Scanlines are the visible horizontal gaps left when a CRT draws only every other row of a frame, and they are not a cosmetic filter. On 15 kHz displays tied to NTSC or PAL timing, the electron beam paints 240 or 288 active picture lines per field, leaving dark space between them. For anyone studying 1977–1999 game hardware, scanlines matter because they change luminance, edge perception, and the apparent size of a sprite. A 320×224 pixel buffer does not reach the eye as a flat grid of square pixels; it arrives as a series of bright lines separated by unlit phosphor. That optical structure was part of the display pipeline, and it shaped how artists chose palette values, dither patterns, and even sprite outlines.

This article looks at the measurable side of that effect: how scanlines alter contrast, how they interact with composite video bandwidth, and how they change the way a player parses a game screen. The goal is not to argue that CRTs were magical. The goal is to treat the scanline as a real signal-processing element, with consequences that can be described in cycles, voltage levels, and pixel rows.

What a Scanline Actually Is in a 15 kHz Display

A scanline is one horizontal sweep of the electron beam across the phosphor-coated face of a CRT. In a standard-definition set, the beam completes one line in about 63.5 microseconds for NTSC or 64 microseconds for PAL. The visible portion of that line is shorter, because the horizontal blanking interval reserves time for the beam to return to the left side. The vertical blanking interval then moves the beam back to the top after each field.

For a 240p game signal, the console does not send a full interlaced frame. It sends 262 or 263 lines per field, with roughly 240 lines carrying active picture data. The display still draws the image as a sequence of horizontal lines, but because the source is progressive, the same line positions are struck on every field. The result is a stable image with dark gaps between active rows. Those gaps are the scanlines people refer to when discussing retro game displays.

The width of a scanline is not fixed in pixels. It depends on beam focus, phosphor pitch, and the bandwidth of the video amplifier. A bright pixel can bloom slightly, widening the line. A dark pixel can leave the line nearly invisible. This means the scanline pattern is not a simple overlay; it is an interaction between the signal and the tube.

Luminance, Contrast, and the Apparent Pixel Grid

Scanlines reduce the total light output of the screen. If every other row is dark, the average luminance drops compared with a modern LCD or OLED showing the same frame at full brightness. That reduction is not uniform across colors. A bright white row loses more absolute luminance than a dark blue row, so the perceived contrast between adjacent colors changes.

This has a direct effect on how we read sprite edges. On a modern display, a one-pixel black outline around a sprite is a thin, continuous border. On a CRT with visible scanlines, that outline is broken into horizontal segments. The eye still groups the segments into a contour, but the contour appears softer. Artists could use this to their advantage by choosing outline colors that were not pure black, because the dark scanline gaps would supply some of the separation for free.

There is also a vertical scaling effect. A sprite that is 16 pixels tall occupies 16 active rows. On a CRT, those 16 rows are separated by 15 dark gaps. The sprite appears taller and slightly less dense than it would on a pixel-perfect LCD. A 16×16 tile does not read as a solid block; it reads as a stack of 16 bright dashes. That changes the visual weight of small characters and UI elements.

How Scanlines Interact with Composite and S-Video

Scanlines are not the only thing shaping the image. The video signal itself has limited bandwidth. Composite video on the NES, for example, runs through a modulator that combines luma and chroma into one signal. The luma bandwidth is roughly 3.5 to 4.2 MHz, which limits how quickly the signal can change from dark to light along a horizontal line. A single pixel at 5.37 MHz is already near or beyond that limit, so sharp vertical edges become soft transitions.

Scanlines add a second layer of filtering. Along the vertical axis, the image is sampled by the line structure of the display. Along the horizontal axis, it is filtered by the video amplifier and the phosphor response. The result is an anisotropic blur: more smoothing horizontally, less vertically. This is why a checkerboard dither pattern can look like a solid mid-tone on a CRT. The horizontal blur merges adjacent pixels, while the scanline gaps reduce the visibility of the pattern’s vertical structure.

S-Video improves the situation by separating luma and chroma, but it does not remove the scanline structure. The luma signal still has bandwidth limits, and the tube still draws discrete lines. RGB on a professional monitor can sharpen the image further, but the scanline gaps remain. The only way to remove them is to use a display that does not draw the image as a series of horizontal sweeps.

Dithering and the Scanline as a Blending Tool

Many 16-bit games used dithering to simulate more colors than the hardware palette allowed. A 50 percent checkerboard of two colors can read as a third color if the display blends the pattern. On a CRT, that blending happens through a combination of horizontal bandwidth limits and the eye’s spatial integration. Scanlines contribute by reducing the contrast of the pattern, making the individual dots less distinct.

Consider a vertical gradient dithered with alternating light and dark pixels. On a modern display, the pattern is obvious. On a CRT, the horizontal blur smears each row, and the scanline gaps separate the rows. The result is a smoother gradient, but with a faint horizontal texture. That texture is not an artifact to be ignored; it is part of the intended look. Artists could count on it to hide banding and to create intermediate tones that were not in the palette.

This is why a raw pixel capture of a game is not a faithful record of what the player saw. The frame buffer is only one stage in a pipeline that includes the video encoder, the cable, the display’s amplifier, the tube, and the viewer’s eye. A screenshot is a useful document, but it is not the final image.

Scanlines and Sprite Design: Outlines, Highlights, and Readability

Sprite artists working on 240p systems had to think about how their pixels would be displayed. A one-pixel horizontal line of highlight color would be drawn as a single bright scanline. A one-pixel vertical line would be drawn as a series of bright dots along a column, with dark gaps between them. The vertical line would appear dimmer and more broken than the horizontal line.

