For image quality analysis, let's start with the main 48MP, which also happens to be the only camera worth discussing on the OnePlus 7.
Both phones have the same sensor and similar optics, so it all comes down to how the software processes the raw data coming off the sensor. First, let's look at some daylight samples.
Looking at images from both cameras, we can see that they both produce excellent images in the daylight scenario but there are a few differences. Both phones have different takes on white balance, with the OnePlus 7 images looking cooler with a magenta tint while the Redmi K20 Pro looking warmer with a greener tint. Both are off slightly but the OnePlus 7 is off by a greater factor. Subjectively speaking, the Redmi K20 Pro's warmer tones also generally look more pleasant even if they aren't super accurate. Apart from that, both phones have good color accuracy.
In terms of detail, both phones capture a good amount but the Redmi K20 Pro applies greater sharpening. This helps the images appear more detailed in low frequency areas and texture in particular looks a bit sharper. However, in high frequency areas such as grass and foliage, the extra sharpening on the Redmi K20 Pro produces some ringing around the edges, which looks unpleasant. The OnePlus 7 renders foliage with greater accuracy but tends to look a bit soft in other areas.
The softer images from the OnePlus 7 may also be due to another factor. We noticed OnePlus 7 images were generally much smaller in file size compared to Redmi K20 Pro images. This despite both having the same resolution and same general content. We noticed some compression artifacts in the sky in the OnePlus 7 images compared to the slightly more grainy but generally more natural-looking Redmi K20 Pro sky. We think OnePlus may be compressing its images a tad too much.
Another thing we noticed was the moire in the Redmi K20 Pro images. Subjects with small repeating patterns such as grilles had a lot of unpleasant moire on the Redmi K20 Pro while the OnePlus 7 software was able to clean it up with greater success.
We also took some 2x zoomed samples, which uses the telephoto lens on the Redmi K20 Pro. The OnePlus 7 uses the center portion of its 48MP sensor to give a 1:1 output, which produces surprisingly good results. You actually get higher resolution 12MP images from the OnePlus 7 in 2x mode compared to the 8MP images from the Redmi K20 Pro.
In terms of detail, the OnePlus 7 manages to come very close to the telephoto camera on the Redmi K20 Pro. The Redmi K20 Pro still manages to edge it out by having generally greater detail and better texture but we were impressed by what the OnePlus 7 managed despite not having a dedicated telephoto sensor.
When it comes to ultra-wide-angle images, there's no trick the OnePlus 7 can employ. Here, the Redmi K20 Pro stands alone with its ultra-wide-angle lens that takes some great looking photos with plenty of detail, excellent colors, good dynamic range and low noise. There is the typical fish-eye distortion but that can be reduced from camera settings. Here we have presented the images without the setting enabled so we can see the full coverage of the lens.
Winner: Redmi K20 Pro. While both phones have comparable image quality in daylight on their main cameras and surprisingly, also in 2x zoomed mode, the OnePlus 7 has no answer to the excellent ultra-wide-angle camera on the Redmi K20 Pro.