Sharp-Bilinear Shaders for Retroarch

This is a collection of shaders for sharp pixels without pixel wobble and minimal blurring in RetroArch/Libretro, based on TheMaister's work.

The following shaders are included:

• "sharp-bilinear-2x-prescale"

This shader does a fixed 2x integer prescale resulting in a small amount of image blurring but no pixelwobble. This is a simple two-pass shader configuration. First, an integer 2x prescale is applied, followed by a bilinear scaling to fullscreen.

• "sharp-bilinear-simple"

This shader does an automatic optimum integer prescale (2x, 3x, 4x etc.), depending on game and screen resolution. I recommend this shader since the autoscaling results in sharper images for some games than the fixed 2x prescale.

• "sharp-bilinear-scanlines"

same as above, but with an overlay of black scanlines (half of each game pixel, vertically is a scanline)

All these shader configurations give sharp pixels with zero pixel wobble in all games.

Compared to the "video smoothing=ON" setting with no shaders, pixels are less blurry. Compared to the "video smoothing=OFF" setting with no shaders, the pixels do not change shape/wobble as they move across the screen.

Installation

To install this shader in RetroPie:

• Copy the contents of the included "Copy_To_RetroPie" folder to /opt/retropie/emulators/retroarch/shader/
• open the RetroPie-Setup menu and choose "Edit RetroPie/RetroArch Configurations"-> "configure basic libretro emulator options"-> "configure default options for all libretro emulators"
• set "Video Shader Enable" to "True"
• set "Video Shader File" to "sharp-bilinear-xxxxx.glslp," choosing xxxxx depending on your preference.

Example Images

shader "sharp-bilinear-simple.glslp" on:

shader off, smoothing on (too much blur):

shader off, smoothing off:

On first glance, the above looks sharp and good, but looking at a detail, we can see pixel wobble that happens if no shader is used. The pixels along the black diagonal should all be the same, square shape, but some of them appear rectangular:

Compare the above detail to the result with the shader on:

Now all pixels have the same shape, at the cost of a slight reduction in sharpness.

Performance

The calculations done by these shaders are trivial. Using any of these shaders with RetroPie on a Raspberry Pi 3 in 1080p full HD resolution, Street Fighter 3 Third Strike (libretro-fba) runs at a steady 60 fps.

Improvements compared to "retro/sharp-bilinear" from the Libretro repository

• There's a small improvement that makes sharp-bilinear-simple work better with vertical games (shmups etc.). The autoscale is calculated separately for both the horizontal and vertical dimension, e.g. the integer prescale could be 4 for the horizontal, and 2 for the vertical. The original sharp-bilinear only used the vertical dimension to calculate the auto-prescale, and then used the same integer for both x and y.
• The autoscaling factors are pre-calculated in the vertex shader, instead of re-calculating for every pixel.
• The sharp-bilinear-2x-prescale filter is very simple compared to all the others. It is a simple shader config that applies two passes of the stock.glsl "Null shader," and therefore contains almost no calculations, and should be extremely fast.