Vulkan Programming Guide: The Official Guide to Learning Vulkan (OpenGL) by Graham Sellers & John Kessenich

Author:Graham Sellers & John Kessenich
Language: eng
Format: epub
Publisher: Pearson Education
Published: 2016-11-07T00:00:00+00:00

Texel Buffers

A texel buffer is a special type of buffer binding used in a shader that can perform format conversion when the data is read. Texel buffers are read-only and are declared in GLSL using a samplerBuffer typed variable, as shown in Listing 6.13. Sampler buffers can return floating-point or signed or unsigned integer data to the shader. An example of each is shown in Listing 6.13.

Listing 6.13: Declaring Texel Buffers in GLSL

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layout (set = 0, binding = 3) uniform samplerBuffer my_float_texel_buffer;

layout (set = 0, binding = 4) uniform isamplerBuffer my_signed_texel_buffer;

layout (set = 0, binding = 5) uniform usamplerBuffer my_unsigned_texel_buffer;

When translated to SPIR-V using the reference compiler, the declarations from Listing 6.13 produce the SPIR-V shader shown in Listing 6.14.

Listing 6.14: Declaring Texel Buffers in SPIR-V

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OpCapability Shader

OpCapability SampledBuffer

%1 = OpExtInstImport "GLSL.std.450"

OpMemoryModel Logical GLSL450

OpEntryPoint Vertex %4 "main"

OpSource GLSL 450

OpName %4 "main"

;; Name our texel buffers.

OpName %10 "my_float_texel_buffer"

OpName %15 "my_signed_texel_buffer"

OpName %20 "my_unsigned_texel_buffer"

;; Assign set and binding decorations.

OpDecorate %10 DescriptorSet 0

OpDecorate %10 Binding 3

OpDecorate %15 DescriptorSet 0

OpDecorate %15 Binding 4

OpDecorate %20 DescriptorSet 0

OpDecorate %20 Binding 5

%2 = OpTypeVoid

%3 = OpTypeFunction %2

;; Declare the three texel buffer variables.

%6 = OpTypeFloat 32

%7 = OpTypeImage %6 Buffer 0 0 0 1 Unknown

%8 = OpTypeSampledImage %7

%9 = OpTypePointer UniformConstant %8

%10 = OpVariable %9 UniformConstant

%11 = OpTypeInt 32 1

%12 = OpTypeImage %11 Buffer 0 0 0 1 Unknown

%13 = OpTypeSampledImage %12

%14 = OpTypePointer UniformConstant %13

%15 = OpVariable %14 UniformConstant

%16 = OpTypeInt 32 0

%17 = OpTypeImage %16 Buffer 0 0 0 1 Unknown

%18 = OpTypeSampledImage %17

%19 = OpTypePointer UniformConstant %18

%20 = OpVariable %19 UniformConstant

...

To fetch from a texel buffer in GLSL, the texelFetch function is used with the sampler variable to read individual texels. A samplerBuffer (or the corresponding signed or unsigned integer variants, isamplerBuffer and usamplerBuffer) can be thought of as a 1D texture that supports only point sampling. However, the maximum size of a texel buffer attached to one of these variables is generally much, much larger than the maximum size of a 1D texture. For example, the minimum required upper bound for a texel buffer in Vulkan is 65,535 elements, whereas the minimum required size for a 1D texture is only 4,096 texels. In some cases, implementations will support texel buffers that are gigabytes in size.

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