Hacker News Reader

Comment by klabb3 | Hacker News Reader

TIL, and hard agree (on face value). I’ve been struck by this with arbitrary rotation of images depending on application, very annoying.

What are the arguments for this? It would seem easier for everyone to rotate and then store exif for the original rotation if necessary.

View on HN

> What are the arguments for this? It would seem easier for everyone to rotate and then store exif for the original rotation if necessary.

Performance. Rotation during rendering is often free, whereas the camera would need an intermediate buffer + copy if it's unable to change the way it samples from the sensor itself.

How is rotation during rendering free?

Pretty much every pixel rendered these days was generated by a shader so gpu side you probably already have way more translation options than just a 90° rotation (likely already being used for a rotation of 0°). You'd likely have to write more code cpu side to handle the case of tell the gpu to rotate this please and handle the UI layout diffrence. Honestly not a lot of code.

For anything GPU-rendered, applying a rotation matrix to a texture sample and/or frame-buffer write is trivially cheap (see also why Vulkan prerotation exists on Android). Even ignoring GPU-rendering, you always are doing a copy as part of rendering and often have some sort of matrix operation anyway at which point concatenating a rotation matrix often doesn't change much of anything.

The cost is paid in different memory access patterns which may or may not be mitigated by the GPU scheduler. It's an insignificant cost either way though, both for the encoder and the renderer. Also depending on the pixel order in sensor, file or frame buffer "rotated" might actually be the native way and the default is where things get flipped around from source to destination.

Access pattern is mitigated by texture swizzling which will happen regardless of how it's ultimately rendered. So even if drawn with an identity matrix you're still "paying" for it regardless just due to the underlying texture layout. GPUs can sample from linear textures, but often it comes with a significant performance penalty unless you stay on a specific, and undefined, path.

Given that rotation sensors have been standard equipment on most cameras (AKA phones) for many years now, I would expect pixel-reordering to be built into supporting ASICs and to impose negligible performance penalties.

Reply on news.ycombinator.com