1 /**************************************************************************
2 *
3 * Copyright 2013 VMware, Inc.
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27
28
29 /**
30 * @file
31 * Format conversion code for srgb formats.
32 *
33 * Functions for converting from srgb to linear and vice versa.
34 * From http://www.opengl.org/registry/specs/EXT/texture_sRGB.txt:
35 *
36 * srgb->linear:
37 * cl = cs / 12.92, cs <= 0.04045
38 * cl = ((cs + 0.055)/1.055)^2.4, cs > 0.04045
39 *
40 * linear->srgb:
41 * if (isnan(cl)) {
42 * Map IEEE-754 Not-a-number to zero.
43 * cs = 0.0;
44 * } else if (cl > 1.0) {
45 * cs = 1.0;
46 * } else if (cl < 0.0) {
47 * cs = 0.0;
48 * } else if (cl < 0.0031308) {
49 * cs = 12.92 * cl;
50 * } else {
51 * cs = 1.055 * pow(cl, 0.41666) - 0.055;
52 * }
53 *
54 * This does not need to be accurate, however at least for d3d10
55 * (http://msdn.microsoft.com/en-us/library/windows/desktop/dd607323%28v=vs.85%29.aspx):
56 * 1) For srgb->linear, it is required that the error on the srgb side is
57 * not larger than 0.5f, which I interpret that if you map the value back
58 * to srgb from linear using the ideal conversion, it would not be off by
59 * more than 0.5f (that is, it would map to the same 8-bit integer value
60 * as it was before conversion to linear).
61 * 2) linear->srgb is permitted 0.6f which luckily looks like quite a large
62 * error is allowed.
63 * 3) Additionally, all srgb values converted to linear and back must result
64 * in the same value as they were originally.
65 *
66 * @author Roland Scheidegger <sroland@vmware.com>
67 */
68
69
70 #include "util/u_debug.h"
71 #include "util/u_math.h"
72
73 #include "lp_bld_type.h"
74 #include "lp_bld_const.h"
75 #include "lp_bld_arit.h"
76 #include "lp_bld_bitarit.h"
77 #include "lp_bld_logic.h"
78 #include "lp_bld_format.h"
79
80
81
82 /**
83 * Convert srgb int values to linear float values.
84 * Several possibilities how to do this, e.g.
85 * - table
86 * - doing the pow() with int-to-float and float-to-int tricks
87 * (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
88 * - just using standard polynomial approximation
89 * (3rd order polynomial is required for crappy but just sufficient accuracy)
90 *
91 * @param src integer (vector) value(s) to convert
92 * (chan_bits bit values unpacked to 32 bit already).
93 */
94 LLVMValueRef
lp_build_srgb_to_linear(struct gallivm_state * gallivm,struct lp_type src_type,unsigned chan_bits,LLVMValueRef src)95 lp_build_srgb_to_linear(struct gallivm_state *gallivm,
96 struct lp_type src_type,
97 unsigned chan_bits,
98 LLVMValueRef src)
99 {
100 struct lp_type f32_type = lp_type_float_vec(32, src_type.length * 32);
101 struct lp_build_context f32_bld;
102 LLVMValueRef srcf, part_lin, part_pow, is_linear, lin_const, lin_thresh;
103 double coeffs[4] = {0.0023f,
104 0.0030f / 255.0f,
105 0.6935f / (255.0f * 255.0f),
106 0.3012f / (255.0f * 255.0f * 255.0f)
107 };
108
109 assert(src_type.width == 32);
110 /* Technically this would work with more bits too but would be inaccurate. */
111 assert(chan_bits <= 8);
112
113 lp_build_context_init(&f32_bld, gallivm, f32_type);
114
115 /*
116 * using polynomial: (src * (src * (src * 0.3012 + 0.6935) + 0.0030) + 0.0023)
117 * ( poly = 0.3012*x^3 + 0.6935*x^2 + 0.0030*x + 0.0023)
118 * (found with octave polyfit and some magic as I couldn't get the error
119 * function right). Using the above mentioned error function, the values stay
120 * within +-0.35, except for the lowest values - hence tweaking linear segment
121 * to cover the first 16 instead of the first 11 values (the error stays
122 * just about acceptable there too).
