How Generative Fill Selections Work in Photoshop – Day 1

Generative Fill Does Three Different Jobs, and Each One Wants a Different Selection

Generative Fill looks like one tool with one box to type in. It is really three jobs sharing an interface: taking something out, putting something in, and swapping one thing for another. Each of them wants a different kind of selection and a different kind of prompt, and getting those two things right is the difference between a result you keep and one you undo.

The rule that matters most is barely mentioned anywhere: Adobe’s own Firefly model works best on a small, local selection, while the partner models like Gemini 2.5 Nano Banana work best when you select the whole canvas. Do it the other way round and both will disappoint you.

The Two Things That Decide the Result

The Selection Sets the Context

Photoshop samples what surrounds your selection to work out what belongs there. A selection drawn tight against an object gives it almost nothing to go on, which is why loose selections consistently beat careful ones here.

An Empty Prompt Means “Remove”

Leave the box blank and Photoshop fills the area with what it thinks should be behind. Type anything and you have asked it to add. That single distinction covers most of what people get wrong.

Different Models Want Different Scopes

Firefly: small selection, local job, three variations at a time. Partner models: select all, instruct in a sentence, usually one result per run.

Remove Something From the Frame

1. Grab the Selection Brush (B) and Paint a Loose Circle Around the Object

   Deliberately sloppy. Include a buffer of the background on all sides — that buffer is the context Photoshop uses to invent the fill, and starving it is the most common cause of a visible patch.

   

2. Open the Contextual Task Bar and Click Generative Fill

   Window > Contextual Task Bar if it isn’t showing.

3. Leave the Prompt Completely Blank and Generate

   No text at all. Describing the background is an instruction to build one, and Photoshop will happily give you a more interesting background than the one you actually had.

   

Add Something That Was Never There

4. Select the Area Where You Want It to Appear

   The selection is doing double duty here: it says where, and its size says how big. Draw it at roughly the scale you want the object to be.

5. Click Generative Fill and Type What You Want

   Short and concrete — “garden gnomes” rather than a paragraph. Check which model is selected before generating.

   

6. Work Through the Variations in the Properties Panel

   Three per run with Firefly. Generating twice and choosing from six is a better use of a minute than accepting the first plausible one.

Replace Something Using a Partner Model

7. Select All With Cmd/Ctrl + A

   This is the part that reads as wrong and isn’t. Partner models such as Gemini 2.5 Nano Banana are built to work on a whole image with an instruction, not on an isolated patch. Give one a small selection and the result is usually worse than Firefly’s.

8. Click Generative Fill and Write an Instruction, Not a Description

   “Change her boots to sunflower boots” — a sentence telling it what to do to the photograph. That phrasing is what partner models respond to, and it is different from the noun-phrase prompts Firefly likes.

9. Choose the Partner Model in the Model Picker, Then Generate

   Expect a single result rather than three. Click Generate again for another option — the variation between runs is real, so a second or third attempt is often worth it.

   

Each run spends generative credits, and partner models are not priced the same as Firefly; our free guide to generative credits breaks down what every AI tool in Photoshop and Lightroom costs.

Common Mistakes

Selecting Too Precisely

A tight selection is the habit you built with every other tool in Photoshop, and it is the wrong habit here. Leave a margin.

Prompting When You Meant to Remove

Blank means remove. Anything typed means add.

Using a Partner Model on a Small Selection

Select all instead, and instruct it. Small local jobs belong to Firefly.

Accepting the First Variation

The spread between generations is wide enough that the first result is rarely the best available.

Asking for Something the Scene Can’t Support

Lighting and perspective come from the surrounding pixels. An object that would need light from a direction the photo doesn’t have will look pasted on no matter how you prompt it.

Prompting Is a Smaller Skill Than It Looks

There is a lot written about prompt craft, and most of it overstates the case for this tool. In Generative Fill the prompt is usually the least important of the three inputs. The selection matters more, the model choice matters more, and the photograph itself matters most — a scene with even light and simple geometry will accept almost anything you generate into it, and a difficult one will fight every attempt.

So the useful habit is not writing better prompts. It is noticing, before you type anything, whether the area you have selected gives Photoshop enough to work with, and whether the job in front of you is a Firefly job or a partner-model job.

Do All Three on One Photograph

Paint a loose selection around something you want gone, hit Generative Fill, and generate with an empty prompt. Download the four free sample images and four PSDs and then do the other two jobs on the same frame — add an object with a short prompt into a selection drawn at the right size, and replace something by selecting all and instructing a partner model. Seeing the three side by side is what makes the selection rule stick. For everything Photoshop does that AI doesn’t, our PRO Photoshop Learning Path runs to 11 courses and 53 hours.

Quick Reference: Keys, Menus and Settings

• Select looselySelection Brush tool, paint a circle around the object and it fills the center
• Show the task barWindow > Contextual Task Bar
• RemoveGenerative Fill, prompt left blank, Firefly Image 3, Generate
• AddGenerative Fill, a short prompt (“garden gnomes”), pick a variation in the Properties panel
• Bring back detailClick the Generative Fill layer mask, B, paint black
• ReplaceSelect > All (Cmd/Ctrl + A), Generative Fill, Gemini 2.5 Nano Banana, “change her boots to sunflower boots”

Firefly gives three variations per run; partner models give one.