16-bit single channel sources images yield RGB TIFF panoramas - why?

[tagi...@gmail.com]

As a relatively inexperienced PTGUI user I've run into several problems creating panoramas using PTGUI version 12.22, and I'm hopeful someone here can offer suggestions on how to get past these issues.

My efforts have used a Windows 10 Pro 21H2 machine that has 32 GB of RAM and also a MacBook Pro with 16 GB running MacOS 11.7.6, . I see similar problems on both of these machines but will confine what I describe here to work with the Windows 10 machine due to its larger amount of RAM.

My source images are 16-bit, single channel TIFF images, each captured using a 60 MP monochrome camera (image file size ~ 120 MB) and with about 10 - 12% overlap between adjacent images, and there are 20 such source images. The subject is a drawing of a human head and shoulders drawn with pencil on good paper. The 20 images can be thought of as roughly forming a grid of 4 rows of 5 columns. The background around the central subject figure is very dark, and there are very few control points in some of these noncentral regions.

I'm able to align, optimize, and create panoramas but I'm usually unable to open or view panoramas using PTGUI or any of the numerous image viewer programs I have (typically I use ImageJ that is set use about 30 GB of the RAM).

The most significant problem is that I can't open or view the created panoramas, with resultant error messages often saying there's a problem with the file format. This may be related to the size of the output panorama image, which can be larger than 6 GB due to the creation of RGB panoramas.

My goal is to created panoramas that are 16-bit single channel TIFFs. I have questions about this:

1. Depending on settings, the created panorama is often an RGB image and that usually cannot be opened. What settings will cause the panorama that is created to be a monochromatic single channel 16-bit image? Am I running into a memory or size issue? My thinking is that creation of a monochrome (i.e. 16-bit single channel) output panorama will require less storage space and RAM. thereby potentially eliminating or reducing any size-related factors and effects.

2. I'm confused about how 16-bit data in the source images may result in a 3-channel RGB panorama image that results in only 8-bit data in each of the R, G, and B channels for a created panorama. Is there a written description that explains how this is done? Here's something else I don't understand: Suppose I want to align and join three adjacent images each of which contains data that fills the 16-bit histogram. In order to match the brightness of each overlapping "join" region requires that the range of values for the joined images together will often exceed what can be represented in 16-bits. I imagine there is some linear transformation used for each pair of adjacent source imagers so that they match well when joined, and to do this the operations may for example temporarily use a 32-bit floating point representation that spans the range of the data after it has been joined before ultimately returning to a 16-bit or 8-bit representation.

Does anyone have suggestions on how I can learn how to correctly create and view such panoramas?

Thanks,

Bill

[PTGui Support]

Hi Bill,

PTGui is hardwired for RGB images. So your 16 bit single channel images
are implicitly converted to 3 channel 16 bit RGB by copying the
channels. The output will also be 3 channels (or 4 if transparency is
included).

The output can be either 8 or 16 bits per pixel: just select the desired
option by clicking on Settings in the Create Panorama tab. All
processing is done internally using floating point data.

The TIFF format was originally limited to 4GB file size, because it's
using 32 bit pointers internally. Some years ago the format was extended
to support larger files (BigTIFF) but unfortunately this is still not
widely supported. PTGui automatically switches to BigTIFF when the file
size exceeds 4 GB but many applications will not be able to load the
resulting image. If you happen to use Photoshop, try generating a PSB
file in PTGui instead, this should work.

Of course part of the problem is that the RGB output from PTGui is 3x as
large as necessary for your application. Adding support for single
channel output is one of the things that's on the wish list for a future
version.

Hope this helps.

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

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