Add some comments for png read interface

2 files changed, 69 insertions, 35 deletions

diff --git a/src/libslic3r/PNGRead.cpp b/src/libslic3r/PNGRead.cpp
index 5b5b9ffec..e66143b84 100644
--- a/src/libslic3r/PNGRead.cpp
+++ b/src/libslic3r/PNGRead.cpp
@@ -41,13 +41,6 @@ bool is_png(const ReadBuf &rb)
     return rb.sz >= PNG_SIG_BYTES && !png_sig_cmp(buf, 0, PNG_SIG_BYTES);
 }
 
-// A wrapper around ReadBuf to be read repeatedly like a stream. libpng needs
-// this form for its buffer read callback.
-struct ReadBufReader {
-    const ReadBuf &rdbuf; size_t pos;
-    ReadBufReader(const ReadBuf &rd): rdbuf{rd}, pos{0} {}
-};
-
 // Buffer read callback for libpng. It provides an allocated output buffer and
 // the amount of data it desires to read from the input.
 void png_read_callback(png_struct *png_ptr,
@@ -55,20 +48,21 @@ void png_read_callback(png_struct *png_ptr,
                        png_size_t  byteCountToRead)
 {
     // Retrieve our input buffer through the png_ptr
-    auto reader = static_cast<ReadBufReader *>(png_get_io_ptr(png_ptr));
+    auto reader = static_cast<IStream *>(png_get_io_ptr(png_ptr));
 
-    if (!reader || byteCountToRead > reader->rdbuf.sz - reader->pos) return;
+    if (!reader || !reader->is_ok()) return;
 
-    auto   buf = static_cast<const png_byte *>(reader->rdbuf.buf);
-    size_t pos = reader->pos;
-
-    std::copy(buf + pos, buf + (pos + byteCountToRead), outBytes);
-    reader->pos += byteCountToRead;
+    reader->read(static_cast<std::uint8_t *>(outBytes), byteCountToRead);
 }
 
-bool decode_png(const ReadBuf &rb, ImageGreyscale &img)
+bool decode_png(IStream &in_buf, ImageGreyscale &out_img)
 {
-    if (!is_png(rb)) return false;
+    static const constexpr int PNG_SIG_BYTES = 8;
+
+    std::vector<png_byte> sig(PNG_SIG_BYTES, 0);
+    in_buf.read(sig.data(), PNG_SIG_BYTES);
+    if (!png_check_sig(sig.data(), PNG_SIG_BYTES))
+        return false;
 
     PNGDescr dsc;
     dsc.png = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr,
@@ -77,26 +71,28 @@ bool decode_png(const ReadBuf &rb, ImageGreyscale &img)
     if(!dsc.png) return false;
 
     dsc.info = png_create_info_struct(dsc.png);
-    if(!dsc.info) return {};
+    if(!dsc.info) return false;
+
+    png_set_read_fn(dsc.png, static_cast<void *>(&in_buf), png_read_callback);
 
-    ReadBufReader reader {rb};
-    png_set_read_fn(dsc.png, static_cast<void *>(&reader), png_read_callback);
+    // Tell that we have already read the first bytes to check the signature
+    png_set_sig_bytes(dsc.png, PNG_SIG_BYTES);
 
     png_read_info(dsc.png, dsc.info);
 
-    img.cols = png_get_image_width(dsc.png, dsc.info);
-    img.rows = png_get_image_height(dsc.png, dsc.info);
+    out_img.cols = png_get_image_width(dsc.png, dsc.info);
+    out_img.rows = png_get_image_height(dsc.png, dsc.info);
     size_t color_type = png_get_color_type(dsc.png, dsc.info);
     size_t bit_depth  = png_get_bit_depth(dsc.png, dsc.info);
 
     if (color_type != PNG_COLOR_TYPE_GRAY || bit_depth != 8)
         return false;
 
-    img.buf.resize(img.rows * img.cols);
+    out_img.buf.resize(out_img.rows * out_img.cols);
 
-    auto readbuf = static_cast<png_bytep>(img.buf.data());
-    for (size_t r = 0; r < img.rows; ++r)
-        png_read_row(dsc.png, readbuf + r * img.cols, nullptr);
+    auto readbuf = static_cast<png_bytep>(out_img.buf.data());
+    for (size_t r = 0; r < out_img.rows; ++r)
+        png_read_row(dsc.png, readbuf + r * out_img.cols, nullptr);
 
     return true;
 }
diff --git a/src/libslic3r/PNGRead.hpp b/src/libslic3r/PNGRead.hpp
index 0e7311f2e..082edd569 100644
--- a/src/libslic3r/PNGRead.hpp
+++ b/src/libslic3r/PNGRead.hpp
@@ -3,30 +3,68 @@
 
 #include <vector>
 #include <string>
+#include <istream>
 
 namespace Slic3r { namespace png {
 
-struct ReadBuf { const void *buf = nullptr; const size_t sz = 0; };
+// Interface for an input stream of encoded png image data.
+struct IStream {
+    virtual ~IStream() = default;
+    virtual size_t read(std::uint8_t *outp, size_t amount) = 0;
+    virtual bool is_ok() const = 0;
+};
 
+// The output format of decode_png: a 2D pixel matrix stored continuously row
+// after row (row major layout).
 template<class PxT> struct Image {
     std::vector<PxT> buf;
     size_t rows, cols;
     PxT get(size_t row, size_t col) const { return buf[row * cols + col]; }
 };
 
-struct RGB { uint8_t r, g, b; };
-
-using ImageRGB = Image<RGB>;
 using ImageGreyscale = Image<uint8_t>;
 
+// Only decodes true 8 bit grayscale png images. Returns false for other formats
+// TODO (if needed): implement transformation of rgb images into grayscale...
+bool decode_png(IStream &stream, ImageGreyscale &out_img);
+
+// TODO (if needed)
+// struct RGB { uint8_t r, g, b; };
+// using ImageRGB = Image<RGB>;
+// bool decode_png(IStream &stream, ImageRGB &img);
+
+
+// Encoded png data buffer: a simple read-only buffer and its size.
+struct ReadBuf { const void *buf = nullptr; const size_t sz = 0; };
+
 bool is_png(const ReadBuf &pngbuf);
 
-// Only decodes true 8 bit grayscale png images. Returns false for other formats
-// TODO: implement transformation of rgb images into grayscale...
-bool decode_png(const ReadBuf &pngbuf, ImageGreyscale &img);
+template<class Img> bool decode_png(const ReadBuf &in_buf, Img &out_img)
+{
+    struct ReadBufStream: public IStream {
+        const ReadBuf &rbuf_ref; size_t pos = 0;
+
+        explicit ReadBufStream(const ReadBuf &buf): rbuf_ref{buf} {}
+
+        size_t read(std::uint8_t *outp, size_t amount) override
+        {
+            if (amount > rbuf_ref.sz - pos) return 0;
+
+            auto buf = static_cast<const std::uint8_t *>(rbuf_ref.buf);
+            std::copy(buf + pos, buf + (pos + amount), outp);
+            pos += amount;
+
+            return amount;
+        }
+
+        bool is_ok() const override { return pos < rbuf_ref.sz; }
+    } stream{in_buf};
+
+    return decode_png(stream, out_img);
+}
+
+// TODO: std::istream of FILE* could be similarly adapted in case its needed...
 
-// TODO
-// bool decode_png(Buffer &&pngbuf, ImageRGB &img);
+}}     // namespace Slic3r::png
 
-}}
 #endif // PNGREAD_HPP