About

Halftone is the reprographic technique that simulates continuous-tone imagery through dots of varying size. Every newspaper photo, screen-printed poster, and offset-printed book uses it. The dot pitch (measured in LPI - lines per inch) and screen angles determine whether the result looks clean or exhibits moiré interference. A misaligned CMYK screen angle of even 2° produces visible rosette artifacts that ruin commercial print runs. This tool computes per-cell luminance from the source bitmap, maps it to dot radius via an inverse relationship, and renders at industry-standard angles: Cyan at 15°, Magenta at 75°, Yellow at 0°, and Key (black) at 45°.

The tool approximates print halftoning for digital preview. Results will differ from actual press output due to ink spread, paper absorption, and dot gain - factors that depend on physical substrate. For grayscale mode, luminance is calculated using BT.709 coefficients: 0.2126R + 0.7152G + 0.0722B. Dot shapes beyond circles (square, diamond, line) alter tonal response curves and are selectable here for creative control.

Formulas

The halftone dot radius for each grid cell is computed from the average luminance of the sampled pixels within that cell.

L = 0.2126 ⋅ R + 0.7152 ⋅ G + 0.0722 ⋅ B

The luminance L is normalized to the range [0, 1]. The dot radius is then:

r = cellSize2 ⋅ √1 − L

The square root produces a perceptually linear relationship between luminance and dot area, since area scales with r². For CMYK decomposition:

K = 1 − max(R′, G′, B′)

C = 1 − R′ − K1 − K

Where R′, G′, B′ are RGB values normalized to [0, 1]. Each CMYK channel is rendered at its designated screen angle. Rotated grid coordinates use standard rotation:

x′ = x ⋅ cos(θ) + y ⋅ sin(θ)

y′ = −x ⋅ sin(θ) + y ⋅ cos(θ)

Where θ is the screen angle in radians, x and y are pixel coordinates, and x′, y′ are rotated coordinates used to determine the grid cell assignment.

Reference Data

Parameter	Typical Print Value	Tool Range	Effect on Output
Dot Size (Grid Cell)	4 - 12 px	3 - 30 px	Smaller cells → finer detail, larger file
Screen Angle (Grayscale)	45°	0 - 180°	Controls dot row orientation
Cyan Angle	15°	Fixed	Industry standard to minimize moiré
Magenta Angle	75°	Fixed	60° offset from Cyan
Yellow Angle	0°	Fixed	Least visible color, least critical angle
Black (Key) Angle	45°	Fixed	Most visible channel, placed at 45° for minimum pattern visibility
Newspaper LPI	65 - 85 LPI	-	Coarse, visible dots
Magazine LPI	133 - 175 LPI	-	Fine detail, smooth gradients
High-Quality Print LPI	200 - 300 LPI	-	Near-photographic reproduction
Dot Shape: Circle	Most common	Selectable	Smooth tonal transitions
Dot Shape: Square	Textile printing	Selectable	Sharper midtones, checkerboard at 50%
Dot Shape: Diamond	Flexographic	Selectable	Better shadow detail
Dot Shape: Line	Engraving / art	Selectable	Directional texture, vintage feel
Dot Gain (Print)	10 - 25%	-	Physical ink spread on paper, not simulated
BT.709 Red Weight	0.2126	Fixed	Perceptual luminance for human vision
BT.709 Green Weight	0.7152	Fixed	Dominant contribution to perceived brightness
BT.709 Blue Weight	0.0722	Fixed	Least contribution to luminance
Moiré Risk: Same Angle	High	-	Channels at same angle create strong interference
Moiré Risk: 30° offset	Low	-	Standard offset eliminates most patterns
Max Recommended Resolution	4000 px	Capped	Browser memory constraint

Frequently Asked Questions

Identical angles cause all four dot grids to overlap perfectly, producing severe moiré interference patterns. The standard offsets (C: 15°, M: 75°, Y: 0°, K: 45°) place the most visible channels (K and M) at maximum angular separation (30° apart), while yellow - the least perceptually visible ink - uses 0° where moiré is most likely but least noticeable. This convention is defined in ISO 12647-2 for offset lithography.

Circle dots produce smooth tonal gradations because their area increases continuously with radius. Square dots create a sudden jump at 50% coverage when adjacent dots merge into a checkerboard. Diamond dots merge at approximately 40% and again at 60%, providing two transition points that can improve shadow detail in flexographic printing. Line dots produce directional texture and do not merge in one axis, creating an engraving-like appearance.

Physical printing introduces dot gain: ink spreads on paper, enlarging dots by 10-25% depending on substrate and press type. Coated stock gains approximately 12%, uncoated stock approximately 22%. This tool renders mathematically precise dots without simulating gain. For prepress accuracy, apply a dot gain compensation curve in professional software before generating halftone screens.

LPI (lines per inch) relates to cell size via the image DPI. At 300 DPI, a 133 LPI screen uses cells of approximately 300 ÷ 133 ≈ 2.3 pixels. Since sub-pixel rendering is impractical, this tool works with integer pixel cell sizes. A cell size of 4-6 px at screen resolution approximates magazine-quality halftoning for on-screen preview. For newspaper simulation, use 10-20 px cells.

Yes, with caveats. Screen printing typically uses 45-65 LPI due to mesh limitations. Use grayscale mode with circle dots and a cell size of 10+ px for best results. Convert to pure black-and-white (1-bit) after export, since screen printing is binary - ink passes through the mesh or it does not. The white background option in this tool produces output ready for direct film positive creation.

In highlights (luminance > 0.95), dots become smaller than 1 px and may disappear entirely - this is expected and matches physical plate behavior where minimum dot size is limited. In shadows (luminance < 0.05), dots exceed cell boundaries and merge into solid fill. Both behaviors are physically correct. If you need to preserve extreme highlight dots, reduce the cell size to increase effective resolution.