123 * Hence: lin = src > 15 ? poly : src / 12.6
124 * This function really only makes sense for vectors, should use LUT otherwise.
125 * All in all (including float conversion) 11 instructions (with sse4.1),
126 * 6 constants (polynomial could be done with 1 instruction less at the cost
127 * of slightly worse dependency chain, fma should also help).
128 */
129 /* doing the 1/255 mul as part of the approximation */
130 srcf = lp_build_int_to_float(&f32_bld, src);
131 if (chan_bits != 8) {
132 /* could adjust all the constants instead */
133 LLVMValueRef rescale_const = lp_build_const_vec(gallivm, f32_type,
134 255.0f / ((1 << chan_bits) - 1));
135 srcf = lp_build_mul(&f32_bld, srcf, rescale_const);
136 }
137 lin_const = lp_build_const_vec(gallivm, f32_type, 1.0f / (12.6f * 255.0f));
138 part_lin = lp_build_mul(&f32_bld, srcf, lin_const);
139
140 part_pow = lp_build_polynomial(&f32_bld, srcf, coeffs, 4);
141
142 lin_thresh = lp_build_const_vec(gallivm, f32_type, 15.0f);
143 is_linear = lp_build_compare(gallivm, f32_type, PIPE_FUNC_LEQUAL, srcf, lin_thresh);
144 return lp_build_select(&f32_bld, is_linear, part_lin, part_pow);
145 }
146
147
148 /**
149 * Convert linear float values to srgb int values.
150 * Several possibilities how to do this, e.g.
151 * - use table (based on exponent/highest order mantissa bits) and do
152 * linear interpolation (https://gist.github.com/rygorous/2203834)
153 * - Chebyshev polynomial
154 * - Approximation using reciprocals
155 * - using int-to-float and float-to-int tricks for pow()
156 * (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
157 *
158 * @param src float (vector) value(s) to convert.
159 */
160 static LLVMValueRef
lp_build_linear_to_srgb(struct gallivm_state * gallivm,struct lp_type src_type,unsigned chan_bits,LLVMValueRef src)161 lp_build_linear_to_srgb(struct gallivm_state *gallivm,
162 struct lp_type src_type,
163 unsigned chan_bits,
164 LLVMValueRef src)
165 {
166 LLVMBuilderRef builder = gallivm->builder;
167 struct lp_build_context f32_bld;
168 LLVMValueRef lin_thresh, lin, lin_const, is_linear, tmp, pow_final;
169
170 lp_build_context_init(&f32_bld, gallivm, src_type);
171
172 src = lp_build_clamp(&f32_bld, src, f32_bld.zero, f32_bld.one);
173
174 if (0) {
175 /*
176 * using int-to-float and float-to-int trick for pow().
177 * This is much more accurate than necessary thanks to the correction,
178 * but it most certainly makes no sense without rsqrt available.
179 * Bonus points if you understand how this works...
180 * All in all (including min/max clamp, conversion) 19 instructions.
181 */
182
183 float exp_f = 2.0f / 3.0f;
184 /* some compilers can't do exp2f, so this is exp2f(127.0f/exp_f - 127.0f) */
185 float exp2f_c = 1.30438178253e+19f;
186 float coeff_f = 0.62996f;
187 LLVMValueRef pow_approx, coeff, x2, exponent, pow_1, pow_2;
188 struct lp_type int_type = lp_int_type(src_type);
189
190 /*
191 * First calculate approx x^8/12
192 */
193 exponent = lp_build_const_vec(gallivm, src_type, exp_f);
194 coeff = lp_build_const_vec(gallivm, src_type,
195 exp2f_c * powf(coeff_f, 1.0f / exp_f));
196
197 /* premultiply src */
198 tmp = lp_build_mul(&f32_bld, coeff, src);
199 /* "log2" */
200 tmp = LLVMBuildBitCast(builder, tmp, lp_build_vec_type(gallivm, int_type), "");
201 tmp = lp_build_int_to_float(&f32_bld, tmp);
202 /* multiply for pow */
203 tmp = lp_build_mul(&f32_bld, tmp, exponent);
204 /* "exp2" */
205 pow_approx = lp_build_itrunc(&f32_bld, tmp);
206 pow_approx = LLVMBuildBitCast(builder, pow_approx,
207 lp_build_vec_type(gallivm, src_type), "");
208
209 /*
210 * Since that pow was inaccurate (like 3 bits, though each sqrt step would
211 * give another bit), compensate the error (which is why we chose another
212 * exponent in the first place).
