user27876: Trying a simple overlay example, getting nasty black fringes:
fmpeg -y -i b.png -i a.png -filter_complex "[0:v]format=rgba[a];
    [1:v]format=rgba[b];
    [a][b]overlay,format=rgba"  normal.png





Expected over Result zoomed in 


user1760: Default pixel format for overlay filter is yuv420p. Use the format option in overlay to set it as auto or rgb.
ffmpeg -y -i b.png -i a.png -filter_complex "[0:v]format=rgba[a]; [1:v]format=rgba[b]; [a][b]overlay=format=auto" normal.png


27876: <p>Trying a simple overlay example, getting nasty black fringes:</p>

<pre><code>fmpeg -y -i b.png -i a.png -filter_complex "[0:v]format=rgba[a];
    [1:v]format=rgba[b];
    [a][b]overlay,format=rgba"  normal.png
</code></pre>

<p><a href="https://i.stack.imgur.com/xTiQY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xTiQY.png" alt="b"></a>
<a href="https://i.stack.imgur.com/TJ12G.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TJ12G.png" alt="a"></a>
<a href="https://i.stack.imgur.com/ZZzAp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZZzAp.png" alt="result"></a>
<a href="https://i.stack.imgur.com/w4Nog.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/w4Nog.png" alt="expected"></a></p>

<p>Expected over Result zoomed in 
<a href="https://i.stack.imgur.com/0UVQo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0UVQo.png" alt="expected vs result"></a></p>
 1760: <p>Default pixel format for <a href="https://ffmpeg.org/ffmpeg-filters.html#overlay" rel="nofollow noreferrer">overlay filte</a>r is yuv420p. Use the <code>format</code> option in overlay to set it as <code>auto</code> or <code>rgb</code>.</p>

<pre><code>ffmpeg -y -i b.png -i a.png -filter_complex "[0:v]format=rgba[a]; [1:v]format=rgba[b]; [a][b]overlay=format=auto" normal.png
</code></pre>
