Hold-And-Modify

A HAM picture that contains up to 4,096 colors in 352×480 resolution, made in 1989

Hold-And-Modify, usually abbreviated as HAM, is a display mode of the Commodore Amiga computer. It uses a highly unusual technique to express the color of pixels, allowing many more colors to appear on screen than would otherwise be possible. HAM mode was commonly used to display digitized photographs or video frames, bitmap art and occasionally animation. At the time of the Amiga's launch in 1985, this near-photorealistic display was unprecedented for a home computer and it was widely used to demonstrate the Amiga's graphical capability. However, HAM has significant technical limitations which prevent it from being used as a general purpose display mode.

Background

The original Amiga chipset uses a planar display with a 12-bit RGB color space that produces 4096 possible colors.

The bitmap of the playfield was held in a section of main memory known as chip RAM, which was shared between the display system and the main CPU. Due to timing considerations, the chipset only had time to read 6 bits per pixel before it was time to draw the next pixel on screen. To reduce the amount of data needed to describe an image and thus fit within this limitation, the display system used an indexed color system with a color palette.

The hardware contained 32 registers that could be set to any of the 4096 possible colors, and the image could access any of these 32 values using 5 bits per pixel. The sixth available bit was used by a display mode known as Extra Half-Brite which reduced the luminosity of that pixel by half, providing an easy way to produce shadowing effects.^[1]

Hold-And-Modify mode

When the Amiga chipset was first being designed, it operated directly in the chroma, hue and luminance values of the NTSC signal, as was common for early home computers for which television sets were used for display. This led to the possibility of greatly reducing the amount of memory needed to display a single-color image like black and white television by setting the color value at the start of a scan line and then having the bitmap values modify the luminance value only. Since analog television images can only change from black to white over a number of pixels, not immediately from one to the next, four pixels would be enough to provide a credible "black and white" image, halving the amount of memory needed to store an image.

As the Amiga design migrated from a games console to a home computer, the chipset likewise moved from HSL color model to the modern RGB color model. To quote Jay Miner:

Hold-And-Modify was essentially a modification of the original luminance-only concept, but this time allowing the modification of one of the three color components, red, green or blue. HAM can be considered a lossy compression technique; under HAM6 mode the playfield is encoded in half the memory normally required for a 12-bit color space. The greater overall color fidelity is achieved at the expense of horizontal chroma resolution. The HAM encoded color space is 'decompressed' by display hardware in realtime as the graphics buffer is being displayed.

Usage

When the Amiga was launched in 1985, HAM mode offered a significant advantage over competing systems. HAM allows display of all 4096 colors simultaneously, though with the aforementioned limitations. This pseudo-photorealistic display was unprecedented for a home computer of the time and allowed display of digitized photographs and rendered 3D images. In comparison the then IBM-PC standard EGA allowed 16 on-screen colors from a palette of 64. EGA's successor VGA released in 1987 with its flagship games mode, Mode 13h, allowed 256 on-screen colors from 262,144. HAM mode was frequently used to demonstrate the Amiga's ability in store displays and trade presentations, since competing hardware could not match the color depth. Due to the limitations described above HAM was mainly used for display of static images and developers largely avoided its use with games or applications requiring animation.

With the introduction of the Advanced Graphics Architecture a conventional planar image could have a palette of 256 colors, offering significantly higher color fidelity. The original HAM mode with its limited color resolution became far less attractive to users of an AGA machine, though it was still included for backward compatibility. The new HAM8 mode was far less useful to the AGA chipset than the HAM mode was to the original chipset, since a planar 256-color palette greatly increased the options to the artist without suffering from the drawbacks of HAM mode. A well-programmed sliced planar mode could prove to be more useful than HAM8. The original purpose of HAM, which was to allow more color resolution despite limited video buffer size and limited memory bandwidth, was no longer as relevant.

As modern computers are capable of high resolution truecolor displays there is no longer any need for display techniques like HAM. Planar graphics are now also obsolete for general purpose computing, since modern graphics hardware has enough memory bandwidth to use packed pixels.

