Display Technology

The Amiga's display hardware was basically built around the technology of the CRT screen.

A CRT screen consisted of a glass screen that was coated with a phosphorous material. An electron beam was shot from the inside at the screen to make the phosphor glow. The beam itself was directed by bending it with electromagnetic fields. The spot that it hit would be constantly moved from left to right and top to bottom across the phosphorous screen.

The beam actually consisted of three beams with one beam per primary color (red, green and blue). But since the beams couldn't have different colors, the phosphorous screen was colored in a red, green and blue dot pattern. Each beam would only hit it's designated phoshorous areas, making it possible to mix the colors by increasing or decreasing the intensity of each beam.

The electron beams couldn't be focused precisely enough to only hit these tiny colored points. Therefore the shadow mask was placed just before the phosphorous screen. It masked out the beams, so that each electron beam would only hit its own coloured phosphorous dots. This was possible because the offset in which the electron guns were placed was inverted to the other side of the shadow mask, just like with a pinhole camera.

All these mechanisms needed to create an image on screen also led to several artefacts, that were prominent in most crt screens:

Blurry pixels

The size of a pixel was defined by the size of the area the electron beam would hit on the screen. It had to be focused to be as small as possible, but it would still be a little blurry.

Scanlines

These were the result of the electron beam scanning the screen line by line. This was also increased because computers of that time didn't usually use interlacing to increase the vertical screen resolution. They displayed 50 full screen refreshes, while televisions displayed 25 half screen refreshes (alternating between even and odd lines, resulting in visible flickering).

Shadow Mask

Just like with modern TFT panels, these are the colored dots, that the image is made of. But with CRT displays these dots were pretty big (especially with TV screens) and the dots were visible to the naked eye, if you got close enough to the screen.

Do-it-yourself CRT effect

Todays screen look very clean and blocky and sometimes it would be nice to just recreate the look of the old crt monitors.

The method I use to create the crt images is a bit complicated. But by analysing the individual artefacts it is possible to rebuild the effect in Photoshop (or any other image manipulation tool).

Preparation

We need more pixels to create the crt artefacts. Minimum 2x the size for the scanlines. Minimum 4x the size for the shadow mask. As we are doing all effects each pixel has to be four pixels wide and four pixels tall.

Take any image (preferably pixel art) and resize it to four times its original size with no interpolation (Photoshop calls this interpolation "Nearest Neighbor").

Make sure the image color space is set to RGB Color (not Indexed Color).

Blurry Pixels

The old crt screens (and especially the crt TVs) didn't have a crisp image resolution. By setting the blurriness after the scaling we can control the amount of blurriness better.

Apply Gaussian Blur with a radius of 1.0 - 1.5 pixels

Scanlines

This layer will "cut out" the horizontal lines from your image. The pattern we are creating here has to be just as tall as the pixels are. In this case four pixels tall.

Create a new layer in front of the actual image.
Make the first row of pixels mid-grey (RGB: 128, 128, 128), the second and fourth light-grey (RGB: 192, 192, 192), the third white (RGB: 255, 255, 255)
Repeat this for all following rows.
Set the blending mode to "Multiply"

Shadow Mask

This layer will add a subtle color pattern to your image that simulates the way the shadow mask creates the colored image on screen. Don't set the opacity too high, as the pattern will become irritating to the eye and it will take away too much brightness from the image.

Create another layer in front of the other layers.
Start in the top left corner of the layer and make the first pixel red, the second green, the third blue and repeat until you have filled the whole row.
Copy the first row to the second row.
Copy the first two rows to the third and fourth, but shift it one pixel to the left (or right).
Repeat this for all following rows.
Now set the layer opacity to 25% and select a nice effect with which the layer is to be multiplied to the image (Photoshop's "Color Dodge" works nicely).

Finalize

All the effects that we applied make the image darker. It is therefore necessary to lighten up the image, even if it means that individual details will be burned out.

Finally lighten up the image a little (Photoshop use Adjust Levels and set the upper input levels from 255 to 192).