213 */
214 /* x * x^(8/12) = x^(20/12) */
215 pow_1 = lp_build_mul(&f32_bld, pow_approx, src);
216
217 /* x * x * x^(-4/12) = x^(20/12) */
218 /* Should avoid using rsqrt if it's not available, but
219 * using x * x^(4/12) * x^(4/12) instead will change error weight */
220 tmp = lp_build_fast_rsqrt(&f32_bld, pow_approx);
221 x2 = lp_build_mul(&f32_bld, src, src);
222 pow_2 = lp_build_mul(&f32_bld, x2, tmp);
223
224 /* average the values so the errors cancel out, compensate bias,
225 * we also squeeze the 1.055 mul of the srgb conversion plus the 255.0 mul
226 * for conversion to int in here */
227 tmp = lp_build_add(&f32_bld, pow_1, pow_2);
228 coeff = lp_build_const_vec(gallivm, src_type,
229 1.0f / (3.0f * coeff_f) * 0.999852f *
230 powf(1.055f * 255.0f, 4.0f));
231 pow_final = lp_build_mul(&f32_bld, tmp, coeff);
232
233 /* x^(5/12) = rsqrt(rsqrt(x^20/12)) */
234 if (lp_build_fast_rsqrt_available(src_type)) {
235 pow_final = lp_build_fast_rsqrt(&f32_bld,
236 lp_build_fast_rsqrt(&f32_bld, pow_final));
237 }
238 else {
239 pow_final = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, pow_final));
240 }
241 pow_final = lp_build_add(&f32_bld, pow_final,
242 lp_build_const_vec(gallivm, src_type, -0.055f * 255.0f));
243 }
244
245 else {
246 /*
247 * using "rational polynomial" approximation here.
248 * Essentially y = a*x^0.375 + b*x^0.5 + c, with also
249 * factoring in the 255.0 mul and the scaling mul.
250 * (a is closer to actual value so has higher weight than b.)
251 * Note: the constants are magic values. They were found empirically,
252 * possibly could be improved but good enough (be VERY careful with
253 * error metric if you'd want to tweak them, they also MUST fit with
254 * the crappy polynomial above for srgb->linear since it is required
255 * that each srgb value maps back to the same value).
256 * This function has an error of max +-0.17. Not sure this is actually
257 * enough, we require +-0.6 but that may include the +-0.5 from integer
258 * conversion. Seems to pass all relevant tests though...
259 * For the approximated srgb->linear values the error is naturally larger
260 * (+-0.42) but still accurate enough (required +-0.5 essentially).
261 * All in all (including min/max clamp, conversion) 15 instructions.
262 * FMA would help (minus 2 instructions).
263 */
264
265 LLVMValueRef x05, x0375, a_const, b_const, c_const, tmp2;
266
267 if (lp_build_fast_rsqrt_available(src_type)) {
268 tmp = lp_build_fast_rsqrt(&f32_bld, src);
269 x05 = lp_build_mul(&f32_bld, src, tmp);
270 }
271 else {
272 /*
273 * I don't really expect this to be practical without rsqrt
274 * but there's no reason for triple punishment so at least
275 * save the otherwise resulting division and unnecessary mul...
