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NAME
sampler_t - A type used to control how elements of an image object are read by read_image{f|i|ui}.
const sampler_t <sampler name> = <value> constant
sampler_t <sampler name> = <value> __constant sampler_t
<sampler name> = <value>
DESCRIPTION
The image read functions take a sampler argument. The sampler can be passed as an argument to the kernel
using clSetKernelArg(3clc), or can be declared in the outermost scope of kernel functions, or it can be a
constant variable of type sampler_t declared in the program source.
Sampler variables in a program are declared to be of type sampler_t. A variable of sampler_t type
declared in the program source must be initialized with a 32-bit unsigned integer constant, which is
interpreted as a bit-field specifiying the following properties:
• Addressing Mode
• Filter Mode
• Normalized Coordinates
These properties control how elements of an image object are read by imageFunctions(3clc).
Samplers can also be declared as global constants in the program source using the syntax shown at the top
of this page.
The sampler fields are described in the table below:
┌───────────────────────────┬──────────────────────────────────────────┐
│Sampler State │ Description │
├───────────────────────────┼──────────────────────────────────────────┤
│ │ │
│ <normalized coords> │ Specifies whether the x, y and │
│ │ z coordinates are passed in as │
│ │ normalized or unnormalized │
│ │ values. This must be a literal │
│ │ value and can be one of the │
│ │ following predefined enums: │
│ │ │
│ │ │
│ │ │
│ │ CLK_NORMALIZED_COORDS_TRUE or │
│ │ CLK_NORMALIZED_COORDS_FALSE. │
│ │ │
│ │ │
│ │ │
│ │ The samplers used with an │
│ │ image in multiple calls to │
│ │ imageFunctions(3clc){f|i|ui} │
│ │ declared in a kernel must use │
│ │ the same value for <normalized │
│ │ coords>. │
├───────────────────────────┼──────────────────────────────────────────┤
│ │ │
│ <addressing mode> │ Specifies the image │
│ │ addressing-mode i.e. how │
│ │ out-of-range image coordinates │
│ │ are handled. This must be a │
│ │ literal value and can be one of │
│ │ the following predefined enums: │
│ │ │
│ │ │
│ │ │
│ │ CLK_ADDRESS_MIRRORED_REPEAT - │
│ │ Flip the image coordinate at │
│ │ every integer junction. This │
│ │ addressing mode can only be used │
│ │ with normalized coordinates. If │
│ │ normalized coordinates are not │
│ │ used, this addressing mode may │
│ │ generate image coordinates that │
│ │ are undefined. │
│ │ │
│ │ │
│ │ │
│ │ CLK_ADDRESS_REPEAT - out-of-range │
│ │ image coordinates are wrapped to │
│ │ the valid range. This address │
│ │ mode can only be used with │
│ │ normalized coordinates. If │
│ │ normalized coordinates are not │
│ │ used, this addressing mode may │
│ │ generate image coordinates that │
│ │ are undefined. │
│ │ │
│ │ │
│ │ │
│ │ CLK_ADDRESS_CLAMP_TO_EDGE - │
│ │ out-of-range image coordinates │
│ │ are clamped to the extent. │
│ │ │
│ │ │
│ │ │
│ │ CLK_ADDRESS_CLAMP - out-of-range │
│ │ image coordinates will return a │
│ │ border color. This is similar to │
│ │ the GL_ADDRESS_CLAMP_TO_BORDER │
│ │ addressing mode. │
│ │ │
│ │ │
│ │ │
│ │ CLK_ADDRESS_NONE - for this │
│ │ addressing mode the programmer │
│ │ guarantees that the image │
│ │ coordinates used to sample │
│ │ elements of the image refer to a │
│ │ location inside the image; │
│ │ otherwise the results are │
│ │ undefined. │
│ │ │
│ │ │
│ │ │
│ │ For 1D and 2D image arrays, the │
│ │ addressing mode applies only to │
│ │ the x and (x, y) coordinates. The │
│ │ addressing mode for the │
│ │ coordinate which specifies the │
│ │ array index is always │
│ │ CLK_ADDRESS_CLAMP_TO_EDGE │
├───────────────────────────┼──────────────────────────────────────────┤
│ │ │
│ <filter mode> │ Specifies the filtering mode to │
│ │ use. This must be a literal value │
│ │ and can be one of the following │
│ │ predefined enums: │
│ │ CLK_FILTER_NEAREST or │
│ │ CLK_FILTER_LINEAR. Refer to │
│ │ section 8.2 for a description of │
│ │ these filter modes. │
└───────────────────────────┴──────────────────────────────────────────┘
NOTES
Note that samplers declared using the constant qualifier are not counted towards the maximum number of
arguments pointing to the constant address space or the maximum size of the constant address space
allowed per device (i.e. CL_DEVICE_MAX_CONSTANT_ARGS and CL_DEVICE_MAX_CONSTANT_BUFFER_SIZE as described
in table 4.3).
The maximum number of samplers that can be declared in a kernel can be queried using the
CL_DEVICE_MAX_SAMPLERS token in clGetDeviceInfo(3clc).
If <addressing mode> in sampler is CLK_ADDRESS_CLAMP, then out-of-range image coordinates return the
border color. The border color selected depends on the image channel order and can be one of the
following values:
• If the image channel order is CL_A, CL_INTENSITY, CL_Rx, CL_RA, CL_RGx, CL_RGBx, CL_ARGB, CL_BGRA, or
CL_RGBA, the border color is (0.0f, 0.0f,0.0f, 0.0f).
• If the image channel order is CL_R, CL_RG, CL_RGB, or CL_LUMINANCE, the border color is (0.0f, 0.0f,
0.0f, 1.0f)
A kernel that uses a sampler with the CL_ADDRESS_CLAMP addressing mode with multiple images may result in
additional samplers being used internally by an implementation. If the same sampler is used with multiple
images called via imageFunctions(3clc), then it is possible that an implementation may need to allocate
an additional sampler to handle the different border color values that may be needed depending on the
image formats being used. These implementation allocated samplers will count against the maximum sampler
values supported by the device and given by CL_DEVICE_MAX_SAMPLERS. Enqueuing a kernel that requires more
samplers than the implementation can support will result in a CL_OUT_OF_RESOURCES error being returned.
Example
const sampler_t samplerA = CLK_NORMALIZED_COORDS_TRUE |
CLK_ADDRESS_REPEAT | CLK_FILTER_NEAREST;
samplerA specifies a sampler that uses normalized coordinates, the repeat addressing mode and a nearest
filter.
SPECIFICATION
OpenCL Specification[1]
SEE ALSO
otherDataTypes(3clc), imageFunctions(3clc), classDiagram(3clc)
AUTHORS
The Khronos Group
COPYRIGHT
Copyright © 2007-2011 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and/or
associated documentation files (the "Materials"), to deal in the Materials without restriction, including
without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Materials, and to permit persons to whom the Materials are furnished to do so, subject to
the condition that this copyright notice and permission notice shall be included in all copies or
substantial portions of the Materials.
NOTES
1. OpenCL Specification
page 292, section 6.12.14.1 - Samplers