Compositing II

• 01
  Refine a Layer Mask Edge with a Soft Brush

  Start with an existing composite from Session 11, or create a rough one now: cut out any subject using Fuzzy Select or By Colour Select, then add a white layer mask (Layer → Mask → Add Layer Mask → White/Full). Click the mask thumbnail to select it. Paint with black at low opacity (10–20%) to soften hard edges — particularly around hair, fur, or fabric where the subject blends into its background. Work from outside inward.

  Layer → Mask → Add Layer Mask → White (full opacity)
• 02
  Use Gaussian Blur on the Mask to Kill the "Cut Out" Look

  With the mask thumbnail selected in the Layers panel, apply a very gentle Gaussian Blur (radius 0.5–1.5 px). This removes the pixel-perfect edge that betrays a selection-based cutout. Do not apply this to the image layer itself — only to the mask. After blurring, use Colours → Levels on the mask to pull the blur back toward clean (Input: move the white point left to restore opacity in the subject's interior).

  Filters → Blur → Gaussian Blur (on mask thumbnail)
• 03
  Remove Colour Fringing with a Darken-Mode Clipping Brush

  When you cut a subject from a bright background, the edges often carry a fringe of the original background colour (green spill from a green screen, white halation from a light background). Create a new transparent layer above the subject layer. Set its blend mode to Darken. Sample a colour from the edge of the subject (not the background) using the Colour Picker. Paint along the fringe at low opacity. Because Darken only affects pixels lighter than the painted colour, the fringe darkens and integrates while clean pixels are untouched.

  New layer above subject → Mode: Darken → Paint with sampled colour
• 04
  Match Light Direction with a Curves Adjustment Layer

  Add a Curves layer clipped to the subject layer only. To clip a layer in GIMP, place the Curves layer directly above the subject, then right-click it in the Layers panel and choose "Set Layer Group" — or, more directly, use a flattened method: flatten the subject layer copy and apply Curves to it directly. The key adjustment: identify the scene's light direction. If light comes from camera-left, lift the Curves midtones slightly and then paint a gradient layer mask on the Curves layer — light on the left, dark on the right — so the subject's brightness follows the scene's logic.

  Colours → Curves on subject layer → gradient mask to control falloff
• 05
  Match Colour Temperature with Colour Balance or Hue-Saturation

  A subject shot under tungsten (warm/orange) placed in a daylight (cool/blue) background will read as wrong immediately. Apply Colours → Colour Balance to the subject layer. Shift the Midtones toward cool (cyan/blue) until the subject stops looking like it was photographed in a different room. Cross-check by sampling a neutral grey from the background and comparing it to a neutral grey on the subject — they should share the same RGB triplet under Colours → Info → Pointer.

  Colours → Colour Balance → Midtones → shift toward scene's temperature
• 06
  Build a Hand-Made Drop Shadow

  Script-Fu's built-in drop shadow is immediately recognisable. Instead, build one manually: duplicate the subject layer (Shift+Ctrl+D). On the duplicate, apply Colours → Hue-Saturation → set Lightness to −100 to make it fully black. Flatten it (merge down the alpha if needed). Apply Filters → Blur → Gaussian Blur (radius 12–30 px depending on image scale). In the Layers panel, drag this shadow layer below the subject but above the background. Use the Move tool to offset it in the direction opposite the light source — if light comes from upper-left, the shadow falls to the lower-right. Set the shadow layer opacity to 50–70%.

  Duplicate subject layer → Hue-Sat Lightness −100 → Gaussian Blur → reposition
• 07
  Add Perspective Distortion to the Shadow

  A flat drop shadow only reads as a shadow cast on a wall. For ground shadows, the shadow must recede away from the viewer — wider and softer at the base of the subject, narrower and sharper at the far end. With the shadow layer selected, use Tools → Transform Tools → Perspective. Drag the top corners inward to taper the shadow away from the subject's feet. After applying, reapply a second Gaussian Blur at half the original radius to add graduated softness: the part of the shadow closest to the subject should be darkest and sharpest; the far end lightest and softest. Add a gradient layer mask (black at far end) to achieve this falloff.

  Tools → Transform Tools → Perspective → then gradient mask for falloff
• 08
  Flatten, Review, and Adjust Global Colour

  Once masking, light matching, and shadow are complete, create a merged visible copy without destroying layers: create a new empty layer at the top of the stack, then use Edit → Copy Visible followed by Edit → Paste In Place, and anchor it. Apply a subtle Curves S-curve to unify the whole composite under one tonal treatment. Even a tiny lift to the midtones (+3 to +5 points) applied globally can make a composite read as a single unified image rather than separate parts.

  Edit → Copy Visible → Paste in Place → Curves S-curve to unify

You'll need two images: a photo of a person (or any object) on a plain background, and a destination background (an outdoor scene, a textured wall, or an interior). Use the Fuzzy Select or Foreground Select tool from Session 5 to extract the subject, then follow these steps in order.

• A
  Extract and mask the subject

  Use Foreground Select to generate a rough selection around the person. Refine using Select → Grow (1 px) then Select → Feather (1 px). Add a white layer mask. Paint the mask to refine any edges where hair or clothing blends with the original background. The mask does not need to be perfect — aim for good enough to read at normal viewing distance.

• B
  Place the subject on the new background

  Open the background image as a layer below the subject (File → Open as Layers, then drag in the Layers panel). Scale and position the subject so the ground contact point is believable — feet should appear to rest on the ground plane, not float above it. Use the Perspective tool if the background has a strong vanishing point.

• C
  Match colour temperature

  Apply Colours → Colour Balance to the subject layer. Identify whether the background is warm or cool and shift the subject toward it. Check by sampling greys in both layers with the Colour Picker and comparing the RGB values in the Pointer Info window.

• D
  Build and position the ground shadow

  Duplicate the subject layer, apply Hue-Saturation Lightness −100, blur by 20 px, position below the subject above the background. Apply the Perspective transform so the shadow tapers away from the feet. Add a black-to-white gradient layer mask so the near end of the shadow is opaque and the far end fades to nothing. Set layer opacity to 60%.

Professional compositors add a rim light — a subtle glow on the edge of the subject that mimics how the background scene's ambient light wraps around an object. No step-by-step guidance. Attempt the following using what you've learned:

• →
  Create a rim-light glow on the subject's edge

  Hint: duplicate the subject layer. On the duplicate, apply Filters → Blur → Gaussian Blur (large radius, 15–30 px). Change this layer's blend mode to Screen and reduce opacity to 20–40%. Move it 3–5 pixels in the direction the background light comes from. This simulates ambient light wrapping the subject. Sample the dominant colour of the background sky or light source and use Colours → Colour Balance to tint the glow layer toward that colour.

• →
  Apply a vignette to unify the whole composite

  Create a new layer at the very top of the stack. Set its mode to Multiply. Use the Blend tool (Gradient) with a black Radial gradient from centre (transparent) to edges (black). Reduce opacity to 20–30%. This darkens the edges of the frame and draws attention to the centre — a technique used in virtually every professional composite to sell the image as a single unified photograph.