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*
* Copyright 2013 VMware, Inc.
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*
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/**
* @file
* Format conversion code for srgb formats.
*
* Functions for converting from srgb to linear and vice versa.
* From http://www.opengl.org/registry/specs/EXT/texture_sRGB.txt:
*
* srgb->linear:
* cl = cs / 12.92, cs <= 0.04045
* cl = ((cs + 0.055)/1.055)^2.4, cs > 0.04045
*
* linear->srgb:
* if (isnan(cl)) {
* Map IEEE-754 Not-a-number to zero.
* cs = 0.0;
* } else if (cl > 1.0) {
* cs = 1.0;
* } else if (cl < 0.0) {
* cs = 0.0;
* } else if (cl < 0.0031308) {
* cs = 12.92 * cl;
* } else {
* cs = 1.055 * pow(cl, 0.41666) - 0.055;
* }
*
* This does not need to be accurate, however at least for d3d10
* (http://msdn.microsoft.com/en-us/library/windows/desktop/dd607323%28v=vs.85%29.aspx):
* 1) For srgb->linear, it is required that the error on the srgb side is
* not larger than 0.5f, which I interpret that if you map the value back
* to srgb from linear using the ideal conversion, it would not be off by
* more than 0.5f (that is, it would map to the same 8-bit integer value
* as it was before conversion to linear).
* 2) linear->srgb is permitted 0.6f which luckily looks like quite a large
* error is allowed.
* 3) Additionally, all srgb values converted to linear and back must result
* in the same value as they were originally.
*
* @author Roland Scheidegger <sroland@vmware.com>
*/
#include "util/u_debug.h"
#include "lp_bld_type.h"
#include "lp_bld_const.h"
#include "lp_bld_arit.h"
#include "lp_bld_bitarit.h"
#include "lp_bld_logic.h"
#include "lp_bld_format.h"
/**
* Convert srgb int values to linear float values.
* Several possibilities how to do this, e.g.
* - table
* - doing the pow() with int-to-float and float-to-int tricks
* (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
* - just using standard polynomial approximation
* (3rd order polynomial is required for crappy but just sufficient accuracy)
*
* @param src integer (vector) value(s) to convert
* (8 bit values unpacked to 32 bit already).
*/
LLVMValueRef
lp_build_srgb_to_linear(struct gallivm_state *gallivm,
struct lp_type src_type,
LLVMValueRef src)
{
struct lp_type f32_type = lp_type_float_vec(32, src_type.length * 32);
struct lp_build_context f32_bld;
LLVMValueRef srcf, part_lin, part_pow, is_linear, lin_const, lin_thresh;
double coeffs[4] = {0.0023f,
0.0030f / 255.0f,
0.6935f / (255.0f * 255.0f),
0.3012f / (255.0f * 255.0f * 255.0f)
};
lp_build_context_init(&f32_bld, gallivm, f32_type);
/*
* using polynomial: (src * (src * (src * 0.3012 + 0.6935) + 0.0030) + 0.0023)
* ( poly = 0.3012*x^3 + 0.6935*x^2 + 0.0030*x + 0.0023)
* (found with octave polyfit and some magic as I couldn't get the error
* function right). Using the above mentioned error function, the values stay
* within +-0.35, except for the lowest values - hence tweaking linear segment
* to cover the first 16 instead of the first 11 values (the error stays
* just about acceptable there too).
* Hence: lin = src > 15 ? poly : src / 12.6
* This function really only makes sense for vectors, should use LUT otherwise.
* All in all (including float conversion) 11 instructions (with sse4.1),
* 6 constants (polynomial could be done with 1 instruction less at the cost
* of slightly worse dependency chain, fma should also help).
*/
/* doing the 1/255 mul as part of the approximation */
srcf = lp_build_int_to_float(&f32_bld, src);
lin_const = lp_build_const_vec(gallivm, f32_type, 1.0f / (12.6f * 255.0f));
part_lin = lp_build_mul(&f32_bld, srcf, lin_const);
part_pow = lp_build_polynomial(&f32_bld, srcf, coeffs, 4);
lin_thresh = lp_build_const_vec(gallivm, f32_type, 15.0f);
is_linear = lp_build_compare(gallivm, f32_type, PIPE_FUNC_LEQUAL, srcf, lin_thresh);
return lp_build_select(&f32_bld, is_linear, part_lin, part_pow);
}
/**
* Convert linear float values to srgb int values.
* Several possibilities how to do this, e.g.
