Merge pull request #27469 from sturkmen72:png-fixes

Fix for 2 channel PNGs #27469

closes #26825

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/imgcodecs/src/grfmt_png.cpp

@@ -508,11 +508,6 @@ bool  PngDecoder::readData( Mat& img )
                 {
                     return false;
                 }
-                // Asking for blend over with no alpha is invalid.
-                if (bop == 1 && mat_cur.channels() != 4)
-                {
-                    return false;
-                }
 
                 memcpy(&m_chunkIHDR.p[8], &chunk.p[12], 8);
 
@@ -1654,7 +1649,8 @@ bool PngEncoder::writeanimation(const Animation& animation, const std::vector<in
             else
                 tmp = animation.still_image;
 
-            cvtColor(tmp, tmp, COLOR_BGRA2RGBA);
+            if (tmp.channels() > 2)
+                cvtColor(tmp, tmp, COLOR_BGRA2RGBA);
             apngFrame.setMat(tmp);
 
             deflateRectOp(apngFrame.getPixels(), x0, y0, w0, h0, bpp, rowbytes, zbuf_size, 0);

modules/imgcodecs/test/test_animation.cpp

@@ -669,14 +669,15 @@ TEST(Imgcodecs_APNG, animation_imread_preview)
 {
     // Set the path to the test image directory and filename for loading.
     const string root = cvtest::TS::ptr()->get_data_path();
-    const string filename = root + "readwrite/033.png";
+    const string filename = root + "readwrite/034.png";
     cv::Mat imread_result;
     cv::imread(filename, imread_result, cv::IMREAD_UNCHANGED);
     EXPECT_FALSE(imread_result.empty());
 
     Animation animation;
-    imreadanimation(filename, animation);
+    ASSERT_TRUE(imreadanimation(filename, animation));
     EXPECT_FALSE(animation.still_image.empty());
+    EXPECT_EQ((size_t)2, animation.frames.size());
 
     EXPECT_EQ(0, cv::norm(animation.still_image, imread_result, cv::NORM_INF));
 }