Meaning of CL_UNORM_INT8 for cl_image_format.image_channel_data_type, and its diffrence with other data types

Question

everyone! I'm not really clear about the meaning of CL_UNORM_INT8, which is one of the available choice of value of cl_image_format.image_channel_data_type; what's specific about this type, and what's its difference with CL_UNSIGNED_INT8?

Answer 1

As far as storage is concerned, these types are identical. In both cases, each pixel channel value will be stored as an 8-bit integer, with values in the range 0-255. The difference comes when reading/writing the image from a kernel.

For the CL_UNSIGNED_INT8 type, you will use the read_imageui and write_imageui functions to access the image. These functions will return (or accept) an unsigned integer, with values in the same range as the storage type.

For the CL_UNORM_INT8 type, you will use the read_imagef and write_imagef functions to access to the image. These functions will return (or accept) a normalised floating point value, in the range 0.0f - 1.0f. Some devices (e.g. most GPUs) have hardware support for normalising texture values, so the conversion between integer and normalised floating point values will be very efficient.