Getting data offset

[bradh]

I'm looking for a way to get the offset (relative to the start of the file) of the data in a dataset, but not seeing this in the API.

The specific case is in an image, where it looks like this (from h5ls):

OnTask                   Dataset {3024/3024, 4032/4032, 3/3}
    Attribute: CLASS scalar
        Type:      6-byte null-terminated ASCII string
    Attribute: IMAGE_MINMAXRANGE {2}
        Type:      native unsigned char
    Attribute: IMAGE_SUBCLASS scalar
        Type:      16-byte null-terminated ASCII string
    Attribute: IMAGE_VERSION scalar
        Type:      4-byte null-terminated ASCII string
    Attribute: INTERLACE_MODE scalar
        Type:      16-byte null-terminated ASCII string
    Location:  1:800
    Links:     1
    Storage:   36578304 logical bytes, 36578304 allocated bytes, 100.00% utilization
    Type:      native unsigned char
    Address:   1400

I'm looking for a way to get what h5ls calls the "Address", which is essentially the first pixel component value.

I do see Dataset::getAddress(), but that gives me what h5ls calls the "Location" value (presumably the start of the dataset structure).

The reason why I'm looking for this is to build a HEIF wrapper around an existing HDF5 file containing images. I don't need the data - I'm trying to use it by reference - but I do need to know where to find it.

[jamesmudd]

Thanks for the question. I think what you want is io.jhdf.api.dataset.ContiguousDataset#getDataAddress you will need to cast the Dataset you have to a ContiguousDataset as getting the address for the data doesn't make sense for all types of Dataset. You might prefer to call io.jhdf.api.dataset.ContiguousDataset#getBuffer to let jHDF give you a ByteBuffer I think you want. Let me know if this answers your question.

[bradh]

I think what you want is io.jhdf.api.dataset.ContiguousDataset#getDataAddress you will need to cast the Dataset you have to a ContiguousDataset as getting the address for the data doesn't make sense for all types of Dataset.

Thanks - that worked well. For the ContiguousDataset I assume the data is physically present in the file between [getDataAddress(), getDataAddress() + getStorageSize() - 1].

You might prefer to call io.jhdf.api.dataset.ContiguousDataset#getBuffer to let jHDF give you a ByteBuffer I think you want.

I need the address (well, the byte range) to store in another file (like a metadata sidecar file) which the image reader application will use as an index, and treat the HDF5 file as an opaque container of bits.

Let me know if this answers your question.

It does (unless the assumption above is wrong).

Thanks for the quick response - much appreciated.