This blog post is one in a series marking the 30th anniversary of the launch of the Hubble Space Telescope. For more information and resources regarding Hubble’s 30th anniversary, please visit hubblesite.
The Hubble Space Telescope recently celebrated its 30th year, looking deep into the cosmos to bring us views that continue to awe and inspire us. The image of this year’s target, NGC 2020, is a fascinating landscape composed of two nebulas that give the impression of an underwater scene, leading to the affectionate name, the Cosmic Reef (check out Joseph DePasquale’s recent post to learn more about the objects and the process of making the image). As the image was being processed and the intricacies of the structure came to light, the visualization team decided that this image is definitely visualization worthy.
And so the process began.
How do you build the Cosmic Reef in 3D?
The visualization team at STScI has had a lot of experience building 3D visualizations of Hubble images and there is one problem that comes up when you try to build out most Hubble images for a 3D visualization: What does this object look like from another angle? We only see the structure from one view, so how do we know how deep the structure is? What’s behind that pillar of dust? To find the answers, we consult with colleagues at STScI who study the objects in the image and similar celestial objects to determine what the scientifically reasonable interpretation of those structures could be. We discuss the structures to create a working picture of the dimensional information of the image, and we create the visualization from there.
Building the Reef
One of the first things the visualization team has to decide on is the camera path. For the visualization, we decided to flip the image compared to the orientation of the image being released for the 30th anniversary. This allowed us to drop the camera to fly by the blue gas in the ‘brain coral’ area to the central star cluster, then over the ridge of NGC 2014, then drop once more to view NGC 2020. We found through this discussion that the 30th image can feel very different when flipped.
The Hubble 30th anniversary image contains two very different looking nebulas, which after discussion with the team, required two different approaches. NGC 2014 would be built in 2.5D rather than full 3D and NGC 2020 would be built as a full 3D model in two different 3D rendering software packages.
NGC 2014 Process
The process for creating NGC 2014 for the visualization is one that has been done before by the team for past Hubble image visualizations. The first step in pulling out the layers of NGC 2014 is to create a version of the image with all of the stars removed.
Next, we begin the process of figuring out where to divide the image into layers, to be cut based on our scientific understanding of the topography of the object.
Each feature marked in the image is isolated, cleaned, and then arranged on planes layered in z-space in 3D-rendering software. These planes are then sculpted to enhance the topography and give it a more believable feel. Any gaps between the planes seen by the camera are painted in with extra fill, based on a reasonable approximation of what we would expect to be an extension of that feature.
We call this 2.5D rather than 3D because the layers that make up the scene are 2 dimensional (for the most part), but the parallax created by the camera and different placement of layers creates the experience of flying through a fully built 3D model.
NGC 2020 Process
We developed a simple mesh model to present to the science team. Since the Hubble image shows a head-on view of 2020, it was essential to consult the astronomers with expertise in this area in order to achieve the most scientifically likely shape. This basic model helped us to discuss structure, density, and other attributes we would have to develop further to execute the final visualization.
Once the shape of 2020 was sculpted as a mesh, the points or vertices were used as a basis for the volumetric data. Converting to volumes followed standard procedures using volume data base files (VDBs) and some shelf tools to develop a basis to build from.
After the model was visualized in a volumetric form, we went back into the original mesh and further separated it into smaller pieces to gain control over specific aspects of NGC 2020. Take for example, the prominent dark spot in the lower left. This area of the mesh had to be removed and organically reshaped to match the Hubble image. Other areas needed additional geometry to push the highlighted areas to their correct emission levels.
When a rough profile of NGC 2020 was fashioned the volumes needed to be adjusted in order to develop a realistic appearance. This was achieved through multiple noise fields, volume layering, and an additional smoke model was created to help smooth and blur some of the attributes.
Finally, lights and shade parameters were adjusted to help hone in on the correct exposure and emission levels. When the team was satisfied with the visual appearance, the frames were sent off to multiple machines for the final render.
Bringing Back the Stars
While the two nebulae are the visually dominant features of the image, the fly-through experience depends heavily on the stars streaming past the camera. Those stars, previously deleted in the 2D images, have to be brought back in the 3D model.
Using the Gaia star catalog, we extracted all stars with three color data within the image region. Gaia does not include all stars as faint as those seen by Hubble, but the density was adequate for a visualization. The look of the stars, called the point-spread function, uses the Tiny Tim program in each of the observed color filters to replicate the appearance of Hubble stars. After analyzing the roughly 7000 stars, we determined that all could be faithfully represented by about 900 template star images.
The stars are distributed in 3D according to a statistical model that has been developed in previous visualizations. The brighter stars are more likely to be in the foreground and the fainter stars are weighted toward the background. Special sets of stars, such as the bright cluster of NGC 2014, are handled separately from the field stars. Additional stars, not in the Hubble image, were added to provide context for the final shot of NGC 2020.
Building the Reef (from home)
As we were finishing up our 3D models for this visualization we were notified that we would be working from home for the foreseen future due to the COVID-19 pandemic. This posed some challenges since we typically do not work from home, and some new workflows would need to be established for the handling of large files, as well as the most computer-intensive part of the visualization: rendering. We worked as a team to develop new workflows and to problem-solve to get the rendering portion spread among the group. We had to try to get the visualization completed before the Hubble 30th anniversary date, and were successful.
The Reef comes together
Once we completed the assets and spent 2000 CPU hours rendering them out, it was time to bring them together.
As mentioned earlier, we built this visualization with many different parts: NGC 2014, using a 2.5D technique comprised of 29 layers positioned in 3D; and NGC 2020 as an actual 3D model comprised of volumetric particles, fluids and gas, and the many individual stars. All the different techniques were rendered in their respective 3D-software packages as individual frames. Their outputs comprised 8 finished layers from the different techniques. The different layers were then composited together.
When composited, the NGC 2014 images were fine-tuned to highlight the structure of the beautiful bilateral structure and to control the overall coloring of the volumetrics. The star layers were inserted into the scene along with the many layers of NGC 2020. Overall color balance was tweaked to match the original, carefully constructed, Hubble image.
What is the theme music to the cosmic reef?
I wanted to give the viewer an immersive and contemplative auditory backdrop to the visuals splashing over their eyes. It spoke to me as being something vaguely orchestral, but with an expansive ambience to give a sense of floating through the scene. The main theme playing as we traverse the nebula came to me as I watched early renders of the camera path over and over. I developed the theme first with piano and then added strings to fill it out. Finally, an airy, delayed, but quickly strummed guitar part acts almost as the stars in the scene to lend dimensionality to the music. I knew early on that we would need some kind of crescendo as we approached the edge of the nebula and drifted off into a new perspective of NGC 2020. The guitars and strings build to a fever pitch and then leave the viewer hanging as the bottom drops out, floating in the vacuum of space, and as we approach NGC 2020, the music lightly returns, morphing into a new theme to reflect our view of NGC 2020. The cellos ground the viewer, bringing back the bottom, driving us towards a new understanding of the true nature of this magnificent image!
We wanted to provide the viewer with a deeper understanding of the structures of the nebulas in the image and to create an engaging experience. We hope you enjoy this dive into the Cosmic Reef!