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Minor LodePNG optimizations #173

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Minor LodePNG optimizations #173

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298c4e9
Silence -Wreturn-type warning in lodepng_benchmark.cpp
woot000 Sep 18, 2022
41349a3
Optimize filter, filterScanline functions
woot000 Sep 18, 2022
e6cd7fc
Optimize getPixelColor* functions
woot000 Sep 18, 2022
0ff2a9c
Optimize lodepng_compute_color_stats, auto_choose_color
woot000 Sep 24, 2022
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37 changes: 15 additions & 22 deletions lodepng.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3256,33 +3256,28 @@ static void getPixelColorRGBA8(unsigned char* r, unsigned char* g,
if(mode->colortype == LCT_GREY) {
if(mode->bitdepth == 8) {
*r = *g = *b = in[i];
if(mode->key_defined && *r == mode->key_r) *a = 0;
else *a = 255;
*a = 255 * !(mode->key_defined && *r == mode->key_r);
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@zvezdochiot zvezdochiot Sep 18, 2022 •
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❓ Why use multiplication where it is unnecessary?:

*a = ((mode->key_defined) && (*r == mode->key_r)) ? 0u : 255u;

Or use array?:

unsigned char wb[2] = {255, 0);
...
*a = wb[(int)((mode->key_defined) && (*r == mode->key_r))];

(for const time).

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Each conversion from if-else statement to multiplication in this case removes one jump instruction, creating code that has a few less branches and is also slightly smaller

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I tested this out with godbolt.org, and it looks like the way you write it doesn't matter, the effect is equivalent and it gets compiled to the same thing:

