Greg Kennedy 2019
Testing ground for various integer scaling routines
This is a C source file containing various methods for doing "nearest neighbor" image scaling. The goal was to test the performance of processes that copy a source array to a destination. Nearest neighbor algorithms will lead to a "pixelated" look but are faster than other options like bilinear, cubic, etc.
A modern computer would simply offload this to a GPU and let it do the scaling, but this is a CPU test. At least, it could be placed in its own thread.
Each method is numbered in the source. Some methods have a letter as well: these are "variations" of the method, which tweak some arrangement but don't affect the overall method itself.
Each method is wrapped in a TEST() macro that sets up a start and end timer,
and executes the function 500 times. The time taken for each operation is
printed to STDOUT.
Some debug functions are available: most helpful is a dump_ppm() which will
write a .ppm file for each method, so the results can be examined for accuracy.
A brief description of each method:
- Straightforward division, as in:
for (y = 0; y < out_height; ++y)
for (x = 0; x < out_width; ++x)
out[y][x] = in[y * in_to_out_ratio][x * in_to_out_ratio];
- Pre-computing two tables of multiplication results
/* table computation */
...
for (y = 0; y < out_height; ++y)
for (x = 0; x < out_width; ++x)
out[y][x] = in[tbl_y[y]][tbl_x[x]];
- Pre-computing one large table of multiplication results
/* table computation */
...
for (i = 0; i < (out_height * out_width); ++i)
out[i] = in[tbl[i]];
- Like 3., but only store when source index should advance.
/* table computation */
...
j = 0;
for (i = 0; i < (out_height * out_width); ++i) {
if (tbl[i]) ++j;
out[i] = in[j];
}
- Fixed-point calculation of delta X and Y.
dy = in_to_out_ratio;
dx = in_to_out_ratio;
s_y = 0;
for (y = 0; y < out_height; ++y) {
s_x = 0;
for (x = 0; x < out_width; ++x) {
out[y][x] = in[s_y][s_x];
s_x += dx;
}
s_y += dy;
}
Finally there's a method_best() which isn't actually the "best", it's a test
function for combining features of the others.
The Makefile should be simple enough to understand, though there are a number
of switchable options at the top for tweaking the CFLAGS of output. These
can be useful for testing the effect of optimization levels or loop unrolling
on any of the algorithms used.