* - use table (based on exponent/highest order mantissa bits) and do
* linear interpolation (https://gist.github.com/rygorous/2203834)
* - Chebyshev polynomial
* - Approximation using reciprocals
* - using int-to-float and float-to-int tricks for pow()
* (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
*
* @param src float (vector) value(s) to convert.
*/
static LLVMValueRef
lp_build_linear_to_srgb(struct gallivm_state *gallivm,
struct lp_type src_type,
LLVMValueRef src)
{
LLVMBuilderRef builder = gallivm->builder;
struct lp_build_context f32_bld;
LLVMValueRef lin_thresh, lin, lin_const, is_linear, tmp, pow_final;
lp_build_context_init(&f32_bld, gallivm, src_type);
src = lp_build_clamp(&f32_bld, src, f32_bld.zero, f32_bld.one);
if (0) {
/*
* using int-to-float and float-to-int trick for pow().
* This is much more accurate than necessary thanks to the correction,
* but it most certainly makes no sense without rsqrt available.
* Bonus points if you understand how this works...
* All in all (including min/max clamp, conversion) 19 instructions.
*/
float exp_f = 2.0f / 3.0f;
/* some compilers can't do exp2f, so this is exp2f(127.0f/exp_f - 127.0f) */
float exp2f_c = 1.30438178253e+19f;
float coeff_f = 0.62996f;
LLVMValueRef pow_approx, coeff, x2, exponent, pow_1, pow_2;
struct lp_type int_type = lp_int_type(src_type);
/*
* First calculate approx x^8/12
*/
exponent = lp_build_const_vec(gallivm, src_type, exp_f);
coeff = lp_build_const_vec(gallivm, src_type,
exp2f_c * powf(coeff_f, 1.0f / exp_f));
/* premultiply src */
tmp = lp_build_mul(&f32_bld, coeff, src);
/* "log2" */
tmp = LLVMBuildBitCast(builder, tmp, lp_build_vec_type(gallivm, int_type), "");
tmp = lp_build_int_to_float(&f32_bld, tmp);
/* multiply for pow */
tmp = lp_build_mul(&f32_bld, tmp, exponent);
/* "exp2" */
pow_approx = lp_build_itrunc(&f32_bld, tmp);
pow_approx = LLVMBuildBitCast(builder, pow_approx,
lp_build_vec_type(gallivm, src_type), "");
/*
* Since that pow was inaccurate (like 3 bits, though each sqrt step would
* give another bit), compensate the error (which is why we chose another
* exponent in the first place).
*/
/* x * x^(8/12) = x^(20/12) */
pow_1 = lp_build_mul(&f32_bld, pow_approx, src);
/* x * x * x^(-4/12) = x^(20/12) */
/* Should avoid using rsqrt if it's not available, but
* using x * x^(4/12) * x^(4/12) instead will change error weight */
tmp = lp_build_fast_rsqrt(&f32_bld, pow_approx);
x2 = lp_build_mul(&f32_bld, src, src);
pow_2 = lp_build_mul(&f32_bld, x2, tmp);
/* average the values so the errors cancel out, compensate bias,
* we also squeeze the 1.055 mul of the srgb conversion plus the 255.0 mul
* for conversion to int in here */
tmp = lp_build_add(&f32_bld, pow_1, pow_2);
coeff = lp_build_const_vec(gallivm, src_type,
1.0f / (3.0f * coeff_f) * 0.999852f *
powf(1.055f * 255.0f, 4.0f));
pow_final = lp_build_mul(&f32_bld, tmp, coeff);
/* x^(5/12) = rsqrt(rsqrt(x^20/12)) */
if (lp_build_fast_rsqrt_available(src_type)) {
pow_final = lp_build_fast_rsqrt(&f32_bld,
lp_build_fast_rsqrt(&f32_bld, pow_final));
}
else {
pow_final = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, pow_final));
}
pow_final = lp_build_add(&f32_bld, pow_final,
lp_build_const_vec(gallivm, src_type, -0.055f * 255.0f));
}
else {
/*
* using "rational polynomial" approximation here.
* Essentially y = a*x^0.375 + b*x^0.5 + c, with also
* factoring in the 255.0 mul and the scaling mul.
* (a is closer to actual value so has higher weight than b.)
* Note: the constants are magic values. They were found empirically,
* possibly could be improved but good enough (be VERY careful with
* error metric if you'd want to tweak them, they also MUST fit with
* the crappy polynomial above for srgb->linear since it is required
* that each srgb value maps back to the same value).
