From 4668221cfe67e32e892cd9a157a43edd426d5b65 Mon Sep 17 00:00:00 2001 From: soaos Date: Thu, 2 Oct 2025 14:47:39 -0400 Subject: Mostly finished the GP text renderer post, need to finish on desktop --- blog/terminal_renderer_mkii/david.png | Bin 0 -> 27218 bytes blog/terminal_renderer_mkii/davidbayer.png | Bin 0 -> 4213 bytes blog/terminal_renderer_mkii/davidthreshold.png | Bin 0 -> 3324 bytes blog/terminal_renderer_mkii/index.html | 156 +++++++++++++++++++++++-- 4 files changed, 148 insertions(+), 8 deletions(-) create mode 100644 blog/terminal_renderer_mkii/david.png create mode 100644 blog/terminal_renderer_mkii/davidbayer.png create mode 100644 blog/terminal_renderer_mkii/davidthreshold.png (limited to 'blog') diff --git a/blog/terminal_renderer_mkii/david.png b/blog/terminal_renderer_mkii/david.png new file mode 100644 index 0000000..6cfa884 Binary files /dev/null and b/blog/terminal_renderer_mkii/david.png differ diff --git a/blog/terminal_renderer_mkii/davidbayer.png b/blog/terminal_renderer_mkii/davidbayer.png new file mode 100644 index 0000000..af4bfc4 Binary files /dev/null and b/blog/terminal_renderer_mkii/davidbayer.png differ diff --git a/blog/terminal_renderer_mkii/davidthreshold.png b/blog/terminal_renderer_mkii/davidthreshold.png new file mode 100644 index 0000000..6c6e014 Binary files /dev/null and b/blog/terminal_renderer_mkii/davidthreshold.png differ diff --git a/blog/terminal_renderer_mkii/index.html b/blog/terminal_renderer_mkii/index.html index bd37efc..78171f1 100644 --- a/blog/terminal_renderer_mkii/index.html +++ b/blog/terminal_renderer_mkii/index.html @@ -19,26 +19,166 @@

Terminal Renderer - Rendering to Text with Compute

October 2, 2025

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This week I brought my terminal renderer to the next level by performing text rendering on the GPU.

+

This week I brought my terminal renderer to the next level by performing text rendering on the GPU. +

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The Stanford Dragon, outlined and rendered as Braille characters in a terminal emulator.
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The Stanford Dragon, outlined and rendered as Braille characters in a terminal emulator. +
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Preamble: Unicode Braille and Ordered Dithering

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Context

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Unicode Braille

- I first messed around with rendering images to the terminal with Braille characters in like 2022 I think? I wrote a simple CLI tool - that applied a threshold to an input image and output it as Braille characters in the terminal. + I first messed around with rendering images to the terminal with Braille characters in like 2022 I + think? I wrote a simple CLI tool + that applied a threshold to an input image and output it as Braille characters in the terminal. Here's a recording I took back + when I did it. +

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+ + + + + + + + + + + + + + + + + + + +
03
14
25
67
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The corresponding bit position for each braille dot.
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+ This effect is pretty cool, and it was pretty easy to implement as well. The trick lies in how the + Unicode Braille block + is laid out. Every 8-dot Braille combination happens to add up to 256 combinations, the perfect amount to + fit in the range between 0x2800 (⠀) and 0x28FF (⣿). In other words, every + character + within the block can be represented by changing the value of a single byte. +

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+ The lowest 6 bits of the pattern map on to a 6-dot braille pattern. However, due + to historical reasons the 8-dot values were tacked on after the fact, which adds + a slightly annoying mapping to the conversion process. Either way, it's a lot easier + than it could be to just read a pixel value, check its brightness, and then use a + bitwise operation to set/clear a dot. +

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Ordered Dithering

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+ Comparing the brightnes of a pixel against a constant threshold is a fine way to + display black and white images, but it's far from ideal and often results in the loss + of a lot of detail from the original image. +

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+ + + +
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From left to right: Original image, threshold, and ordered dither. Wikipedia
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By using ordered dithering, + we + can preserve much more of the subtleties of the original image. While not the "truest" version of + dithering possible, + ordered dithering (and Bayer dithering in particular) provides a few advantages that make it very + well suited to realtime computer graphics: +

+ Feel free to read up on the specifics of threshold maps and stuff, but for the purposes of this little + explanation it's + enough to know that it's basically just a matrix of 𝓃⨉𝓃 values between 0 and 1, and then to determine + whether a pixel (𝓍,𝓎) + is white or black, you check the brightness against the threshold value at (𝓍%𝓃,𝓎%𝓃) in the map.

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Generating and Parsing Logs

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The old way™

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+ My first attempt at realtime terminal graphics with ordered dithering + (I put a video up at the time) + ran entirely on the CPU. I pre-calculated the threshold map at the beginning of execution and ran each + frame + through a sequential function to dither it and convert it to Braille characters. +

+ To be honest, I never noticed + any significant performance issues doing this, as you can imagine the image size required to fill a + terminal + screen is signficantly smaller than a normal window. However, I knew I could easily perform the + dithering on the GPU + as a post-processing effect, so I eventually wrote a shader to do that. In combination with another + effect I used to + add outlines to objects, I was able to significantly improve the visual fidelity of the experience. A + good example of + where the renderer was at until like a week ago can be seen in this video. +

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+ Until now I hadn't really considered moving the text conversion to the GPU. I mean, GPU is for + graphics, + right? I just copied the entire framebuffer back onto the CPU after dithering + and used the same sequential conversion algorithm. Then I had an idea that would drastically reduce the + amount + of copying necessary. +

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Compute post-processing

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+ What if, instead of extracting and copying the framebuffer every single frame, we "rendered" the text on + the GPU + and read that back instead? Assuming each pixel in a texture is 32 bits (RGBA8), and knowing that + each braille + character is a block of 8 pixels, could we not theoretically shave off at least 7/8 of the bytes + copied? +

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+ As it turns out, it's remarkably easy to do. I'm using the Bevy engine, + and hooking in a compute node to my existing post-processing render pipeline worked right out of the + box. + I allocated a storage buffer large enough to hold the necessary amount of characters, read it back each + frame, and dumped + the contents into the terminal. +

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+ I used UTF-32 encoding on the storage buffer because I knew I could easily convert a "wide string" into + UTF-8 before printing it, and + 32 bits provides a consistent space to fill for each workgroup in the shader versus a variable-length + encoding like UTF-8. + Although now that I think about it, I could probably switch to using UTF-16 since all the Braille + characters could be represented + in 2 bytes, and that would be half the size of the UTF-32 text, which is half empty bytes anyways. +

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Okay so I went and did that, seems to work great. Wow. This little side quest has been a part of my + broader efforts to revive a project I + spent a lot of time on. I'm taking the opportunity to really dig in and rework some of the stuff I'm not + totally happy with. So there might be quite a few of this kind of post in the near future. Stay tuned.

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