276 */
277 x05 = lp_build_sqrt(&f32_bld, src);
278 }
279
280 tmp = lp_build_mul(&f32_bld, x05, src);
281 if (lp_build_fast_rsqrt_available(src_type)) {
282 x0375 = lp_build_fast_rsqrt(&f32_bld, lp_build_fast_rsqrt(&f32_bld, tmp));
283 }
284 else {
285 x0375 = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, tmp));
286 }
287
288 a_const = lp_build_const_vec(gallivm, src_type, 0.675f * 1.0622 * 255.0f);
289 b_const = lp_build_const_vec(gallivm, src_type, 0.325f * 1.0622 * 255.0f);
290 c_const = lp_build_const_vec(gallivm, src_type, -0.0620f * 255.0f);
291
292 tmp = lp_build_mul(&f32_bld, a_const, x0375);
293 tmp2 = lp_build_mad(&f32_bld, b_const, x05, c_const);
294 pow_final = lp_build_add(&f32_bld, tmp, tmp2);
295 }
296
297 /* linear part is easy */
298 lin_const = lp_build_const_vec(gallivm, src_type, 12.92f * 255.0f);
299 lin = lp_build_mul(&f32_bld, src, lin_const);
300
301 lin_thresh = lp_build_const_vec(gallivm, src_type, 0.0031308f);
302 is_linear = lp_build_compare(gallivm, src_type, PIPE_FUNC_LEQUAL, src, lin_thresh);
303 tmp = lp_build_select(&f32_bld, is_linear, lin, pow_final);
304
305 if (chan_bits != 8) {
306 /* could adjust all the constants instead */
307 LLVMValueRef rescale_const = lp_build_const_vec(gallivm, src_type,
308 ((1 << chan_bits) - 1) / 255.0f);
309 tmp = lp_build_mul(&f32_bld, tmp, rescale_const);
310 }
311
312 f32_bld.type.sign = 0;
313 return lp_build_iround(&f32_bld, tmp);
314 }
315
316
317 /**
318 * Convert linear float soa values to packed srgb AoS values.
319 * This only handles packed formats which are 4x8bit in size
320 * (rgba and rgbx plus swizzles), and 16bit 565-style formats
321 * with no alpha. (In the latter case the return values won't be
322 * fully packed, it will look like r5g6b5x16r5g6b5x16...)
323 *
324 * @param src float SoA (vector) values to convert.
325 */
326 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)327 lp_build_float_to_srgb_packed(struct gallivm_state *gallivm,
328 const struct util_format_description *dst_fmt,
329 struct lp_type src_type,
330 LLVMValueRef *src)
331 {
332 LLVMBuilderRef builder = gallivm->builder;
333 unsigned chan;
334 struct lp_build_context f32_bld;
335 struct lp_type int32_type = lp_int_type(src_type);
336 LLVMValueRef tmpsrgb[4], alpha, dst;
337
338 lp_build_context_init(&f32_bld, gallivm, src_type);
339
340 /* rgb is subject to linear->srgb conversion, alpha is not */
341 for (chan = 0; chan < 3; chan++) {
342 unsigned chan_bits = dst_fmt->channel[dst_fmt->swizzle[chan]].size;
343 tmpsrgb[chan] = lp_build_linear_to_srgb(gallivm, src_type, chan_bits, src[chan]);
344 }
345 /*
346 * can't use lp_build_conv since we want to keep values as 32bit
347 * here so we can interleave with rgb to go from SoA->AoS.
348 */
349 alpha = lp_build_clamp_zero_one_nanzero(&f32_bld, src[3]);
350 alpha = lp_build_mul(&f32_bld, alpha,
351 lp_build_const_vec(gallivm, src_type, 255.0f));
352 tmpsrgb[3] = lp_build_iround(&f32_bld, alpha);
353
354 dst = lp_build_zero(gallivm, int32_type);
355 for (chan = 0; chan < dst_fmt->nr_channels; chan++) {
356 if (dst_fmt->swizzle[chan] <= PIPE_SWIZZLE_W) {
357 unsigned ls;
358 LLVMValueRef shifted, shift_val;
359 ls = dst_fmt->channel[dst_fmt->swizzle[chan]].shift;
360 shift_val = lp_build_const_int_vec(gallivm, int32_type, ls);
361 shifted = LLVMBuildShl(builder, tmpsrgb[chan], shift_val, "");
362 dst = LLVMBuildOr(builder, dst, shifted, "");
363 }
364 }
365 return dst;
366 }
367