How to flip image vertically using Boost::GIL

Question

I'm trying to generate an image icon. Based on some restrictions, I need to get a one-dimensional array of int32_t pixel values in order {r, g, b, a, r1, g1, b1, a1, ...}. In this case, the order of the rows should be the same as with OpenGL UV (bottom to top). For this, I used the following, verified, it works:

const int image_width = image.width();
const int image_height = image.height();
if (!(image_width && image_height)) { return 1; }

int preview_width, preview_height;
if (image_width > image_height) {
    preview_width = PREVIEW_SIZE;
    preview_height = PREVIEW_SIZE * (float)image_height / (float)image_width;
}
else {
    preview_width = PREVIEW_SIZE * (float)image_width / (float)image_height;
    preview_height = PREVIEW_SIZE;
}

bg::rgba8_image_t preview_square(preview_width, preview_height);
const bg::rgba8_view_t& ps_viewer = bg::view(preview_square);
bg::resize_view(bg::const_view(image), ps_viewer, bg::bilinear_sampler());

std::vector<int32_t> arr(preview_width * preview_height);
memcpy(&arr[0], &ps_viewer[0], sizeof(int32_t) * arr.size());

Based on the storage order of pixels in the GIL, I need to flip the image along the y-axis. Please tell me how to implement this?

sehe · Accepted Answer · 2020-05-25T23:37:38.303

1

You can use a flipped view to do the transform:

#include <boost/gil.hpp>
#include <boost/gil/algorithm.hpp>
#include <boost/gil/dynamic_step.hpp>
#include <boost/gil/extension/io/jpeg.hpp>
#include <boost/gil/extension/io/png.hpp>

namespace bg = boost::gil;

int main() {
    bg::rgba8_image_t v;
    bg::read_and_convert_image("/tmp/VIplm.jpg", v, bg::jpeg_tag{});

    using Pixel = bg::rgba8_pixel_t;
    std::vector<Pixel> vec(v.width() * v.height());

    auto dest = bg::interleaved_view(v.width(), v.height(), vec.data(), v.width() * sizeof(Pixel));
    bg::copy_and_convert_pixels(
        bg::flipped_left_right_view(bg::const_view(v)),
        dest);

    bg::write_view("/tmp/pixels.png", dest, bg::png_tag{});
}

The png is here:

I left out the resize operation.

edited May 25 '20 at 23:37

answered May 25 '20 at 20:47

sehe

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Yes, I already tried this `bg::flipped_up_down_view(bg::const_view(v));` But in this case, the use of `memcpy` does not work correctly(the output looks like solid noise). I think I made a mistake trying to just copy the memory from the view to the vector? – ivpe May 25 '20 at 23:08
Hmm. I rather glossed over the rest of the code seeing that the `flipped_..._view` was missing, thinking that'd be all. I'm not exactly sure what you are doing with the manual pixel buffer. I think you'd like to have a raw view from the flipped image instead? See **updated** answer code – sehe May 25 '20 at 23:38

How to flip image vertically using Boost::GIL

1 Answers1