Skia Fiddle

void print_data(const SkData* data, const char* name) {
    if (data) {
        SkDebugf("\nxxd -r -p > %s << EOF", name);
        size_t s = data->size();
        const uint8_t* d = data->bytes();
        for (size_t i = 0; i < s; ++i) {
            if (i % 40 == 0) { SkDebugf("\n"); }
            SkDebugf("%02x", d[i]);
        }
        SkDebugf("\nEOF\n\n");
    }
}

sk_sp<SkData> Encode_PPM_B(const SkPixmap& src) {
        if (src.width() <= 0 || src.height() <= 0 || !src.addr() ||
            src.colorType() == kUnknown_SkColorType) {
            return nullptr;
        }
        SkString s = SkStringPrintf("P6\n%d %d\n255\n", src.width(), src.height());
        auto result = SkData::MakeUninitialized(s.size() + 3 * src.width() * src.height());
        uint8_t* ptr = static_cast<uint8_t*>(result->writable_data());
        memcpy(ptr, s.c_str(), s.size());
        ptr += s.size();
        for (int y = 0; y < src.height(); ++y) {
            for (int x = 0; x < src.height(); ++x) {
                SkColor c = src.getColor(x, y);
                *ptr++ = SkColorGetR(c);
                *ptr++ = SkColorGetG(c);
                *ptr++ = SkColorGetB(c);
            }
        }
        return result;
}

SkBitmap ToBitmap(SkImage* img) {
    SkBitmap bitmap;
    (void)img->asLegacyBitmap(&bitmap, SkImage::kRO_LegacyBitmapMode);
    return bitmap;
}

void draw(SkCanvas*) {
    SkBitmap bitmap = ToBitmap(image.get());
    sk_sp<SkData> data = Encode_PPM_B(bitmap.pixmap());
    print_data(data.get(), "foo.ppm");
}

Skia API Documentation