--- /dev/null
+/**************************************************************************
+ *
+ * Copyright 2013 VMware, Inc.
+ * All Rights Reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sub license, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the
+ * next paragraph) shall be included in all copies or substantial portions
+ * of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT.
+ * IN NO EVENT SHALL VMWARE AND/OR ITS SUPPLIERS BE LIABLE FOR
+ * ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+ * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ **************************************************************************/
+
+
+/**
+ * @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)
+ };
+
+ assert(src_type.width == 32);
+
+ 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.
+ */
+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;
+ 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(127.0f / exp_f - 127.0f) *
+ 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);
+}
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