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.
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.
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.
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