on the test state, the stencil value and the stencil write masks. The test is enabled or disabled by the stencilTestEnable member of VkPipelineDepthStencilStateCreateInfo.

When disabled, the stencil test and associated modifications are not made, and the sample’s coverage is not modified.

The stencil test is controlled with the front and back members of VkPipelineDepthStencilStateCreateInfo which are of type VkStencilOpState.

The VkStencilOpState structure is defined as:

typedef struct VkStencilOpState {
VkStencilOp    failOp;
VkStencilOp    passOp;
VkStencilOp    depthFailOp;
VkCompareOp    compareOp;
uint32_t       reference;
} VkStencilOpState;

• failOp is the action performed on samples that fail the stencil test.
• passOp is the action performed on samples that pass both the depth and stencil tests.
• depthFailOp is the action performed on samples that pass the stencil test and fail the depth test.
• compareOp is the comparison operator used in the stencil test.
• compareMask selects the bits of the unsigned integer stencil values participating in the stencil test.
• writeMask selects the bits of the unsigned integer stencil values updated by the stencil test in the stencil framebuffer attachment.
• reference is an integer reference value that is used in the unsigned stencil comparison.

There are two sets of stencil-related state, the front stencil state set and the back stencil state set. Stencil tests and writes use the front set of stencil state when processing fragments rasterized from non-polygon primitives (points and lines) and front-facing polygon primitives while the back set of stencil state is used when processing fragments rasterized from back-facing polygon primitives. For the purposes of stencil testing, a primitive is still considered a polygon even if the polygon is to be rasterized as points or lines due to the current VkPolygonMode. Whether a polygon is front- or back-facing is determined in the same manner used for face culling (see Basic Polygon Rasterization).

The operation of the stencil test is also affected by the compareMask, writeMask, and reference members of VkStencilOpState set in the pipeline state object if the pipeline state object is created without the VK_DYNAMIC_STATE_STENCIL_COMPARE_MASK, VK_DYNAMIC_STATE_STENCIL_WRITE_MASK, and VK_DYNAMIC_STATE_STENCIL_REFERENCE dynamic states enabled, respectively.

If the pipeline state object is created with the VK_DYNAMIC_STATE_STENCIL_COMPARE_MASK dynamic state enabled, then to dynamically set the stencil compare mask call:

void vkCmdSetStencilCompareMask(
VkCommandBuffer                             commandBuffer,
uint32_t                                    compareMask);

• commandBuffer is the command buffer into which the command will be recorded.
• faceMask is a bitmask specifying the set of stencil state for which to update the compare mask. Bits which can be set include:

typedef enum VkStencilFaceFlagBits {
VK_STENCIL_FACE_FRONT_BIT = 0x00000001,
VK_STENCIL_FACE_BACK_BIT = 0x00000002,
VK_STENCIL_FRONT_AND_BACK = 0x00000003,
} VkStencilFaceFlagBits;

• VK_STENCIL_FACE_FRONT_BIT indicates that only the front set of stencil state is updated.
• VK_STENCIL_FACE_BACK_BIT indicates that only the back set of stencil state is updated.
• VK_STENCIL_FRONT_AND_BACK is the combination of VK_STENCIL_FACE_FRONT_BIT and VK_STENCIL_FACE_BACK_BIT and indicates that both sets of stencil state are updated.
• compareMask is the new value to use as the stencil compare mask.

If the pipeline state object is created with the VK_DYNAMIC_STATE_STENCIL_WRITE_MASK dynamic state enabled, then to dynamically set the stencil write mask call:

void vkCmdSetStencilWriteMask(
VkCommandBuffer                             commandBuffer,
uint32_t                                    writeMask);

• commandBuffer is the command buffer into which the command will be recorded.
• faceMask is a bitmask of VkStencilFaceFlagBits specifying the set of stencil state for which to update the write mask, as described above for vkCmdSetStencilCompareMask.
• writeMask is the new value to use as the stencil write mask.

