Making two sources agree

What the eye is actually detecting

Show someone a composite and they will say it “looks off” without being able to say why. They are almost never responding to the content. They are responding to two images having different physical characteristics, which the visual system reads as two surfaces rather than one.

Five characteristics, all matchable.

Grain and noise

Every photograph has noise (sensor noise, film grain) with a particular size and colour. Generated images have their own, usually finer and more even, and often almost none after upscaling.

The fix: flatten the composite and add a single grain layer over everything. In GIMP that is a new layer filled with 50% grey, Filters > Noise > RGB Noise at a low value, set to Overlay, opacity 10 to 25%.

Doing it over the whole image rather than per element is the important part. A unified grain layer makes disparate sources belong to one photograph, and it is the cheapest, most effective integration step there is. This alone rescues most composites.

Sharpness and depth of field

If the generated scene has a shallow depth of field and your product is pin-sharp, the product will not sit in the space. Work out which plane the object occupies and blur it to match, using a graduated blur if it is large enough to span planes.

The opposite happens too: an upscaled generated background can be unnaturally crisp everywhere, sharper than any real lens, and a normally-photographed product looks soft against it. Then you blur the background slightly rather than sharpening the product.

Chromatic aberration

Real lenses split colour slightly at high-contrast edges: a faint red-cyan or blue-yellow fringe, strongest at the frame’s corners. Generated images often have it, inherited from training data. A cut-out product will not.

If the scene has visible fringing, add a trace to the product: offset the red and blue channels by a pixel. If it does not, remove any the product has. Either way, both sources must agree, and this is a detail that separates careful work from adequate work.

Vignette and falloff

Real lenses darken toward the corners. Add one vignette over the flattened composite rather than carrying two.

Colour temperature and black level

Two sources will have different whites and different blacks. Match the black point first (one Curves adjustment lifting or lowering the darkest values) then the white balance. People do it the other way round and then cannot get the shadows to agree, because a mismatched black level looks like a colour cast and is not one.

The order

1. Place and scale the elements.
2. Match black point, then white balance, per element.
3. Match depth of field and sharpness, per element.
4. Match chromatic aberration, per element.
5. Add contact shadows and cast shadows.
6. Flatten a copy.
7. Apply grain, vignette and final grade to the whole thing.

Steps six and seven are the ones that do most of the work and the ones most often skipped. Global treatment over a flattened image is what makes separate sources become one photograph.

Free tools, all of it

GIMP, Krita and Photopea between them do every step. GIMP has Curves, RGB Noise, Gaussian blur, channel offsets for aberration, and a gradient tool for vignettes. If you want a more film-like grain, Filters > Noise > HSV Noise is usually better than RGB noise because it varies hue slightly, which is closer to how film behaves.

For a graded look, GIMP supports colour lookup tables, and free cinematic LUTs are widely available: one applied at reduced opacity over the flattened composite unifies everything and gives the set a house look for free.

What this cannot fix

Matching signatures fixes integration. It does not fix geometry. If the product was photographed from eye level and the scene is seen from above, no amount of grain matching will make it sit in the space: the viewer reads perspective before texture, and a perspective mismatch is visible at thumbnail size while a grain mismatch is not.

So perspective and light direction are decided when you photograph, and are effectively unfixable afterwards. Grain, sharpness, aberration and grade are fixable at any point. Knowing which category a problem falls into tells you whether to open the editor or to reshoot, and reshooting a product against a window takes ten minutes, which is less than an hour of failing to correct perspective in software.