user15971: I'm trying to capture various frames within a x264-encoded video files into png files (in python), and then compare the quality of them:
cap = cv2.VideoCapture(filename);
//get frame_number for saving, and save the frame
cap.set(cv2.CAP_PROP_POS_FRAMES,frame_no);
ret, frame = cap.read()

cv2.imwrite(dir_path+'uniform_frame_'+str(frame_no)+".png", frame);

Now I was wondering if I fetch an I-Frame, does it mean that it has a higher quality than other frames, e.g. P-frames and B-Frames? (inherently, keyframes are not compressed, so they must have higher quality and less noise).

user18155: Intra frames (I-frames, keyframes) are independent of other frames - they contain the complete information for reconstructing the frame's image. There is no need for reading other frames and computing from them.
So it's about a speed, not about a quality. On the other hand, more intra frames, grater size of the file. (Known tradeoff speed for space.)
Intra frames are preferably used in most media players for so called fast seek (fast because of skipping the necessity of reading other frames and computing from them) - see for example 
View -> Options -> Tweaks -> Fast seek (on keyframe)

in Media Player Classic (from K-Lite Codec Pack):

Adding more intra frames is important for relatively slow devices (e. g. smartphones) because after jump to non-intra frame they played back ugly colored and broken video while they meet the next intra frame - so for common density of 1 intra frame for 250 frames and the 25 fps the user may wait almost 10 second for restoring the image.

user3908: "I, B and P" don't refer to the viewed frames, they refer to the encoded, stored forms of the frames. However the frame is encoded in the file or stream, when it appears on the screen or is extracted as a still it is a complete frame, reconstructed from the IBP information as needed.

15971: <p>I'm trying to capture various frames within a x264-encoded video files into <code>png</code> files (in <code>python</code>), and then compare the quality of them:</p>

<pre><code>cap = cv2.VideoCapture(filename);
//get frame_number for saving, and save the frame
cap.set(cv2.CAP_PROP_POS_FRAMES,frame_no);
ret, frame = cap.read()

cv2.imwrite(dir_path+'uniform_frame_'+str(frame_no)+".png", frame);
</code></pre>

<p>Now I was wondering if I fetch an <strong>I-Frame</strong>, does it mean that it has a higher quality than other frames, e.g. <strong>P-frames</strong> and <strong>B-Frames</strong>? (inherently, keyframes are not compressed, so they must have higher quality and less noise).</p>
 18155: <p>Intra frames (I-frames, keyframes) are <em>independent of other frames</em> - they contain the complete information for reconstructing the frame's image. There is no need for reading other frames and computing from them.</p>

<p>So it's about a <em>speed</em>, not about a quality. On the other hand, more intra frames, grater size of the file. (Known tradeoff speed for space.)</p>

<p>Intra frames are preferably used in most media players for so called <em>fast seek</em> (fast because of skipping the necessity of reading other frames and computing from them) - see for example </p>

<pre><code>View -&gt; Options -&gt; Tweaks -&gt; Fast seek (on keyframe)
</code></pre>

<p>in Media Player Classic (from <a href="https://www.codecguide.com/" rel="nofollow noreferrer">K-Lite Codec Pack</a>):</p>

<p><a href="https://i.stack.imgur.com/TgXAh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TgXAh.png" alt="enter image description here"></a></p>

<p>Adding more intra frames is important for relatively slow devices (e. g. smartphones) because after jump to non-intra frame <em>they played back ugly colored and broken video</em> while they meet the next intra frame - so for common density of 1 intra frame for 250 frames and the 25 fps the user may wait almost 10 second for restoring the image.</p>
 3908: <p>"I, B and P" don't refer to the viewed frames, they refer to the encoded, stored forms of the frames. However the frame is encoded in the file or stream, when it appears on the screen or is extracted as a still it is a complete frame, reconstructed from the IBP information as needed.</p>
