read_imageh (1D, Half)

Do an element lookup in the 1D image object, returning half floating-point values.

half4 read_imageh ( image1d_t  image ,
  sampler_t  sampler ,
  int  coord )
half4 read_imageh ( image1d_t  image ,
  sampler_t  sampler ,
  float  coord )
half4 read_imageh ( image1d_t  image ,
  int  coord )
half4 read_imageh ( image1d_buffer_t  image ,
  int  coord )
half4 read_imageh ( image1d_array_t  image ,
  int2  coord )
half4 read_imageh ( image1d_array_t  image ,
  sampler_t  sampler ,
  int2  coord )
half4 read_imageh ( image1d_array_t  image ,
  sampler_t  sampler ,
  float2  coord )

Description

An application that wants to use half and halfn types will need to include the #pragma OPENCL EXTENSION cl_khr_fp16 : enable directive.

For forms that take an image1d_t, use coord to do an element lookup in the 1D image object specified by image.

For forms that take an image1d_array_t, Use coord.x to do an element lookup in the 1D image identified by coord.y in the 1D 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 precision 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 precision 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 precision 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.

Note that image writes to sRGB images are only supported if the cl_khr_srgb_image_writes extension is supported; otherwise the behavior of writing to a sRGB image is undefined.

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

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.

sRGB Images

The built-in image read functions will perform sRGB to linear RGB conversions if the image is an sRGB image. Writing to sRGB images from a kernel is an optional extension. The cl_khr_srgb_image_writes extension will be reported in the CL_DEVICE_EXTENSIONS string if a device supports writing to sRGB images using write_imagef. clGetSupportedImageFormats will return the supported sRGB images if CL_MEM_READ_WRITE or CL_MEM_WRITE_ONLY is specified in flags argument and the device supports writing to an sRGB image. If cl_khr_srgb_image_writes is supported, the built-in image write functions will perform the linear to sRGB conversion.

Only the R, G and B components are converted from linear to sRGB and vice-versa. The alpha component is returned as is.

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.

Mapping image channels to color values

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 Order float4, 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, CL_sRGB, CL_sRGBx (r, g, b, 1.0)
CL_RGBA, CL_BGRA, CL_ARGB, CL_ABGR, CL_sRGBA, CL_sBGRA (r, g, b, a)
CL_INTENSITY (I, I, I, I)
CL_LUMINANCE (L, L, L, 1.0)

For CL_DEPTH images, a scalar value is returned by read_imagef or supplied to write_imagef.

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.

CL_SAMPLER_NORMALIZED_COORDS must be CL_TRUE for built-in functions described in the table above that read from a mip-mapped image; otherwise the behavior is undefined. The value specified in the lod argument is clamped to the minimum of (actual number of mip-levels – 1) in the image or value specified for CL_SAMPLER_LOD_MAX.

Specification

OpenCL Specification

Also see

Image Functions, cl_khr_mipmap_image, sampler_t

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