Tag Archives: rectified

Photogrammetry, point clouds and stained glass

Last weekend I met with Jules Mominee of Mominee Studios [nationally renowned designers of fine stained glass and restorers of historic art glass] to conduct a work shop demonstrating how photogrammetry can add value to his work. We visited Trinity Episcopal Church in Staunton, VA to choose a test subject from their rich collection of stained glass windows – and selected the triptych behind the allar which was designed by the celebrated Tiffany Glass and Decorating Co. of New York in 1897.

My goal was to demonstrate how photographing the windows with a calibrated camera+lens combination could produce a valuable documentary record of these important heirlooms above and beyond standard photography. I would show how we could use the photographs as the basis for rectified scale-able photographs (with all lens and parallax distortion removed). I also wanted to show how we could “go into” the photographs and extract precise 3D point measurements as needed to create measured drawings and such.

This blog post will try to cover what we did.

Photographing the triptych with telescoping tripod. Note surveyor’s rod (to establish real world distance in the photos) and a white balance target.

The first step involved shooting overlapping photographs of the subject with a different lenses. Some of the shots captured the scene in its entirety while others captured  smaller regions in greater detail (for use later as pieces of a mosaic).

The variety of images shot loaded into photogrammetric software

Next up, we processed the photographs using software that automatically calculates the relationship of the camera stations to one another and creates a point cloud describing features in common captured by the photographs.

Point Cloud representing the stained glass (in true color) and the relative 3D locations for each photograph used.

The point cloud is essentially flat (due to what it is depicting) but nonetheless consists of an agglomeration of precise 3D measurements. Here is an animation showing its three dimensional nature:

An animation showing the point cloud depicting the stained glass triptych and the camera stations (in red)

Then we chose a handful of “smart points” relating to specific locations on the glass in order to establish a meaningful coordinate system. These points are shown on the images below.

Location of “smart points” on center window (lower portion)

Location of “smart points” on center window (upper portion)

Location of “smart points” on right window

Once these ponts were chosen and used to define our principal plane, we recalculated the model (with “smart points” on our surveying rod to establish real world dimensions).  Here are the x, y, and z values for our smart points:

Object Point Calculation Table

If you look closely at these values you’ll find that the average error value for this small batch of points is calculated to be about one one hundredth of an inch. Photogrammetric analysis (esp. when using controls and targets) can greatly exceed this level of accuracy – but this is already well beyond what would be required to replicate this design.

On to image rectification… The next step is to use these same 3D coordinates to define theoretical planes onto which the individual photographs will be projected so that the resulting images match precisely the real world conditions of the glass surface.

Defining a rectification plane with four or more points

The window above shows a plane formed by points 5, 6, 7 and 8 that has a maximum error value of about a sixteenth of an inch (which means that this portion of the window is pretty flat – if there were buckling and such, as will happen with windows over a hundred years old, this value would be greater…). So this will be the spatial plane onto which we will rectify the photo of the center window, lower portion.

Next up, we made a lasso of the area of the photo that we want to rectify since not all of the image corresponds to our rectification plane.

Lasso indicating extent of image that is coplanar to the rectification plane.

Then we were ready to create our rectified image of the triptych in its entirety by creating a mosaic of four smaller rectified images. In the way that we shot this example, we were able to create a rectified image that would respect the graphic scale of 3″=1′-0″ (1:4) when printed at 150 dots per inch. This ‘resolution’ can be increased by shooting more images that are in closer range to the surface being documented.

Creating a mosaic of individually rectified regions

And here is an overview of our finished result:

with some additional images “zoomed in”:

100% crop

400% crop

So this is the level of detail available across the entire surface of the three windows. If the image were printed on several sheets full size, we would produce, effectively, the same type of document as if we were able to make a high quality “rubbing” of the window – with out having to remove it and in a fraction of the time (and in color!!!)

Continuing on, I showed how the image could also be brought into a CAD program  (such as AutoCAD) in order to create a highly detailed measured drawing in vector format. In this scenario one can directly query the image to get real world dimensions.

Overview of Triptych in CAD

Closer up view in CAD

Detail view in CAD

So, in the end we showed the value of photogrammetry as a high quality AND cost effective tool for documenting heritage artifacts such as stained glass both for restoration purposes as well as for insurance purpose to provide a reliable document in the event of catastrophic loss. It also can provide a way to share the unmatched artistry of these windows to any who would like to have a closer look.