If the pipeline state object is created with the VK_DYNAMIC_STATE_STENCIL_REFERENCE dynamic state enabled, then to dynamically set the stencil reference value call:

void vkCmdSetStencilReference(
VkCommandBuffer                             commandBuffer,
uint32_t                                    reference);

• commandBuffer is the command buffer into which the command will be recorded.
• faceMask is a bitmask of VkStencilFaceFlagBits specifying the set of stencil state for which to update the reference value, as described above for vkCmdSetStencilCompareMask.
• reference is the new value to use as the stencil reference value.

reference is an integer reference value that is used in the unsigned stencil comparison. Stencil comparison clamps the reference value to $[0,2^s-1]$ , where $s$ is the number of bits in the stencil framebuffer attachment. The $s$ least significant bits of compareMask are bitwise ANDed with both the reference and the stored stencil value, and the resulting masked values are those that participate in the comparison controlled by compareOp. Let $R$ be the masked reference value and $S$ be the masked stored stencil value. compareOp is a symbolic constant that determines the stencil comparison function:

typedef enum VkCompareOp {
VK_COMPARE_OP_NEVER = 0,
VK_COMPARE_OP_LESS = 1,
VK_COMPARE_OP_EQUAL = 2,
VK_COMPARE_OP_LESS_OR_EQUAL = 3,
VK_COMPARE_OP_GREATER = 4,
VK_COMPARE_OP_NOT_EQUAL = 5,
VK_COMPARE_OP_GREATER_OR_EQUAL = 6,
VK_COMPARE_OP_ALWAYS = 7,
} VkCompareOp;

• VK_COMPARE_OP_NEVER: the test never passes.
• VK_COMPARE_OP_LESS: the test passes when $R \lt S$ .
• VK_COMPARE_OP_EQUAL: the test passes when $R = S$ .
• VK_COMPARE_OP_LESS_OR_EQUAL: the test passes when $R \leq S$ .
• VK_COMPARE_OP_GREATER: the test passes when $R \gt S$ .
• VK_COMPARE_OP_NOT_EQUAL: the test passes when $R \neq S$ .
• VK_COMPARE_OP_GREATER_OR_EQUAL: the test passes when $R \geq S$ .
• VK_COMPARE_OP_ALWAYS: the test always passes.

As described earlier, the failOp, passOp, and depthFailOp members of VkStencilOpState indicate what happens to the stored stencil value if this or certain subsequent tests fail or pass. Each enum is of type VkStencilOp, which is defined as:

typedef enum VkStencilOp {
VK_STENCIL_OP_KEEP = 0,
VK_STENCIL_OP_ZERO = 1,
VK_STENCIL_OP_REPLACE = 2,
VK_STENCIL_OP_INCREMENT_AND_CLAMP = 3,
VK_STENCIL_OP_DECREMENT_AND_CLAMP = 4,
VK_STENCIL_OP_INVERT = 5,
VK_STENCIL_OP_INCREMENT_AND_WRAP = 6,
VK_STENCIL_OP_DECREMENT_AND_WRAP = 7,
} VkStencilOp;

The possible values are:

• VK_STENCIL_OP_KEEP keeps the current value.
• VK_STENCIL_OP_ZERO sets the value to 0.
• VK_STENCIL_OP_REPLACE sets the value to reference.
• VK_STENCIL_OP_INCREMENT_AND_CLAMP increments the current value and clamps to the maximum representable unsigned value.
• VK_STENCIL_OP_DECREMENT_AND_CLAMP decrements the current value and clamps to 0.
• VK_STENCIL_OP_INVERT bitwise-inverts the current value.
• VK_STENCIL_OP_INCREMENT_AND_WRAP increments the current value and wraps to 0 when the maximum value would have been exceeded.
• VK_STENCIL_OP_DECREMENT_AND_WRAP decrements the current value and wraps to the maximum possible value when the value would go below 0.

For purposes of increment and decrement, the stencil bits are considered as an unsigned integer.

If the stencil test fails, the sample’s coverage bit is cleared in the fragment. If there is no stencil framebuffer attachment, stencil modification cannot occur, and it is as if the stencil tests always pass.

If the stencil test passes, the writeMask member of the VkStencilOpState structures controls how the updated stencil value is written to the stencil framebuffer attachment.

The least significant $s$ bits of writeMask, where $s$ is the number of bits in the stencil framebuffer attachment, specify an integer mask. Where a $1$ appears in this mask, the corresponding bit in the stencil value in the depth/stencil attachment is written; where a $0$ appears, the bit is not written. The writeMask value uses either the front-facing or back-facing state based on the facing-ness of the fragment. Fragments generated by front-facing primitives use the front mask and fragments generated by back-facing primitives use the back mask.