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Move invariant variables to their respective parts of the loop. #2

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Move invariant variables to their respective parts of the loop. #2

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37 changes: 27 additions & 10 deletions src/read.c
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Original file line number Diff line number Diff line change
Expand Up @@ -131,16 +131,33 @@ SEXP read_jpeg(SEXP sFn, SEXP sNative) {
Rf_setAttrib(res, Rf_install("channels"), Rf_ScalarInteger(pln));
UNPROTECT(1);
} else {
int x, y, p, pls = width * height;
double *data;

res = PROTECT(Rf_allocVector(REALSXP, height * rowbytes));
data = REAL(res);

for(y = 0; y < height; y++)
for (x = 0; x < width; x++)
for (p = 0; p < pln; p++)
data[y + x * height + p * pls] = ((double)image[y * rowbytes + x * pln + p]) / 255.0;
res = PROTECT(Rf_allocVector(REALSXP, height * rowbytes));

{
double *data;
data = REAL(res);

int const pls = width * height;
for (int y = 0; y < height; y++) {
int const i_y_part = y * rowbytes;
for (int x = 0; x < width; x++) {
int const i_xy_part = x * pln + i_y_part;
int const d_xy_part = y + x * height;
for (int p = 0; p < pln; p++) {
data[d_xy_part + p * pls] = ((double)image[i_xy_part + p]) / 255.0;
}
}
}
}

/*
* This is the fastest way.
* It takes about 60% of the time for a 4.2 MB file.
* However, the dimentions needs to be in a different order: pls, width, height. Unfortunately, that would surely break old code.
*
for (size_t i = 0; i < last_index; ++i) {
data[i] = ((double) image[i]) / 255.0;
*/

dim = Rf_allocVector(INTSXP, (pln > 1) ? 3 : 2);
INTEGER(dim)[0] = height;
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