resize.cpp

#include <stb_image_resize.h>

static void build_mips(uint8_t* data, size_t base, uint32_t w, uint32_t h)
{
    uint32_t mw = w;
    uint32_t mh = h;
    uint32_t pw = w;
    uint32_t ph = h;
    size_t prev_start = base;
    size_t last_size = size_t(w) * size_t(h) * 4;
    while (mw > 1 || mh > 1) {
        mw = std::max(1u, mw >> 1);
        mh = std::max(1u, mh >> 1);
        stbir_resize_uint8(
            &data[prev_start], pw, ph, 0,
            &data[base + last_size], mw, mh, 0, 4
        );
        pw = mw;
        ph = mh;
        prev_start = base + last_size;
        last_size += size_t(mw) * size_t(mh) * 4;
    }
}

static size_t mips_required_memory(uint32_t w, uint32_t h)
{
    size_t layer_bytes_0 = size_t(w) * size_t(h) * 4;
    size_t layer_bytes = layer_bytes_0;
    uint32_t mw = w;
    uint32_t mh = h;
    while (mw > 1 || mh > 1) {
        mw = std::max(1u, mw >> 1);
        mh = std::max(1u, mh >> 1);
        layer_bytes += size_t(mw) * size_t(mh) * 4;
    }
    return layer_bytes;
}

void example(std::string const& path)
{
    uint32_t w = 0;
    uint32_t h = 0;
    auto data = load_image(path, w, h); // last two are refs

    size_t s = mips_required_memory(w, h);
    const bgfx::Memory* mem = bgfx::alloc(uint32_t(s));

    memcpy(&mem->data[0], data.data(), s);
    
    build_mips(&mem->data[0], 0, w, h);
}