Limitations

HAM mode places restrictions on the value of adjacent pixels on each horizontal line of the playfield. In order to render two arbitrary colors adjacently, it may take up to two intermediary pixels to change to the intended color (if the red, green and blue components must all be modified). In the worst case this reduces the horizontal chroma resolution from 704 pixels to as low as 240 (for comparison, contemporary technologies like VHS had chroma resolution of 40 at 0.4 MHz). This problem is particularly noticeable at areas of sharp contrast (strong horizontal image gradients), where an undesirable multi-hued artifact or "fringe" may appear. Various rendering techniques were used to minimize the impact of "fringing" and HAM displays were often designed to incorporate subtle horizontal color gradients, avoiding vertical edges and contrasts.

Displaying a full color image in HAM mode requires some careful preprocessing. Because HAM can only modify one of the RGB components at a time, rapid color transitions along a scan line may be best achieved by using one of the preset color registers for these transitions. To render an arbitrary image, a programmer may choose to first examine the original image for the most noticeable of these transitions and then assign those colors to one of the registers, a technique known as adaptive palettes. However, with only 16 available registers in the original HAM mode, some loss in color fidelity is common.

Additionally, HAM mode does not easily permit arbitrary animation of the display. For example, if an arbitrary portion of the playfield is to be moved to another on-screen position, the Hold-and-Modify values may have to be recomputed on all source and target lines in order to display the image correctly (an operation not well-suited to animation). Specifically, if the left-most edge of the animated object contains any 'modify' pixels, or if the image immediately to the right of the object contains any 'modify' pixels, then those Hold-and-Modify values must be recomputed. An attempt to move an object around the screen (such as with the use of the blitter) will create noticeable fringing at the left and right borders of that image, unless the graphics are specially designed to avoid this. In order to avoid recomputing Hold-and-Modify values and circumvent fringing, the programmer would have to ensure the left-most pixel of every blitter object and the left-most pixel of every line of a scrolling playfield is a 'set' pixel. The palette would have to be designed so that it incorporates every such left-most pixel. Alternatively, a HAM display can be animated by generating pixel values through procedural generation, though this is generally useful for synthetic images only, for example, the 'rainbow' effects used in demos.

Note, however, that Hold-and-Modify only applies to playfield pixels. 128 pixels of sprite data (in DMA mode) per scanline are still available for placement on top of the HAM playfield.

Implementations

Original Chip Set HAM mode (HAM6)

HAM6 mode, named for the 6 bits of data per pixel, was introduced with the Original Chip Set and was retained in the later Enhanced Chip Set and Advanced Graphics Architecture. HAM6 allows up to 4096 colors to be displayed simultaneously at resolutions from 320×200 to 360×576.

HAM6 encoding uses six bits per pixel: two bits for control and four bits for data. If the two control bits are both set to zero, the four remaining bits are used to index one of the 16 preset color registers, operating in the fashion of a normal indexed bitmap. The other three possible control bit patterns indicate that the color of the previous pixel (to the left) on the scanline should be used and the data bits should instead be used to modify the value of the red, green or blue component. Consequently, there are four possibilities:^[1]

• Set: Use the 4 bits of data to index a color from the 16 color palette. Use that color for this pixel.
• Modify Red: Hold the green and blue color components of the previous pixel. Use the 4 bits of data as the new red color component of this pixel.
• Modify Green: Hold the red and blue color components of the previous pixel. Use the 4 bits of data as the new green color component of this pixel.
• Modify Blue: Hold the red and green color components of the previous pixel. Use the 4 bits of data as the new blue color component of this pixel.

A similar mode, HAM5, is also available but it's rather useless. In HAM5 the sixth bit is always zero, so only the blue color component can be modified.^[1]

Sliced HAM mode (SHAM)

The Original Amiga Chipset included a support chip known as the "Copper" that handled interrupts and other timing and housekeeping duties independently of the CPU and the video system. Using the Copper, it was possible to call back into the CPU to run short tasks while the video display was handling the vertical or horizontal blank interrupt (HBI). This allowed programmers to tie machine code to the display without causing visual artifacts, which makes many video effects much simpler to implement.

