Description
Use the coordinate (coord.x, coord.y, coord.z) to do an element lookup in the 3D image object specified by image. coord.w is ignored.
read_imagef returns 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_imagef returns 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_imagef returns floating-point values for image objects created with image_channel_data_type set to CL_HALF_FLOAT or CL_FLOAT.
The read_imagef 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_imagef for image objects with image_channel_data_type values not specified in the description above are undefined.
Notes
The built-in functions defined in this section can only be used with image memory objects created with clCreateImage2D, or clCreateImage3D. 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 __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.
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 image read functions take a sampler argument. The sampler can be passed as an argument to the kernel using clSetKernelArg, or it can be a constant variable of type sampler_t declared in the program source.
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 unsigned int4 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.
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