Gary Lucas created IMAGING-312:
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Summary: Alpha-channel setting not interpreted from ExtraSamples
tag
Key: IMAGING-312
URL: https://issues.apache.org/jira/browse/IMAGING-312
Project: Commons Imaging
Issue Type: Bug
Components: Format: TIFF
Affects Versions: 1.0-alpha2
Environment:
Reporter: Gary Lucas
Commons Imaging sometimes misinterprets TIFF files that have 4-byte RGB samples
but do not define alpha. In some cases, these images are treated as
semi-transparent when they should be opaque. Commons Imaging is not unique in
this regard... Windows Photo Viewer does the same thing.
The TIFF specification allows RGB images to be encoded with 4-bytes per pixel.
It would be natural to assume (as Commons Imaging does) that the 4th byte is
the alpha channel and that it would have values of 0xff in the case where
pixels were opaque. However, the interpretation of the 4th byte depends on
information in the TIFF "ExtraSamples" tag.
It turns out that there are images in-the-wild that use 4 bytes, but populate
the 4th byte with junk values. For example, there are a number of older aerial
photographs from the US Geological Survey (USGS) that do this. These images
give an ExtraSamples tag with a value of zero. But the TIFF specification
calls for images to be treated as having alpha channels only if the
ExtraSamples field carries a value of either 1 or 2. When ExtraSamples has a
value of 0, the 4th byte is to be ignored.
There are many examples of this phenomenon on the USGS Earth Explorer website.
One specific example:
* High Resolution Orthoimagery
* Dataset: 201203_connecticut_state_lot1_ct_0x3000m_utm_cnir
* Entity: 2818289_18TYL425825
* File: 18tyl425825.tif
*Proposed Fix*
I propose to do the following:
* Extend the TiffImageParser logic for detecting alpha to assume hasAlpha is
true if and only if the ExtraSamples tag is supplied and contains values 1 or
2.
* Provide a hasAlpha accessor for the ImageBuilder class (is should really have
one anyway)
* Enhance the DataReaderStrips and DataReaderTiles classes to check hasAlpha
when processing RGB images that have 4 samples per pixel samples.
*Concerns*
At this time, I am not sure what to do if an RGB TIFF image uses 4-samples per
pixel but the ExtraSamples tag is not provided. At this time, I have not seen
an example of this, but my collection of sample TIFF files is rather narrow and
I would not rule it out.
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