1 /**************************************************************************
2 *
3 * Copyright 2013 VMware, Inc.
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11 * permit persons to whom the Software is furnished to do so, subject to
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13 *
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26 **************************************************************************/
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
72 #include "lp_bld_type.h"
73 #include "lp_bld_const.h"
74 #include "lp_bld_arit.h"
75 #include "lp_bld_bitarit.h"
76 #include "lp_bld_logic.h"
77 #include "lp_bld_format.h"
78
79
80
81 /**
82 * Convert srgb int values to linear float values.
83 * Several possibilities how to do this, e.g.
84 * - table
85 * - doing the pow() with int-to-float and float-to-int tricks
86 * (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
87 * - just using standard polynomial approximation
88 * (3rd order polynomial is required for crappy but just sufficient accuracy)
89 *
90 * @param src integer (vector) value(s) to convert
91 * (chan_bits bit values unpacked to 32 bit already).
92 */
93 LLVMValueRef
lp_build_srgb_to_linear(struct gallivm_state * gallivm,struct lp_type src_type,unsigned chan_bits,LLVMValueRef src)94 lp_build_srgb_to_linear(struct gallivm_state *gallivm,
95 struct lp_type src_type,
96 unsigned chan_bits,
97 LLVMValueRef src)
98 {
99 struct lp_type f32_type = lp_type_float_vec(32, src_type.length * 32);
100 struct lp_build_context f32_bld;
101 LLVMValueRef srcf, part_lin, part_pow, is_linear, lin_const, lin_thresh;
102 double coeffs[4] = {0.0023f,
103 0.0030f / 255.0f,
104 0.6935f / (255.0f * 255.0f),
105 0.3012f / (255.0f * 255.0f * 255.0f)
106 };
107
108 assert(src_type.width == 32);
109 /* Technically this would work with more bits too but would be inaccurate. */
110 assert(chan_bits <= 8);
111
112 lp_build_context_init(&f32_bld, gallivm, f32_type);
113
114 /*
115 * using polynomial: (src * (src * (src * 0.3012 + 0.6935) + 0.0030) + 0.0023)
116 * ( poly = 0.3012*x^3 + 0.6935*x^2 + 0.0030*x + 0.0023)
117 * (found with octave polyfit and some magic as I couldn't get the error
118 * function right). Using the above mentioned error function, the values stay
119 * within +-0.35, except for the lowest values - hence tweaking linear segment
120 * to cover the first 16 instead of the first 11 values (the error stays
121 * just about acceptable there too).
122 * Hence: lin = src > 15 ? poly : src / 12.6
123 * This function really only makes sense for vectors, should use LUT otherwise.
124 * All in all (including float conversion) 11 instructions (with sse4.1),
125 * 6 constants (polynomial could be done with 1 instruction less at the cost
126 * of slightly worse dependency chain, fma should also help).
127 */
128 /* doing the 1/255 mul as part of the approximation */
129 srcf = lp_build_int_to_float(&f32_bld, src);
130 if (chan_bits != 8) {
131 /* could adjust all the constants instead */
132 LLVMValueRef rescale_const = lp_build_const_vec(gallivm, f32_type,
133 255.0f / ((1 << chan_bits) - 1));
134 srcf = lp_build_mul(&f32_bld, srcf, rescale_const);
135 }
136 lin_const = lp_build_const_vec(gallivm, f32_type, 1.0f / (12.6f * 255.0f));
137 part_lin = lp_build_mul(&f32_bld, srcf, lin_const);
138
139 part_pow = lp_build_polynomial(&f32_bld, srcf, coeffs, 4);
140
141 lin_thresh = lp_build_const_vec(gallivm, f32_type, 15.0f);
142 is_linear = lp_build_compare(gallivm, f32_type, PIPE_FUNC_LEQUAL, srcf, lin_thresh);
143 return lp_build_select(&f32_bld, is_linear, part_lin, part_pow);
144 }
145
146
147 /**
148 * Convert linear float values to srgb int values.
149 * Several possibilities how to do this, e.g.
150 * - use table (based on exponent/highest order mantissa bits) and do
151 * linear interpolation (https://gist.github.com/rygorous/2203834)
152 * - Chebyshev polynomial
153 * - Approximation using reciprocals
154 * - using int-to-float and float-to-int tricks for pow()
155 * (http://stackoverflow.com/questions/6475373/optimizations-for-pow-with-const-non-integer-exponent)
156 *
157 * @param src float (vector) value(s) to convert.
158 */
159 static LLVMValueRef
lp_build_linear_to_srgb(struct gallivm_state * gallivm,struct lp_type src_type,unsigned chan_bits,LLVMValueRef src)160 lp_build_linear_to_srgb(struct gallivm_state *gallivm,
161 struct lp_type src_type,
162 unsigned chan_bits,
163 LLVMValueRef src)
164 {
165 LLVMBuilderRef builder = gallivm->builder;
166 struct lp_build_context f32_bld;
167 LLVMValueRef lin_thresh, lin, lin_const, is_linear, tmp, pow_final;
168
169 lp_build_context_init(&f32_bld, gallivm, src_type);
170
171 src = lp_build_clamp(&f32_bld, src, f32_bld.zero, f32_bld.one);
172
173 if (0) {
174 /*
175 * using int-to-float and float-to-int trick for pow().
