CImg library creates distorted images on rotation

Question

I want to use the CImg library (http://cimg.sourceforge.net/) to rotate an image with an arbitrary angle (the image is read by Qt which should not perform the rotation):

QImage img("sample_with_alpha.png");
img = img.convertToFormat(QImage::Format_ARGB32);

float angle = 45;

cimg_library::CImg<uint8_t> src(img.bits(), img.width(), img.height(), 1, 4);
cimg_library::CImg<uint8_t> out = src.get_rotate(angle);

// Further processing:
// Data: out.data(), out.width(), out.height(), Stride: out.width() * 4

The final data in "out.data()" is ok when the the angle is set to 0. But for other angles the output data is distorted. I assume that the CImg library changes the output format and/or stride during rotation?

Regards,

Answer 1

CImg does not store the pixel buffer of an image in interleaved mode, as RGBARGBARGBA... but uses a channel by channel structure RRRRRRRR.....GGGGGGGGG.......BBBBBBBBB.....AAAAAAAAA. I assume your img.bits() pointer points to pixels with interleaved channels, so if you want to pass this to CImg, you'll need to permute the buffer structure before you can apply any of the CImg method. Try this :

cimg_library::CImg<uint8_t> src(img.bits(), 4,img.width(), img.height(), 1);
src.permute_axes("yzcx");
cimg_library::CImg<uint8_t> out = src.get_rotate(angle);
// Here, the out image should be OK, try displaying it with out.display();
// But you still need to go back to an interleaved image pointer if you want to
// get it back in Qt.
out.permute_axes("cxyz");   // Do the inverse permutation.
const uint8_t *p_out = out.data();  // Interleaved result.

I guess this should work as expected.