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Add some comments for png read interface

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This commit is contained in:
tamasmeszaros 2020-08-04 10:13:01 +02:00
parent ad0df8fd09
commit 79567a1958
2 changed files with 69 additions and 35 deletions
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46
src/libslic3r/PNGRead.cpp
View file

@ -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;
ReadBufReader reader {rb};
png_set_read_fn(dsc.png, static_cast<void *>(&reader), png_read_callback);
png_set_read_fn(dsc.png, static_cast<void *>(&in_buf), 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;
}

58
src/libslic3r/PNGRead.hpp
View file

@ -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;
// TODO
// bool decode_png(Buffer &&pngbuf, ImageRGB &img);
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...
}} // namespace Slic3r::png
}}
#endif // PNGREAD_HPP