* This function has an error of max +-0.17 (and we'd only require +-0.6),
* for the approximated srgb->linear values the error is naturally larger
* (+-0.42) but still accurate enough (required +-0.5 essentially).
* All in all (including min/max clamp, conversion) 15 instructions.
* FMA would help (minus 2 instructions).
*/
LLVMValueRef x05, x0375, a_const, b_const, c_const, tmp2;
if (lp_build_fast_rsqrt_available(src_type)) {
tmp = lp_build_fast_rsqrt(&f32_bld, src);
x05 = lp_build_mul(&f32_bld, src, tmp);
}
else {
/*
* I don't really expect this to be practical without rsqrt
* but there's no reason for triple punishment so at least
* save the otherwise resulting division and unnecessary mul...
*/
x05 = lp_build_sqrt(&f32_bld, src);
}
tmp = lp_build_mul(&f32_bld, x05, src);
if (lp_build_fast_rsqrt_available(src_type)) {
x0375 = lp_build_fast_rsqrt(&f32_bld, lp_build_fast_rsqrt(&f32_bld, tmp));
}
else {
x0375 = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, tmp));
}
a_const = lp_build_const_vec(gallivm, src_type, 0.675f * 1.0622 * 255.0f);
b_const = lp_build_const_vec(gallivm, src_type, 0.325f * 1.0622 * 255.0f);
c_const = lp_build_const_vec(gallivm, src_type, -0.0620f * 255.0f);
tmp = lp_build_mul(&f32_bld, a_const, x0375);
tmp2 = lp_build_mul(&f32_bld, b_const, x05);
tmp2 = lp_build_add(&f32_bld, tmp2, c_const);
pow_final = lp_build_add(&f32_bld, tmp, tmp2);
}
/* linear part is easy */
lin_const = lp_build_const_vec(gallivm, src_type, 12.92f * 255.0f);
lin = lp_build_mul(&f32_bld, src, lin_const);
lin_thresh = lp_build_const_vec(gallivm, src_type, 0.0031308f);
is_linear = lp_build_compare(gallivm, src_type, PIPE_FUNC_LEQUAL, src, lin_thresh);
tmp = lp_build_select(&f32_bld, is_linear, lin, pow_final);
f32_bld.type.sign = 0;
return lp_build_iround(&f32_bld, tmp);
}
/**
* Convert linear float soa values to packed srgb AoS values.
* This only handles packed formats which are 4x8bit in size
* (rgba and rgbx plus swizzles).
*
* @param src float SoA (vector) values to convert.
*/
LLVMValueRef
lp_build_float_to_srgb_packed(struct gallivm_state *gallivm,
const struct util_format_description *dst_fmt,
struct lp_type src_type,
LLVMValueRef *src)
{
LLVMBuilderRef builder = gallivm->builder;
unsigned chan;
struct lp_build_context f32_bld;
struct lp_type int32_type = lp_int_type(src_type);
LLVMValueRef tmpsrgb[4], alpha, dst;
lp_build_context_init(&f32_bld, gallivm, src_type);
/* rgb is subject to linear->srgb conversion, alpha is not */
for (chan = 0; chan < 3; chan++) {
tmpsrgb[chan] = lp_build_linear_to_srgb(gallivm, src_type, src[chan]);
}
/*
* can't use lp_build_conv since we want to keep values as 32bit
* here so we can interleave with rgb to go from SoA->AoS.
*/
alpha = lp_build_clamp(&f32_bld, src[3], f32_bld.zero, f32_bld.one);
alpha = lp_build_mul(&f32_bld, alpha,
lp_build_const_vec(gallivm, src_type, 255.0f));
tmpsrgb[3] = lp_build_iround(&f32_bld, alpha);
dst = lp_build_zero(gallivm, int32_type);
for (chan = 0; chan < dst_fmt->nr_channels; chan++) {
if (dst_fmt->swizzle[chan] <= UTIL_FORMAT_SWIZZLE_W) {
unsigned ls;
LLVMValueRef shifted, shift_val;
ls = dst_fmt->channel[dst_fmt->swizzle[chan]].shift;
shift_val = lp_build_const_int_vec(gallivm, int32_type, ls);
shifted = LLVMBuildShl(builder, tmpsrgb[chan], shift_val, "");
dst = LLVMBuildOr(builder, dst, shifted, "");
}
}
return dst;
}