How exactly is the PSNR of a model is calculated?

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I know that PSNR is calculated from the quality of reconstructed images by comparing them to ground truth images, but how is calculated the PSNR of the whole gaussian splatting model?

Furthermore I did a test where took photos of a room in a hemisphere camera position and constructed a gaussian splatting model from it. My first dataset had 20 photos. After the training the value of the PSNR was 35. I did the same training with 100 photos of the same room and the PSNR was just 26. Shouldn't it be better, because I had more details from the room?