image

So the original with if/else looks more readable I'd say

} else if(mode->bitdepth == 16) {
*r = *g = *b = in[i * 2 + 0];
if(mode->key_defined && 256U * in[i * 2 + 0] + in[i * 2 + 1] == mode->key_r) *a = 0;
else *a = 255;
*a = 255 * !(mode->key_defined && 256U * in[i * 2 + 0] + in[i * 2 + 1] == mode->key_r);
} else {
unsigned highest = ((1U << mode->bitdepth) - 1U); /*highest possible value for this bit depth*/
size_t j = i * mode->bitdepth;
unsigned value = readBitsFromReversedStream(&j, in, mode->bitdepth);
*r = *g = *b = (value * 255) / highest;
if(mode->key_defined && value == mode->key_r) *a = 0;
else *a = 255;
*a = 255 * !(mode->key_defined && value == mode->key_r);
}
} else if(mode->colortype == LCT_RGB) {
if(mode->bitdepth == 8) {
*r = in[i * 3 + 0]; *g = in[i * 3 + 1]; *b = in[i * 3 + 2];
if(mode->key_defined && *r == mode->key_r && *g == mode->key_g && *b == mode->key_b) *a = 0;
else *a = 255;
*a = 255 * !(mode->key_defined && *r == mode->key_r && *g == mode->key_g && *b == mode->key_b);
} else {
*r = in[i * 6 + 0];
*g = in[i * 6 + 2];
*b = in[i * 6 + 4];
if(mode->key_defined && 256U * in[i * 6 + 0] + in[i * 6 + 1] == mode->key_r
*a = 255 * !(mode->key_defined && 256U * in[i * 6 + 0] + in[i * 6 + 1] == mode->key_r
&& 256U * in[i * 6 + 2] + in[i * 6 + 3] == mode->key_g
&& 256U * in[i * 6 + 4] + in[i * 6 + 5] == mode->key_b) *a = 0;
else *a = 255;
&& 256U * in[i * 6 + 4] + in[i * 6 + 5] == mode->key_b);
}
} else if(mode->colortype == LCT_PALETTE) {
unsigned index;
Expand Down Expand Up @@ -3337,21 +3332,21 @@ static void getPixelColorsRGBA8(unsigned char* LODEPNG_RESTRICT buffer, size_t n
if(mode->key_defined) {
buffer -= numpixels * num_channels;
for(i = 0; i != numpixels; ++i, buffer += num_channels) {
if(buffer[0] == mode->key_r) buffer[3] = 0;
buffer[3] *= !(buffer[0] == mode->key_r);
}
}
} else if(mode->bitdepth == 16) {
for(i = 0; i != numpixels; ++i, buffer += num_channels) {
buffer[0] = buffer[1] = buffer[2] = in[i * 2];
buffer[3] = mode->key_defined && 256U * in[i * 2 + 0] + in[i * 2 + 1] == mode->key_r ? 0 : 255;
buffer[3] = 255 * !(mode->key_defined && 256U * in[i * 2 + 0] + in[i * 2 + 1] == mode->key_r);
}
} else {
unsigned highest = ((1U << mode->bitdepth) - 1U); /*highest possible value for this bit depth*/
size_t j = 0;
for(i = 0; i != numpixels; ++i, buffer += num_channels) {
unsigned value = readBitsFromReversedStream(&j, in, mode->bitdepth);
buffer[0] = buffer[1] = buffer[2] = (value * 255) / highest;
buffer[3] = mode->key_defined && value == mode->key_r ? 0 : 255;
buffer[3] = 255 * !(mode->key_defined && value == mode->key_r);
}
}
} else if(mode->colortype == LCT_RGB) {
Expand All @@ -3363,18 +3358,18 @@ static void getPixelColorsRGBA8(unsigned char* LODEPNG_RESTRICT buffer, size_t n
if(mode->key_defined) {
buffer -= numpixels * num_channels;
for(i = 0; i != numpixels; ++i, buffer += num_channels) {
if(buffer[0] == mode->key_r && buffer[1]== mode->key_g && buffer[2] == mode->key_b) buffer[3] = 0;
buffer[3] *= !(buffer[0] == mode->key_r && buffer[1]== mode->key_g && buffer[2] == mode->key_b);
}
}
} else {
for(i = 0; i != numpixels; ++i, buffer += num_channels) {
buffer[0] = in[i * 6 + 0];
buffer[1] = in[i * 6 + 2];
buffer[2] = in[i * 6 + 4];
buffer[3] = mode->key_defined
buffer[3] = 255 * !(mode->key_defined
&& 256U * in[i * 6 + 0] + in[i * 6 + 1] == mode->key_r
&& 256U * in[i * 6 + 2] + in[i * 6 + 3] == mode->key_g
&& 256U * in[i * 6 + 4] + in[i * 6 + 5] == mode->key_b ? 0 : 255;
&& 256U * in[i * 6 + 4] + in[i * 6 + 5] == mode->key_b);
}
}
} else if(mode->colortype == LCT_PALETTE) {
Expand Down Expand Up @@ -3497,17 +3492,15 @@ static void getPixelColorRGBA16(unsigned short* r, unsigned short* g, unsigned s
const unsigned char* in, size_t i, const LodePNGColorMode* mode) {
if(mode->colortype == LCT_GREY) {
*r = *g = *b = 256 * in[i * 2 + 0] + in[i * 2 + 1];
if(mode->key_defined && 256U * in[i * 2 + 0] + in[i * 2 + 1] == mode->key_r) *a = 0;
else *a = 65535;
*a = 65535 * !(mode->key_defined && 256U * in[i * 2 + 0] + in[i * 2 + 1] == mode->key_r);
} else if(mode->colortype == LCT_RGB) {
*r = 256u * in[i * 6 + 0] + in[i * 6 + 1];
*g = 256u * in[i * 6 + 2] + in[i * 6 + 3];
*b = 256u * in[i * 6 + 4] + in[i * 6 + 5];
if(mode->key_defined
*a = 65535 *!(mode->key_defined
&& 256u * in[i * 6 + 0] + in[i * 6 + 1] == mode->key_r
&& 256u * in[i * 6 + 2] + in[i * 6 + 3] == mode->key_g
&& 256u * in[i * 6 + 4] + in[i * 6 + 5] == mode->key_b) *a = 0;
else *a = 65535;
&& 256u * in[i * 6 + 4] + in[i * 6 + 5] == mode->key_b);
} else if(mode->colortype == LCT_GREY_ALPHA) {
*r = *g = *b = 256u * in[i * 4 + 0] + in[i * 4 + 1];
*a = 256u * in[i * 4 + 2] + in[i * 4 + 3];
Expand Down