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rk27xx - use more correct function to map between percived lightness and PWM fill factor when setting backlight brightness

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git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30168 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Marcin Bukat 2011-07-19 18:29:30 +00:00
parent 5d9b230168
commit 31b7ecb09b
1 changed files with 20 additions and 7 deletions
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27
firmware/target/arm/rk27xx/backlight-rk27xx.c
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@ -26,11 +26,24 @@
#include "system.h" #include "system.h"
static int brightness = DEFAULT_BRIGHTNESS_SETTING; static int brightness = DEFAULT_BRIGHTNESS_SETTING;
static const unsigned short log_brightness[] = {
0x2710, 0x27ac, 0x2849, 0x2983, 0x2abd, 0x2c93, 0x2e6a, 0x30dd, /* Based on http://www.poynton.com/PDFs/SMPTE93_Gamma.pdf
0x3351, 0x3661, 0x3971, 0x3d1f, 0x40cc, 0x4516, 0x4960, 0x4e47, * CIE standarized function that relates physical luminance
0x532e, 0x58b1, 0x5e35, 0x6456, 0x6a76, 0x7134, 0x77f1, 0x7f4c, * to perceived lightness
0x86a6, 0x8e9d, 0x9695, 0x9f29, 0xa7bd, 0xb0ee, 0xba1f, 0xc350 *
* 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) 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 */ /* set pwm frequency to 500Hz - my lcd panel can't cope more reliably */
/* (apb_freq/pwm_freq)/pwm_div = (50 000 000/500)/2 */ /* (apb_freq/pwm_freq)/pwm_div = (50 000 000/500)/2 */
PWMT0_LRC = 50000; PWMT0_LRC = 50000;
PWMT0_HRC = log_brightness[brightness]; PWMT0_HRC = lin_brightness[brightness];
/* reset counter */ /* reset counter */
PWMT0_CNTR = 0x00; PWMT0_CNTR = 0x00;
@ -88,5 +101,5 @@ void _backlight_off(void)
void _backlight_set_brightness(int val) void _backlight_set_brightness(int val)
{ {
brightness = val & 0x1f; brightness = val & 0x1f;
PWMT0_HRC = log_brightness[brightness]; PWMT0_HRC = lin_brightness[brightness];
} }