This asymmetry explains some common sprite design choices. Characters often have strong horizontal features, such as belts, capes, or shoulder pads, because those features align with the scanline direction and read clearly. Vertical details, such as thin sword blades or staff edges, needed to be at least two pixels wide to survive the scanline gaps. A one-pixel vertical line could disappear into the dark space between rows, especially on a low-contrast background.

Outlines followed a similar logic. A dark outline around a sprite helped separate it from the background, but the outline’s effectiveness depended on its orientation. Horizontal outline segments were reinforced by the scanline gaps. Vertical outline segments had to compete with the gaps, so they often needed to be thicker or paired with a lighter interior color. This is not a rule that artists wrote down; it is a constraint that emerged from the display technology.

Color Choices Under Scanline Conditions

Scanlines also affected color perception. Because the dark gaps reduce overall luminance, colors appear darker and slightly less saturated on a CRT than they do in a raw pixel capture. A bright red that looks garish on an LCD might look rich on a CRT. A dark blue that is barely visible on an LCD might disappear entirely on a CRT, because the scanline gaps push it below the threshold of visibility.

This is why many games used high-contrast palettes for important gameplay elements. Health bars, power-up icons, and player characters were often drawn with bright, saturated colors that could survive the luminance loss. Backgrounds could use darker, less saturated colors, because they did not need to be read quickly. The scanline structure acted as a kind of global contrast control, and artists compensated by pushing important elements toward the bright end of the palette.

There is also a color bleeding effect. On a CRT, a bright scanline can bleed into the dark gap above or below it, especially if the beam is slightly defocused. This bleeding softens the scanline pattern and adds a small amount of vertical blending. The amount of bleeding depends on the tube’s focus, the brightness setting, and the age of the phosphor. Two identical consoles could look different on two different CRTs, even if both were properly adjusted.

Scanlines in Emulation and Modern Displays

Modern emulators often include scanline filters, but these filters are usually simple overlays. They darken every other row of the output image, which mimics the most obvious part of the effect. What they do not simulate is the horizontal blur, the phosphor response, the beam bloom, and the color bleeding. A simple scanline overlay can make an image look darker and more textured, but it does not reproduce the full display pipeline.

More sophisticated shaders attempt to model the CRT more accurately. They simulate the electron beam’s Gaussian profile, the shadow mask or aperture grille, and the phosphor’s decay time. These shaders can produce images that are much closer to a real CRT, but they require significant GPU power and careful calibration. The difference between a simple scanline overlay and a full CRT simulation is the difference between a screenshot and a measurement.

For anyone documenting retro game graphics, this is an important distinction. A pixel-perfect screenshot is a useful reference for tile maps and sprite sheets, but it is not the image the player saw. A CRT simulation is closer, but it is still a simulation. The only way to see the original image is to use original hardware and a properly adjusted CRT, and even then, the image will vary from tube to tube.

Practical Takeaways for Reverse-Engineering Game Art

When examining a game’s graphics, it helps to ask how the scanline structure would have affected each element. A sprite that looks too dark in a raw capture might have been perfectly readable on a CRT. A dither pattern that looks noisy in a screenshot might have blended into a smooth gradient. A one-pixel vertical line that seems to disappear might have been a deliberate choice, because the artist knew it would be reinforced by the scanline gaps.

This is not a call to romanticize CRTs. It is a call to treat the display as part of the system. The console’s video output, the cable, the display, and the viewer’s eye are all stages in a pipeline. A game’s graphics were designed for that pipeline, not for a raw pixel buffer. Understanding the pipeline makes it easier to explain why games look the way they do, and why some design choices that seem odd today made sense in context.

One useful exercise is to compare a raw screenshot, a simple scanline overlay, and a full CRT simulation side by side. The differences reveal which parts of the image were shaped by the display and which parts were shaped by the artist. That comparison is a good starting point for a deeper study of a specific game’s art style. For more on how the NES PPU maps its palette to the composite signal, see the NESdev wiki entry on PPU palettes. The RetroRGB site also documents how different video encoders and cables change the final image on real hardware.

FAQ

Why do scanlines make sprites look taller?

Scanlines insert dark gaps between active rows. A sprite that is 16 pixels tall occupies 16 bright rows separated by 15 dark gaps. The eye perceives the total vertical extent as larger than 16 solid rows, so the sprite appears taller and slightly less dense than it would on a pixel-perfect display.

Do scanlines affect all colors equally?

No. Scanlines reduce total luminance, but the effect is more noticeable on bright colors. A bright white row loses more absolute light than a dark blue row, so the perceived contrast between colors changes. Dark colors can also become harder to distinguish, because the scanline gaps push them closer to black.

Can a scanline filter in an emulator reproduce the original CRT look?

A simple scanline overlay only darkens every other row. It does not simulate horizontal blur, beam bloom, phosphor decay, or color bleeding. More advanced CRT shaders can get much closer, but they are still simulations. The original look depends on the specific tube, its calibration, and the video signal path.

Why did some games use dithering if the hardware palette was limited?

Dithering creates intermediate tones by alternating two colors in a pattern. On a CRT, horizontal blur and scanline gaps blend the pattern, making it read as a third color. This allowed artists to simulate more colors than the hardware could display directly, at the cost of a faint horizontal texture.