🏔️ Down Yosemite Valley and Back

Milos Popovic shipped forge3d 1.40.1 yesterday, with a new example: a camera flythrough of Bryce Canyon. I wanted to try it right away, and there was only ever one place to point it. I grew up in Groveland, on the road to Yosemite, so it had to be the valley.

The plan: start at Half Dome, drop into the valley, fly it end to end, turn around at Tunnel View, and come back to the classic Half Dome view. On a June evening, because that’s when the granite glows.

The flight

• Half Dome. It opens hovering over Tenaya Canyon, facing the sheer north-west face with the low sun full on it, then slides down along the face into the valley.
• Down the valley. About 400 metres off the floor, it flies west past Curry Village, Sentinel Bridge and Yosemite Falls, then between El Capitan and the spires of Cathedral Rocks, then past Bridalveil Fall.
• Tunnel View. It turns over the Merced gorge and comes around on one of the most photographed views in the park: El Cap on the left, Half Dome in the middle, Cathedral Rocks on the right.
• Back to Half Dome. It flies back up the valley and slows to a long-lens hold on Half Dome from near Sentinel Bridge.

How it works: what’s real and what’s painted

It looks like a drone shot, but nothing here was filmed. It’s three layers stacked on a laptop GPU.

1. The shape is lidar. In 2019 an aircraft flew over Yosemite firing laser pulses at the ground, hundreds of thousands a second, and timed the echoes. From those the USGS built a bare-earth elevation model at 1-metre detail. I averaged seven of its tiles to a 3-metre grid, about 32 million heights, and forge3d turns that grid into a 3D surface. That’s why El Cap and Half Dome have their real shapes.
2. The colour is a photo. NAIP is aerial photography the US flies over the whole country every couple of years. It’s draped over the surface like a tablecloth. The forest you see is a photo of treetops from 1.5 metres a pixel. There are no 3D trees; from 400 metres up you can’t tell.
3. The light is computed. For every frame the GPU works out where the sun is (June 20, 7:15 PM, low in the north-west), which way each slope faces it, which ridges throw shadows across the valley, and how much darker the folds and creases should be. Then a few lines of Python paint the evening sky, warm up the highlights and add a little haze toward the horizon.

The camera path is just fifteen keyframes, each a position, a point to look at and a lens, smoothed into one curve. A check against the lidar lifts the camera wherever it would pass within 110 metres of the ground.

Three things that went wrong first

• I flew it into El Capitan. My first path ran a kilometre too far north, right over El Cap’s rim. The clearance check had to lift it 760 metres to avoid hitting the rock. So I traced the low line of the valley floor from the lidar itself and laid the path along that.
• The cliffs were the wrong colour. An aerial photo looks straight down, so a 900-metre wall is only a few pixels wide in it, and NAIP is shot around midday, when Half Dome’s face is in shadow. Draped on the cliffs, the photo came out as dark grey smears. Anything steeper than about 50° now gets a granite tone instead, and the evening sun lights it. That’s the glow you see on Half Dome.
• Half Dome started as a blob. I first opened the video from directly above the summit, which turns one of the most famous rocks in America into a shapeless lump. Moving the camera out over Tenaya Canyon, facing the cut face, fixed it.

Last thing

Yosemite Valley has been at the end of my road since I was a kid in Groveland. Flying it like this with the newest version of Milos’s tool, from the face of Half Dome all the way down and back, was a real treat. Same tools as the Humphreys Peak orbit and the heat dome, and huge thanks again to Milos for forge3d and for the Bryce Canyon example that started this one.

Data: USGS 3D Elevation Program 1 m lidar DEM, project CA_YosemiteNP_2019, and 1/3 arc-second DEM; USDA NAIP imagery via the USGS National Map. Rendering: forge3d 1.40.1 by Milos Popovic. Sun position from forge3d’s solar calculator.

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