176 * This is much more accurate than necessary thanks to the correction,
177 * but it most certainly makes no sense without rsqrt available.
178 * Bonus points if you understand how this works...
179 * All in all (including min/max clamp, conversion) 19 instructions.
180 */
181
182 float exp_f = 2.0f / 3.0f;
183 /* some compilers can't do exp2f, so this is exp2f(127.0f/exp_f - 127.0f) */
184 float exp2f_c = 1.30438178253e+19f;
185 float coeff_f = 0.62996f;
186 LLVMValueRef pow_approx, coeff, x2, exponent, pow_1, pow_2;
187 struct lp_type int_type = lp_int_type(src_type);
188
189 /*
190 * First calculate approx x^8/12
191 */
192 exponent = lp_build_const_vec(gallivm, src_type, exp_f);
193 coeff = lp_build_const_vec(gallivm, src_type,
194 exp2f_c * powf(coeff_f, 1.0f / exp_f));
195
196 /* premultiply src */
197 tmp = lp_build_mul(&f32_bld, coeff, src);
198 /* "log2" */
199 tmp = LLVMBuildBitCast(builder, tmp, lp_build_vec_type(gallivm, int_type), "");
200 tmp = lp_build_int_to_float(&f32_bld, tmp);
201 /* multiply for pow */
202 tmp = lp_build_mul(&f32_bld, tmp, exponent);
203 /* "exp2" */
204 pow_approx = lp_build_itrunc(&f32_bld, tmp);
205 pow_approx = LLVMBuildBitCast(builder, pow_approx,
206 lp_build_vec_type(gallivm, src_type), "");
207
208 /*
209 * Since that pow was inaccurate (like 3 bits, though each sqrt step would
210 * give another bit), compensate the error (which is why we chose another
211 * exponent in the first place).
212 */
213 /* x * x^(8/12) = x^(20/12) */
214 pow_1 = lp_build_mul(&f32_bld, pow_approx, src);
215
216 /* x * x * x^(-4/12) = x^(20/12) */
217 /* Should avoid using rsqrt if it's not available, but
218 * using x * x^(4/12) * x^(4/12) instead will change error weight */
219 tmp = lp_build_fast_rsqrt(&f32_bld, pow_approx);
220 x2 = lp_build_mul(&f32_bld, src, src);
221 pow_2 = lp_build_mul(&f32_bld, x2, tmp);
222
223 /* average the values so the errors cancel out, compensate bias,
224 * we also squeeze the 1.055 mul of the srgb conversion plus the 255.0 mul
225 * for conversion to int in here */
226 tmp = lp_build_add(&f32_bld, pow_1, pow_2);
227 coeff = lp_build_const_vec(gallivm, src_type,
228 1.0f / (3.0f * coeff_f) * 0.999852f *
229 powf(1.055f * 255.0f, 4.0f));
230 pow_final = lp_build_mul(&f32_bld, tmp, coeff);
231
232 /* x^(5/12) = rsqrt(rsqrt(x^20/12)) */
233 if (lp_build_fast_rsqrt_available(src_type)) {
234 pow_final = lp_build_fast_rsqrt(&f32_bld,
235 lp_build_fast_rsqrt(&f32_bld, pow_final));
236 }
237 else {
238 pow_final = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, pow_final));
239 }
240 pow_final = lp_build_add(&f32_bld, pow_final,
241 lp_build_const_vec(gallivm, src_type, -0.055f * 255.0f));
242 }
243
244 else {
245 /*
246 * using "rational polynomial" approximation here.
247 * Essentially y = a*x^0.375 + b*x^0.5 + c, with also
248 * factoring in the 255.0 mul and the scaling mul.
249 * (a is closer to actual value so has higher weight than b.)
250 * Note: the constants are magic values. They were found empirically,
251 * possibly could be improved but good enough (be VERY careful with
252 * error metric if you'd want to tweak them, they also MUST fit with
253 * the crappy polynomial above for srgb->linear since it is required
254 * that each srgb value maps back to the same value).
255 * This function has an error of max +-0.17. Not sure this is actually
256 * enough, we require +-0.6 but that may include the +-0.5 from integer
257 * conversion. Seems to pass all relevant tests though...
258 * For the approximated srgb->linear values the error is naturally larger
259 * (+-0.42) but still accurate enough (required +-0.5 essentially).
260 * All in all (including min/max clamp, conversion) 15 instructions.
261 * FMA would help (minus 2 instructions).
