From 31b7ecb09b31d7fb1e95758d5c7b3b40b38437ed Mon Sep 17 00:00:00 2001 From: Marcin Bukat Date: Tue, 19 Jul 2011 18:29:30 +0000 Subject: [PATCH] rk27xx - use more correct function to map between percived lightness and PWM fill factor when setting backlight brightness git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30168 a1c6a512-1295-4272-9138-f99709370657 --- firmware/target/arm/rk27xx/backlight-rk27xx.c | 27 ++++++++++++++----- 1 file changed, 20 insertions(+), 7 deletions(-) diff --git a/firmware/target/arm/rk27xx/backlight-rk27xx.c b/firmware/target/arm/rk27xx/backlight-rk27xx.c index e759220dff..cb669aecff 100644 --- a/firmware/target/arm/rk27xx/backlight-rk27xx.c +++ b/firmware/target/arm/rk27xx/backlight-rk27xx.c @@ -26,11 +26,24 @@ #include "system.h" static int brightness = DEFAULT_BRIGHTNESS_SETTING; -static const unsigned short log_brightness[] = { - 0x2710, 0x27ac, 0x2849, 0x2983, 0x2abd, 0x2c93, 0x2e6a, 0x30dd, - 0x3351, 0x3661, 0x3971, 0x3d1f, 0x40cc, 0x4516, 0x4960, 0x4e47, - 0x532e, 0x58b1, 0x5e35, 0x6456, 0x6a76, 0x7134, 0x77f1, 0x7f4c, - 0x86a6, 0x8e9d, 0x9695, 0x9f29, 0xa7bd, 0xb0ee, 0xba1f, 0xc350 + +/* Based on http://www.poynton.com/PDFs/SMPTE93_Gamma.pdf + * CIE standarized function that relates physical luminance + * to perceived lightness + * + * L* = 116*(Y/Yn)^(1/3) - 16 (Y/Yn > 0 008856) + * where Yn is luminance of reference white + * + * Actual function is lightly tweaked to account for the fact + * that fill factor of the PWM below ~15% gives black. + * So the function used to calculate the values in the matrix was: + * f(x) = 42000 * ((100*x/31 + 16)/116)^3 + 8000 + */ +static const unsigned short lin_brightness[] = { + 8110, 8191, 8304, 8455, 8649, 8892, 9189, 9545, + 9966, 10457, 11024, 11671, 12406, 13232, 14156, 15182, + 16316, 17565, 18932, 20423, 22045, 23801, 25699, 27742, + 29937, 32289, 34803, 37485, 40340, 43374, 46592, 50000 }; bool _backlight_init(void) @@ -50,7 +63,7 @@ bool _backlight_init(void) /* set pwm frequency to 500Hz - my lcd panel can't cope more reliably */ /* (apb_freq/pwm_freq)/pwm_div = (50 000 000/500)/2 */ PWMT0_LRC = 50000; - PWMT0_HRC = log_brightness[brightness]; + PWMT0_HRC = lin_brightness[brightness]; /* reset counter */ PWMT0_CNTR = 0x00; @@ -88,5 +101,5 @@ void _backlight_off(void) void _backlight_set_brightness(int val) { brightness = val & 0x1f; - PWMT0_HRC = log_brightness[brightness]; + PWMT0_HRC = lin_brightness[brightness]; }