3 ms·

> Use of float makes it unlikely that you’d run it on the PlayStation, which has no FPU. I think this sentence just made me realize what made the PSX graphics

> Use of float makes it unlikely that you’d run it on the PlayStation, which has no FPU. I think this sentence just made me realize what made the PSX graphics look so "PSX", I'm guessing all the positions/translations and similar stuff were actually not floating point which they typically are (today at least), hence the classic look of triangles/meshes kind of "jumping"? Huh...

anthk 2mo ago
Integers. You could almost emulate a PSX in a potato (Pentium MMX) with -ffast-math -O3 except for Alpha channel (transparencies) in textures.
ndepoel 2mo ago
The jumpy polygons are because the PSX's GPU only accepts vertex positions in screen coordinates, meaning that vertices always snap to hard integer values. There is no sub-pixel accuracy. That's not a limitation caused by the PSX's lack of an FPU though. A large part of the render pipeline and standard libraries are built around the use of fixed point math, i.e. using integers to simulate decimal values. The GTE co-processor which handles 3D math makes use of standardized fixed-point types. So the PSX would have been capable of calculating sub-pixel coordinate values, it's just that the communication between CPU and GPU was designed to use only integer screen coordinate values.
spicyjpeg 2mo ago
It's slightly more complicated than that. The PlayStation's geometry pipeline uses fixed-point coordinates with a decent amount of fractional precision (12 bits), however the GPU lacks support for subpixel rendering and operates entirely in screen space using integer X/Y pixel coordinates; all transformed vertices thus have to be rounded CPU-side before they reach the GPU. The GPU is also a 2D rasterizer only, with no perspective correction nor depth buffering, so transformed Z values are used on the CPU to sort polygons back to front (typically using hardware-assisted linked list bucket sorting) and dropped afterwards [1]. [1] https://github.com/spicyjpeg/ps1-bare-metal/blob/main/src/08_spinningCube/main.c#L252-L296 https://github.com/spicyjpeg/ps1-bare-metal/blob/main/src/08...
meindnoch 2mo ago
No. The reason is that the PlayStation doesn't have perspective-correct texcoord interpolation.
- unleaded 2mo ago
  That's a separate issue from the wobbling.
- klodolph 2mo ago
  Well, that’s half of it, and the other half of it is no sub-pixel coordinates.

anthk 2mo ago

Integers. You could almost emulate a PSX in a potato (Pentium MMX) with -ffast-math -O3 except for Alpha channel (transparencies) in textures.

ndepoel 2mo ago

The jumpy polygons are because the PSX's GPU only accepts vertex positions in screen coordinates, meaning that vertices always snap to hard integer values. There is no sub-pixel accuracy. That's not a limitation caused by the PSX's lack of an FPU though. A large part of the render pipeline and standard libraries are built around the use of fixed point math, i.e. using integers to simulate decimal values. The GTE co-processor which handles 3D math makes use of standardized fixed-point types. So the PSX would have been capable of calculating sub-pixel coordinate values, it's just that the communication between CPU and GPU was designed to use only integer screen coordinate values.

spicyjpeg 2mo ago

It's slightly more complicated than that. The PlayStation's geometry pipeline uses fixed-point coordinates with a decent amount of fractional precision (12 bits), however the GPU lacks support for subpixel rendering and operates entirely in screen space using integer X/Y pixel coordinates; all transformed vertices thus have to be rounded CPU-side before they reach the GPU. The GPU is also a 2D rasterizer only, with no perspective correction nor depth buffering, so transformed Z values are used on the CPU to sort polygons back to front (typically using hardware-assisted linked list bucket sorting) and dropped afterwards [1]. [1] https://github.com/spicyjpeg/ps1-bare-metal/blob/main/src/08_spinningCube/main.c#L252-L296 https://github.com/spicyjpeg/ps1-bare-metal/blob/main/src/08...

meindnoch 2mo ago

No. The reason is that the PlayStation doesn't have perspective-correct texcoord interpolation.

unleaded 2mo ago
That's a separate issue from the wobbling.
klodolph 2mo ago
Well, that’s half of it, and the other half of it is no sub-pixel coordinates.