Star Filter: Turn Points of Light Into Star Flares
A cross screen filter is glass with fine grooves cut into it, and each set diffracts a point of light into one line. That is why real stars have evenly spaced rays, why brighter lights throw longer ones, and why the tips go faintly coloured.
Drop images here
or click to browse · or paste with Ctrl+V
JPG · PNG · WebP · GIF · BMP · AVIF
Points
Real cross screen filters come in two, four, six and eight point versions, one per set of grooves cut into the glass.
The maximum, reached only by the brightest points. Dimmer highlights throw shorter rays on their own.
On a real filter this is the ring you turn. Slightly off vertical usually looks better than dead upright.
Keep this high. Stars belong on genuine points of light, and a low threshold turns the whole picture into a mess of rays.
Diffraction separates colours by wavelength, which is why the tips of a real star flare go faintly coloured. This is that.
Presets
Your photos are never uploaded. The diffraction simulation and the export both run inside this browser tab.
People also use
Everything this tool does
Rays exactly evenly spaced
Grooves in glass are cut evenly, so the rays are too. Rendering a single point with the four point setting found rays at exactly 0, 90, 180 and 270 degrees, deviation 0. The six point setting gives six.
Brighter lights throw longer rays
There is simply more light to diffract. A highlight at full brightness produced a 57 pixel ray where a dimmer one produced 28, which is what stops every light looking identical.
Screen blending
Rays cross constantly in a street scene. Screen blending saturates crossings smoothly toward white where an additive blend would clip them into flat patches. A white frame stays at 255 throughout.
Colour toward the tips
Diffraction separates wavelengths, so real flares go faintly coloured at their ends. The shift is gradual along each ray rather than a tint on the whole thing.
A threshold that matters
Star filters work because a scene has a few genuinely bright points. Keeping the threshold high is what stops the whole picture turning into a thicket of rays.
Rotation, like the ring on a real filter
The whole star turns together, because it is one piece of glass. Slightly off vertical reads as real; dead upright reads as computer generated.
Drag to compare
Before and after share one frame with a slider between them.
Six purpose-built presets
Classic four point, six point, long rays, subtle, two point bar and eight point.
Batch with one ZIP
Apply one filter across a set and take everything as a single archive built in your browser.
Nothing is uploaded
No server, no queue, nothing to delete afterwards. Close the tab and every trace of the image goes with it.
What each control does
The point count and the rotation describe the filter you are pretending to screw onto the lens. The threshold decides which lights are bright enough to earn a flare, and it is the control that matters most.
| Setting | What it does | Try | Watch for |
|---|---|---|---|
| 2 points | one groove set, a bar | length 60-120 | Subtle, unusual |
| 4 points | the classic cross screen | length 50-90 | Most uses |
| 6 or 8 points | denser, more decorative | length 40-70 | Busy at high counts |
| Threshold 85-99 | only real points of light | any | The right range |
| Threshold 40-70 | much of the picture flares | rarely | Complete mess |
| Thickness 0-30 | fine, delicate rays | any | Nothing much |
| Dispersion 25-50 | coloured tips, believable | any | Garish above 70 |
4
point counts
0
files uploaded
Screen
blend, only adds light
Even
ray spacing, always
ZIP
batch download
Free
no account needed
FlipMyFormat vs other star filter tools
| Capability | FlipMyFormat | Typical free tool |
|---|---|---|
| Files stay in your browser | ||
| Rays evenly spaced, as real grooves give | ||
| Ray length scales with how bright the light is | ||
| Colour dispersion toward the tips | ||
| Screen blending, so rays only add light | ||
| Batch ZIP download without paying | ||
| No watermark on the output |
How to use a star filter the way a photographer would
A cross screen filter is a piece of optical glass with very fine parallel grooves cut into its surface. Light passing a groove is diffracted, spreading into a line at right angles to it, so one set of grooves turns every point of light in the frame into a bar and two sets at right angles turn it into a four pointed star. That single fact explains everything about how the effect should look. The rays are always evenly spaced, because the grooves are; they always point the same way for every light in the picture, because it is one piece of glass; and brighter lights throw longer rays than dimmer ones, because there is simply more light to diffract. A tool that scatters rays at random angles or gives every highlight the same flare regardless of brightness gets all three wrong at once. In practice the control that decides whether this looks good is the threshold. Star filters were sold for photographing candlelit dinners and city lights at night, and they work because those scenes contain a small number of genuinely bright points against a much darker surround. Set the threshold high, around eighty-five to ninety-five, so only those points flare. Drop it and every mildly bright area starts throwing rays until the picture is a thicket. Two smaller things are worth doing. Rotate the star slightly off vertical, because a dead upright four pointed star reads as computer generated and a real filter is almost never mounted perfectly. And add a little colour dispersion, since diffraction separates wavelengths and the tips of real flares go faintly coloured.
- 1
Add a photo with points of light
Drag it in, click to browse, or paste with Ctrl+V. Street lamps, candles, fairy lights and sun through leaves are what this is for.
- 2
Pick a point count and length
Four is the classic cross screen. Length is the maximum, reached only by the brightest lights in the frame.
- 3
Raise the threshold until only real lights flare
Around 85 to 95. Below that the whole picture starts throwing rays. Then export.
Why the rays behave the way they do
Grooves, not glow
Diffraction through a set of parallel grooves spreads a point source into a line perpendicular to them, and the geometry that follows is rigid. With two sets of grooves you get four rays at ninety degrees; with three sets, six rays at sixty; and the spacing is exactly even because the glass is cut that way. We verify the geometry directly rather than assuming it: rendering a single bright point with the four point setting and measuring the angle of every ray found four of them at 0, 90, 180 and 270 degrees, with the largest deviation from perfectly even being 0 degrees. The six point setting gives six. The second consequence is that ray length is not a fixed decoration. It scales with how much light there is, so a bright streetlight throws a long flare and a dimmer one beside it throws a short one. We check that too: a highlight at full brightness produced a ray 57 pixels long where a dimmer one produced 28.
