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NAME

       read_imageh_(2D,_Half) - Do an element lookup in the 2D image object, returning half
       floating-point values.

       half4 read_imageh(image2d_t image, sampler_t sampler, int2 coord);

       half4 read_imageh(image2d_t image, sampler_t sampler, float2 coord);

       half4 read_imageh(image2d_t image, int2 coord);

       half4 read_imageh(image2d_array_t image, int4 coord);

       half4 read_imageh(image2d_array_t image, sampler_t sampler, int4 coord);

       half4 read_imageh(image2d_array_t image, sampler_t sampler, float4 coord);

DESCRIPTION

       For the forms that take an image2d_t, use the coordinate (x, y) to do an element lookup in
       the 2D image object specified by image.

       For the forms that take an image2d_array_t, Use coord.xy to do an element lookup in the 2D
       image identified by coord.z in the 2D image array specified by image.

       The samplerless read image functions behave exactly as the corresponding read image
       functions that take integer coordinates and a sampler with filter mode set to
       CLK_FILTER_NEAREST, normalized coordinates set to CLK_NORMALIZED_COORDS_FALSE and
       addressing mode to CLK_ADDRESS_NONE.

       read_imageh returns half floating-point values in the range [0.0... 1.0] for image objects
       created with image_channel_data_type set to one of the predefined packed formats or
       CL_UNORM_INT8 or CL_UNORM_INT16.

       read_imageh returns half floating-point values in the range [-1.0... 1.0] for image
       objects created with image_channel_data_type set to CL_SNORM_INT8, or CL_SNORM_INT16.

       read_imageh returns half floating-point values for image objects created with
       image_channel_data_type set to CL_HALF_FLOAT.

       The read_imageh calls that take integer coordinates must use a sampler with filter mode
       set to CLK_FILTER_NEAREST, normalized coordinates set to CLK_NORMALIZED_COORDS_FALSE and
       addressing mode set to CLK_ADDRESS_CLAMP_TO_EDGE, CLK_ADDRESS_CLAMP or CLK_ADDRESS_NONE;
       otherwise the values returned are undefined.

       Values returned by read_imageh for image objects with image_channel_data_type values not
       specified in the description above are undefined.

NOTES

       General information about the read_image* functions follows.

       The built-in functions defined in this section can only be used with image memory objects.
       An image memory object can be accessed by specific function calls that read from and/or
       write to specific locations in the image.

       Image memory objects that are being read by a kernel should be declared with the
       accessQualifiers(3clc) qualifier.  write_image calls to image memory objects declared with
       the accessQualifiers(3clc) qualifier will generate a compilation error. Image memory
       objects that are being written to by a kernel should be declared with the
       accessQualifiers(3clc) qualifier.  read_image calls to image memory objects declared with
       the accessQualifiers(3clc) qualifier will generate a compilation error.  read_image and
       write_image calls to the same image memory object in a kernel are not supported.

       The read_image calls returns a four component floating-point, integer or unsigned integer
       color value. The color values returned by read_image are identified as x, y, z, w where x
       refers to the red component, y refers to the green component, z refers to the blue
       component and w refers to the alpha component.

       The following table describes the mapping of the number of channels of an image element to
       the appropriate components in the float4, int4 or uint4 vector data type for the color
       values returned by read_image{f|i|ui} or supplied to write_image{f|i|ui}. The unmapped
       components will be set to 0.0 for red, green and blue channels and will be set to 1.0 for
       the alpha channel.

                     ┌──────────────────────────┬──────────────────────────────────┐
                     │      Channel Orderfloat4, int4 or uint4 components │
                     │                          │ of channel data                  │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │       CL_R, CL_Rx(r, 0.0, 0.0, 1.0)        │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │          CL_A(0.0, 0.0, 0.0, a)        │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │      CL_RG, CL_RGx(r, g, 0.0, 1.0)         │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │          CL_RA(r, 0.0, 0.0, a)         │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │     CL_RGB, CL_RGBx(r, g, b, 1.0)          │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │CL_RGBA, CL_BGRA, CL_ARGB(r, g, b, a)           │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │      CL_INTENSITY(I, I, I, I)           │
                     ├──────────────────────────┼──────────────────────────────────┤
                     │      CL_LUMINANCE(L, L, L, 1.0)          │
                     └──────────────────────────┴──────────────────────────────────┘

       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 read_image{f|i|ui}, 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.

SPECIFICATION

       OpenCL Specification[1]

SEE ALSO

       imageFunctions(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 27, section 9.5.8 - Image Read and Write Functions (half)