fix(st7789): improve PWM backlight dimming color accuracy - #4
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…ss bias Original code expanded RGB565 to 8-bit channels, multiplied with truncating division (/256), then shifted back — causing consistent darkening and minor hue errors, especially noticeable at 40–80% brightness. Improved version: - Extracts components in native precision: 5-bit red/blue, 6-bit green - Scales with proper rounding: (val * backlight + 127) >> 8 - Packs directly back to RGB565 format Improves visual quality dramatically: - Grays/whites stay neutral instead of going too dark or greenish - Better preservation of shadow detail - More natural fade-to-black behavior overall
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Again, this work was done by the https://micropythonos.com/ team to allow running a normal MicroPythonOS build (which works on many boards, including the LilyGo T-Display-S3) on an emulated esp32s3. Thank you for all your work on QEMU! |
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Thanks, I hadn't noticed this.
ThomasFarstrike
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The path VBE_setPalette -> SDL_SetColors converted Duke3D's 6-bit palette values through an unnecessary 8-bit intermediate, creating an asymmetric precision issue analogous to the QEMU st7789 backlight fix (a159x36/qemu#4): Old: r6/b6 -> 8-bit via (v<<2)|(v>>4) -> >>3 -> r5 (net: v6>>1, floor) g6 -> 8-bit via (v<<2)|(v>>4) -> >>2 -> g6 (net: identity, lossless) For any palette entry with odd source values, R5=B5=floor(v/2)=0 while G6=v remains non-zero — producing a pure-green pixel instead of near-black. This is visible as a greenish cast in dark/shadowed areas. Fix: add SDL_SetPalette565() which converts directly from the 6-bit source, zeroing the green LSB to give it the same effective 5-bit precision as R/B: r5 = v6 >> 1 g6 = (v6 >> 1) << 1 // 5-bit precision in the 6-bit RGB565 green field b5 = v6 >> 1 For neutral input (r=g=b=val): r5/31 == (g6>>1)/31 == b5/31 -> neutral gray. At full brightness (val=63): r5=31, g6=62, b5=31 — green is 1/63 (~1.6%) shy of maximum, which is perceptually indistinguishable. Also removes the 1KB static SDL_Color fmt_swap[256] intermediate buffer and the SDL_SetColors() call, reducing per-palette-update work by ~50%.
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Original code expanded RGB565 to 8-bit channels, multiplied with truncating division (/256), then shifted back, causing consistent darkening and minor hue errors, especially noticeable at 40–80% brightness.
Improved version:
Improves visual quality dramatically:
Before:
at 90% brightness, the gray and blue turned greenish
After:
at 90% brightness, all the colors look as they should