pngdetail -r … crashes

[chris0e3]

Hello,

Thanks for all your work on LodePNG. It’s appreciated.

On macOS, compiled with Clang, pngdetail -r ‹PNG-file› always crashes when given a 256×256 or 512×512 RGBA-8 PNG file.
[It probably crashes on any power-of-two multiples of these sizes & many other sizes too. And any pixel format.]

I’ve tracked it down to the rescale function in pngdetail.cpp.
The function appears to always access 1 element beyond the end of the std::vector.
[You were often getting away with it but for these images the vector’s data size is an exact (large-ish) power-of-two probably resulting in a full virtual memory page being allocated, so the array[size+1] access causes a page fault/illegal address exception.]

Changing:

lodepng/pngdetail.cpp

Lines 470 to 471 in 0b1d9cc

	for(int x0 = xa; x0 <= xb; x0++) {
	int index0 = x0 * numchannels + y * w0 * numchannels;

lodepng/pngdetail.cpp

Lines 491 to 492 in 0b1d9cc

	for(int y0 = ya; y0 <= yb; y0++) {
	int index0 = x * numchannels + y0 * w1 * numchannels;

to

          for(int x0 = xa; x0 <= xb; x0++) {
            int index0 = x0 * numchannels + y * w0 * numchannels;
            if (size_t(index0) >= in.size()) continue;
...
          for(int y0 = ya; y0 <= yb; y0++) {
            int index0 = x * numchannels + y0 * w1 * numchannels;
            if (size_t(index0) >= temp.size()) continue;

appears to fix the problem. [BTW index0 should probably have been declared size_t.]

Also the std::vectors passed to & returned by rescale appear to always contain RGBA-16 big-endian images. However, only the high bytes of the RGBA components are used/needed. So you could effectively double the performance by changing:

lodepng/pngdetail.cpp

Line 458 in 0b1d9cc

for (int c = 0; c < numchannels; c++) {

to

    for (int c = 0; c < numchannels; c += 2) {

Also

lodepng/pngdetail.cpp

Line 747 in 0b1d9cc

size_t countColors(std::vector<unsigned char> image, unsigned w, unsigned h,

should probably be:

size_t countColors(const std::vector<unsigned char>& image, unsigned w, unsigned h,

And

lodepng/pngdetail.cpp

Line 1507 in 0b1d9cc

if(options.rendersize >= 1 && options.rendersize <= 4096) options.rendersize = size;

should probably be:

        if(size >= 1 && size <= 4096) options.rendersize = size;

Finally adding data.loadInspect(); as the first line in:

lodepng/pngdetail.cpp

Line 1434 in 0b1d9cc

void showInfos(Data& data, const Options& options) {

encourages pngdetail -s ‹file› & pngdetail -c ‹file› to print some output.