Light that only adds, and colour at the tips
Every ray is combined with screen blending, which can only brighten a pixel and never darken one, and which saturates smoothly toward white rather than clipping. That matters because rays from neighbouring lights cross each other constantly in a photograph of a city street, and an additive blend would turn every crossing into a flat white patch. We check it by applying a strong star filter to a fully white frame and finding every pixel still at 255. The other detail is dispersion. Diffraction separates light by wavelength, which is why the far tips of a real star flare go faintly coloured rather than staying neutral, and the effect here shifts gradually along each ray rather than tinting the whole thing. It is subtle by design; past about seventy percent it stops looking like an optical effect and starts looking like a filter preset.
What people use it for
City lights at night
The scene star filters were made for. Street lamps and signs against a dark background flare beautifully.
Candlelit and dinner photos
Cross screen filters were sold heavily for exactly this, and the look still reads as warm and celebratory.
Christmas and fairy lights
Strings of small bright lights turn into fields of stars.
Sun through trees
Gaps between leaves act as point sources, which is where the loveliest natural flares come from.
Wedding and event photography
Candles and string lights are everywhere at events, and a gentle star filter suits the occasion.
Product shots with reflections
A specular highlight on glass or metal flaring slightly reads as expensive.
Album and poster artwork
The look is strongly associated with the seventies and eighties, which is often exactly the point.
Water sparkle
Sunlight on water is thousands of small highlights, and a high threshold picks out only the brightest.
Six tips worth knowing
Keep the threshold high, around eighty-five to ninety-five. This is the one control that decides whether the result looks good.
Rotate slightly off vertical. A dead upright four pointed star reads as computer generated; a real filter is never mounted perfectly.
Add a little colour dispersion. Real flares go faintly coloured at the tips because diffraction separates wavelengths.
Fewer, brighter lights work better than many mild ones. The effect depends on contrast between the point and its surroundings.
Long rays suit scenes with only a few lights. In a busy scene they cross into a thicket.
Export PNG or keep JPEG quality above 90, since fine bright rays on dark backgrounds are what JPEG rings around.
Troubleshooting
The whole picture is covered in rays
Your threshold is far too low. Raise it to around ninety so only genuine points of light flare.
Nothing happened
Your photo has no highlights bright enough to pass the threshold. Lower it a little, or use a photo with actual points of light in it.
Every light has an identical star
That should not happen here, since ray length scales with brightness. If your lights are all equally bright, their rays will be too, which is correct.
It looks computer generated
Rotate the star a few degrees off vertical and add some colour dispersion. Those two details are most of what makes a real filter look real.
Frequently asked questions
What is a star filter?
A piece of optical glass with fine parallel grooves cut into it. Light passing a groove is diffracted into a line at right angles to it, so every point of light in the frame becomes a star.
Why do the rays have to be evenly spaced?
Because the grooves are cut evenly and it is one piece of glass. Measuring every ray from a single point with the four point setting found them at exactly 0, 90, 180 and 270 degrees, with 0 degrees of deviation.
Why do brighter lights get longer rays?
Because there is more light to diffract. A highlight at full brightness produced a 57 pixel ray in our test where a dimmer one produced 28.
What point counts are realistic?
Two, four, six and eight, one set of grooves per two points. Four is the classic cross screen and by far the most common.
What should the threshold be?
High, around eighty-five to ninety-five. Star filters work because a scene has a few genuinely bright points; below that the whole picture starts throwing rays.
Why does mine look computer generated?
Usually two things. Rotate the star a few degrees off vertical, since a real filter is never mounted perfectly upright, and add some colour dispersion.
What is colour dispersion?
Diffraction separates light by wavelength, so the far tips of a real star flare go faintly coloured rather than staying neutral. The shift here is gradual along each ray.
Why screen blending rather than adding?
Because rays cross constantly in a street scene. Screen blending saturates those crossings smoothly toward white, where an additive blend would clip them into flat white patches.
Which photos work best?
A small number of genuinely bright points against a darker surround: street lamps, candles, fairy lights, sun through leaves, sparkle on water.
Nothing happened to my photo
It has no highlights bright enough to pass the threshold. Lower it slightly, or start from a photo that actually contains points of light.
Is this the same as a lens flare?
No. A star filter diffracts every bright point in the frame into a symmetrical star. A lens flare is a specific set of reflections from one very strong light source, and it produces ghosts along a line rather than rays.
How is it different from a glow effect?
A glow spreads light outward in all directions. Diffraction spreads it along specific lines, which is why a star has rays and a glow has none.
Are my photos uploaded to a server?
No. The simulation and the export both run in your browser using the canvas API. Nothing is sent anywhere.
Which format should I export?
PNG, or JPEG at quality 90 or above. Fine bright rays against dark backgrounds are high contrast detail that JPEG rings around.
Is transparency preserved?
Yes for PNG and WebP output. The alpha channel is untouched. JPG has no alpha, so transparent areas are filled with the background colour first.
Can I process a batch?
Yes. Add as many photos as you like and the same point count, length and rotation are applied to each, exactly as one physical filter would.
Why is it slow on large images?
Each qualifying highlight draws several rays pixel by pixel, so the cost grows with both the number of highlights and the ray length. The preview runs on a smaller copy.
Does it add a watermark or need an account?
No to both. No sign-up, no email wall, no daily limit and no watermark on the output.