262 */
263
264 LLVMValueRef x05, x0375, a_const, b_const, c_const, tmp2;
265
266 if (lp_build_fast_rsqrt_available(src_type)) {
267 tmp = lp_build_fast_rsqrt(&f32_bld, src);
268 x05 = lp_build_mul(&f32_bld, src, tmp);
269 }
270 else {
271 /*
272 * I don't really expect this to be practical without rsqrt
273 * but there's no reason for triple punishment so at least
274 * save the otherwise resulting division and unnecessary mul...
275 */
276 x05 = lp_build_sqrt(&f32_bld, src);
277 }
278
279 tmp = lp_build_mul(&f32_bld, x05, src);
280 if (lp_build_fast_rsqrt_available(src_type)) {
281 x0375 = lp_build_fast_rsqrt(&f32_bld, lp_build_fast_rsqrt(&f32_bld, tmp));
282 }
283 else {
284 x0375 = lp_build_sqrt(&f32_bld, lp_build_sqrt(&f32_bld, tmp));
285 }
286
287 a_const = lp_build_const_vec(gallivm, src_type, 0.675f * 1.0622 * 255.0f);
288 b_const = lp_build_const_vec(gallivm, src_type, 0.325f * 1.0622 * 255.0f);
289 c_const = lp_build_const_vec(gallivm, src_type, -0.0620f * 255.0f);
290
291 tmp = lp_build_mul(&f32_bld, a_const, x0375);
292 tmp2 = lp_build_mad(&f32_bld, b_const, x05, c_const);
293 pow_final = lp_build_add(&f32_bld, tmp, tmp2);
294 }
295
296 /* linear part is easy */
297 lin_const = lp_build_const_vec(gallivm, src_type, 12.92f * 255.0f);
298 lin = lp_build_mul(&f32_bld, src, lin_const);
299
300 lin_thresh = lp_build_const_vec(gallivm, src_type, 0.0031308f);
301 is_linear = lp_build_compare(gallivm, src_type, PIPE_FUNC_LEQUAL, src, lin_thresh);
302 tmp = lp_build_select(&f32_bld, is_linear, lin, pow_final);
303
304 if (chan_bits != 8) {
305 /* could adjust all the constants instead */
306 LLVMValueRef rescale_const = lp_build_const_vec(gallivm, src_type,
307 ((1 << chan_bits) - 1) / 255.0f);
308 tmp = lp_build_mul(&f32_bld, tmp, rescale_const);
309 }
310
311 f32_bld.type.sign = 0;
312 return lp_build_iround(&f32_bld, tmp);
313 }
314
315
316 /**
317 * Convert linear float soa values to packed srgb AoS values.
318 * This only handles packed formats which are 4x8bit in size
319 * (rgba and rgbx plus swizzles), and 16bit 565-style formats
320 * with no alpha. (In the latter case the return values won't be
321 * fully packed, it will look like r5g6b5x16r5g6b5x16...)
322 *
323 * @param src float SoA (vector) values to convert.
324 */
325 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)326 lp_build_float_to_srgb_packed(struct gallivm_state *gallivm,
327 const struct util_format_description *dst_fmt,
328 struct lp_type src_type,
329 LLVMValueRef *src)
330 {
331 LLVMBuilderRef builder = gallivm->builder;
332 unsigned chan;
333 struct lp_build_context f32_bld;
334 struct lp_type int32_type = lp_int_type(src_type);
335 LLVMValueRef tmpsrgb[4], alpha, dst;
336
337 lp_build_context_init(&f32_bld, gallivm, src_type);
338
339 /* rgb is subject to linear->srgb conversion, alpha is not */
340 for (chan = 0; chan < 3; chan++) {
341 unsigned chan_bits = dst_fmt->channel[dst_fmt->swizzle[chan]].size;
342 tmpsrgb[chan] = lp_build_linear_to_srgb(gallivm, src_type, chan_bits, src[chan]);
343 }
344 /*
345 * can't use lp_build_conv since we want to keep values as 32bit
346 * here so we can interleave with rgb to go from SoA->AoS.
347 */
348 alpha = lp_build_clamp_zero_one_nanzero(&f32_bld, src[3]);
349 alpha = lp_build_mul(&f32_bld, alpha,
350 lp_build_const_vec(gallivm, src_type, 255.0f));
351 tmpsrgb[3] = lp_build_iround(&f32_bld, alpha);
352
353 dst = lp_build_zero(gallivm, int32_type);
354 for (chan = 0; chan < dst_fmt->nr_channels; chan++) {
355 if (dst_fmt->swizzle[chan] <= PIPE_SWIZZLE_W) {
356 unsigned ls;
357 LLVMValueRef shifted, shift_val;
358 ls = dst_fmt->channel[dst_fmt->swizzle[chan]].shift;
359 shift_val = lp_build_const_int_vec(gallivm, int32_type, ls);
360 shifted = LLVMBuildShl(builder, tmpsrgb[chan], shift_val, "");
361 dst = LLVMBuildOr(builder, dst, shifted, "");
362 }
363 }
364 return dst;
365 }
366