CRT Pixel Shader Filter for SNES Emulation

Update 05/18/2011: More screenshots for more new filters in my new post. :-)

Update 05/02/11: After many changes, it looks like the CRT shader development has settled down, so there's less need for me to maintain the older versions of the shaders. From here on out, I recommend visiting Screwtape's git repo for all of your XML shader needs (my mediafire account will still be there, but Screwtape has all of mine and more).

CRT.OpenGL.shader and CRT-flat.OpenGL.shader are similar to the shader covered in this post, though they run slightly faster and have no visible artifacts. CRT-simple.OpenGL.shader is a simplified rewrite that should be usable on much older, slower machines.

Here is the original post for informational purposes:

This post covers the use of filters to upscale pixel art--specifically as it applies to SNES emulation--with special attention to CRT reproduction. If you just want the pictures and to download the filters, skip to the bottom of the post.

Background:

As everyone who dabbles in old-school emulation knows, artwork that was intended for a 480i CRT television that has been upscaled to an HD resolution looks like absolute garbage on an LCD monitor. The chunky sprites with their often thick, cartoony outlines just weren't designed to be reproduced with sharp edges resulting from nearest-neighbor upscaling.

To get around this ugly upscaling effect, many emulators now include upscaling interpolation filters, which apply complex mathematical algorithms to the original picture to fill in the gaps between things that are impossible to represent in chunky low-res, such as curves and smooth diagonal lines. You're probably familiar with some of the more common and popular interpolating filters, such as SuperEagle, SuperSaI and HQ2x. Unfortunately, none of these filters gets everything quite right, especially numbers and letters, which can look bubbly or overly smoothed (you can learn more about pixel art scaling algorithms here).

Purists have long been turned off by the inaccuracies of interpolating filters and have instead used scanline masks to try and capture the effect of an interlaced display, relying on the human brain's natural ability to recognize patterns and fill in the gaps between lines (you can learn everything you ever wanted to know about scanlines here). However, this too falls short from a true representation of a CRT display, as it ignores the existence of phosphors--the tiny red-, green- and blue-colored lenses that the electron gun in the back of a CRT tube shoots with a beam of electrons to recreate a colored pixel--and the color bleed that naturally occurs in these displays.

Recently, a number of determined individuals have set out to try and capture all of the different effects of a CRT display, warts and all, to truly reproduce classic pixel art the way it was meant to be viewed.

The Comparisons:

(Each of these images is presented as it would be displayed onscreen, at a resolution of approximately 800x600, then again at 400% scale without any interpolation used when scaling; as always, click the thumbnail to embiggen)

First, we should look at the baseline. This was scaled up to size using nearest-neighbor and is otherwise untouched:

Next, we'll add blargg's NTSC filter, which emulates the noise and color bleed of an NTSC video signal (this filter has several presets; I will only be showing the RGB preset, which reproduces the look of an SNES hooked up via RGB connection [not available in the U.S.], and the RF preset, which reproduces the look of the SNES RF modulator attachment, respectively):

(<- Look at that noisy RF signal!)

As a note, blargg's NTSC filter is so accurate that byuu, the author of bsnes, recommends its use along with bsnes' accuracy profile to achieve proper blending on games that use halftones to simulate transparency (Jurassic Park and Kirby's Dreamland, for example).

Next up, we'll look at cgwg's CRT shader, which includes a phosphor mask and barrel distortion to simulate the screen curvature of a CRT television (just look at those RGB phophors!):

Similarly, there is a version of cgwg's CRT shader, which doesn't include the barrel distortion and represents an idealized flat CRT (actual flat CRTs tended to have slight blurring at the edges where the tube curvature would normally be). Incidentally, this version also has no visible garbage pixels (the occasional black specs that are visible in the curved version):

Pixel Shaders vs. Software Filters

cgwg's CRT shader is a special kind of filter known as a pixel shader. Unlike regular filters, which rely on the CPU to do all of the complex upscaling calculations, pixel shaders draw on the awesome computing power of the video card to do the calculations, thereby leaving the CPU to focus on emulating the SNES. Additionally, since the pixel shader is calculated separately from the filter in bsnes, you can stack blargg's NTSC filter with cgwg's CRT shader:

Finally, for non-purists, we'll look at the combination of cgwg's CRT shader with the popular SuperSaI filter, which creates a pleasing--though not quite as accurate--output:

As amazing as cgwg's CRT shader is already, there is still some room for improvement. For example, the current implementation misses the intensity-based bloom effect on individual phosphors that can be seen in a true CRT. DOLLS (J) [!], one of the contributors to the CRT reproduction effort, intends to write a more complete CRT emulation shader in the future that will incorporate these and other idiosyncracies.

Click here to download cgwg's CRT shader (Also includes the 'flat' version for those who don't like the tube-style curvature; UPDATE: fixed dead link), which is compatible with bsnes and the newest release of snes9x. For more information on CRT emulation, you can check out this highly informative thread on the bsnes forum. For other bsnes-compatible shaders that are not included with the official download, check out my mediafire account.

UPDATE (3/4/2011): Themaister did a rewrite of the flat version of cgwg's CRT shader, moving many of the calculations from fragment to vertex, which provides a substantial ~20% increase in speed (making it usable on many older and less powerful video cards). This rewrite also appears to conform more rigidly to the GLSL shader spec, making it compatible with more cards from different vendors. I have labeled it v4 of cgwg's CRT Flat, and it is available in the aforemented mediafire account.