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287 Broadway

Building at Broadway & Reade Street in Manhattan

“Prominently situated on a comer site, the 287 Broadway Building is notable for its combination of the ltalianate and French Second Empire styles as executed in cast iron, and one of the few surviving examples in New York City.” – From the 1987 NYCLPC Landmark Designation Report

Construction of a new building to the south of 287 Broadway has caused this historic structure to fail.  I was asked to prepare rectified photography and a point cloud file/drawing that will at once capture the existing conditions and composition of the cast iron facades – and to provide some precise measurements as to how severly the building is leaning and in what manner.

Reade Street Elevation

Broadway Elevation

Two elevations of 287 Broadway

Point identities (that correspond to a "smart" point cloud) are noted on the Photographs

Streetscape Methodology

This is a screenshot from AutoCAD: The lower image is a series of rectified, scalable images of the east side of Augusta Street in Staunton, VA - with an aerial view of the same street grabbed from Google Earth inserted above.

This week I have been developing a methodology for capturing and representing a large volume of structures quickly as I will be doing a project charged with just this task next week in the Russian sea port city Vladivostok. Above is the sample streetscape I created using Augusta Street in Staunton as my test subject.  The photography work took about 15 minutes. I did not use a surveying rod as I typically do, nor did I collect control dimensions with my laser.  In stead, I scaled my model using dimensions extracted from satellite imagery available on Google Earth. I was looking to find a way to obtain accuracy sufficient for 1:200 prints that was also very fast.  To check my work I measured a distance from the model prepared in this way and checked it with the real world conditions. Incredibly, the error was 1/4″!

Scalable at 1/16"=1'-0" when printed at 300 dpi

Above is an individual image extracted from the group that comprises the streetscape. As noted the photo will behave like a measured drawing at the ratio of 1:192 when printed at 300dpi – which means that it is 1:96, or 1/8″=1′-0″ when printed at 150dpi and so on… My opinion is that this is very good considering how fast I made myself complete this work!

The streetscape in my test project represents about 1,000 linear feet. It will help me to extrapolate what can be achieved when tasked with documenting an entire historic district under tight time constraints.

Childs Restaurant -2

Sheet 1 with rectified images "thawed"

Sheet 2 with rectified images "frozen"

Childs Restaurant

Sheet 3 with some rectified images "thawed", others "frozen"

The Provident Mutual Insurance Building (2)

The Provident Mutual Insurance Building

This facade was captured and rendered into a single measured line drawing, enhanced by four additional sheets bearing a mosaic of rectified photographs to convery additional detail about the building’s existing conditions.

The Cosmos Club Ballroom

Ballroom at the Cosmo Club

Here is a new project that I shot last week: documentation of the existing conditions of the interior surfaces in the ballroom at the Cosmos Club in Washington DC (formerly the Townsend Mansion). This is going to be a great opportunity to show what the photo + measured line drawing approach to documentation can yield (with a limited budget).

Mosaic Images from multiple view points simultaneously

Maria Mitchell Hybrid Drawing

Maria Mitchell Hybrid Drawing

Above is what I like to call a “hybrid drawing” of the historic Maria Mitchell House located on the island of Nantucket. A hybrid drawing is one of two things, or possible both: It is a photographic image that behaves like a measured drawing (it is scalable and can provide quantifiable data), or it is a measured drawing that is rich in the way a photographic image is (materials, colors, actual as-found existing conditions are depicted photographically).

Line drawing with key to individual rectified images

Line drawing with key to individual rectified images

Above is a view of the line drawing with all of the rectified images that compose the hybrid drawing “frozen”. In other words, in the CAD drawing, these layers of information are turned off and made invisible so as to see just the line-work itself and a series of polygons that correspond to bit map/raster images that are referenced by the drawing.

Below are a series of images showing each individual rectified photographs as it is situated in the context of the drawing.  These added together make the composite image at the top of this post.

Maria Mitchell 10Maria Mitchell 9

Maria Mitchell 8Maria Mitchell 7Maria Mitchell 6Maria Mitchell 5

Maria Mitchell 11