Using this technique, programmers developed the Sliced HAM or SHAM mode, also known as dynamic HAM. SHAM used computer code that was called on selected scan lines, or all of them, that changed the palette during the HBI. This meant that every scan line could have its own set of 16 base colors. This made it much simpler to choose the palette, which could now be done per-line instead of per-image. The only downside to this approach is that the image was no longer held solely in chip RAM, as the controlling program had to hold multiple palettes, and there was some complexity to setting up the SHAM mode.

This technique was not limited to HAM, and was widely used with the machine's more conventional graphics modes as well. Dynamic HiRes uses a similar palette changing technique to produce 16 colors per line in the high resolution models, whereas HAM is limited to low resolution but allows both 16 indexed colors as well as modifications of them.

The SHAM idea was deprecated when HAM8 was introduced,^[2] since even an unsliced HAM8 image has far more color resolution than a sliced HAM6 image. However, SHAM remains the best available HAM mode on those Amigas with the original chipset.

Advanced Graphics Architecture HAM mode (HAM8)

With the release of the Advanced Graphics Architecture (AGA) in 1992, the original HAM mode was renamed "HAM6", and a new "HAM8" mode was introduced (the numbered suffix represents the bitplanes used by the respective HAM mode). With AGA, instead of 4 bits per color component, the Amiga now had up to 8 bits per color component, resulting in 16,777,216 possible colors (24-bit color space).

HAM8 operates in the same way as HAM6, using two "control" bits per pixel, but with six bits of data per pixel instead of four. The set operation selects from a palette of 64 colors instead of 16. The modify operation modifies the six most significant bits of either the red, green or blue color component - the two least significant bits of the color cannot be altered by this operation and remain as set by the most recent set operation. Compared to HAM6, HAM8 can display many more on-screen colors. The maximum number of on-screen colors using HAM8 was widely reported to be 262,144 colors (18-bit RGB color space). In fact, the maximum number of unique on-screen colors can be greater than 262,144, depending on the two least significant bits of each color component in the 64 color palette. In theory, all 16.7 million colors could be displayed with a large enough screen and an appropriate base palette, but in practice the limitations in achieving full precision mean that the two least significant bits are typically ignored. In general, the perceived HAM8 color depth is roughly equivalent to a high color display.

The vertical display resolutions for HAM8 are the same as for HAM6. The horizontal resolution can be 320 (360 with overscan) as before, doubled to 640 (720 with overscan) or even quadrupled to 1280 pixels (1440 with overscan). The AGA chipset also introduced even higher resolutions for the traditional planar display modes. The total number of pixels in a HAM8 image cannot exceed 829,440 (1440×576) using PAL modes but can exceed 1,310,720 (1280×1024) using third-party display hardware (Indivision AGA flicker-fixer).

Like the original HAM mode, a HAM8 screen cannot display any arbitrary color at any arbitrary position, since every pixel relies on either a limited palette or relies on up to two color components of the previous pixel. As with the original HAM mode, designers may also choose to 'slice' the display (see below) in order to circumvent some of these restrictions. A HAM7 mode is also available but not widely used.

HAM emulation

HAM is unique to the Amiga and its distinct chipsets. To allow direct rendering of legacy images encoded in HAM format software-based HAM emulators have been developed which do not require the original display hardware. Pre-4.0 versions of AmigaOS can use HAM mode in the presence of the native Amiga chipset. AmigaOS 4.0 and up, designed for radically different hardware, provides HAM emulation for use on modern chunky graphics hardware. Dedicated Amiga emulators running on non-native hardware are able to display HAM mode by emulation of the display hardware. However, since no other computer architecture used the HAM technique, viewing a HAM image on any other architecture requires programmatic interpretation of the image file. Faithful software-based decoding will produce identical results, setting aside variations in color fidelity between display setups.

However, if the goal is merely to display a SHAM image on a non-Amiga platform, the required color values may be pre-calculated based on the palette entries that are programmed via the copper-list, regardless of whether the palette is modified in the middle of a scanline. It is always possible to up-convert a HAM or SHAM image losslessly to a 32-bit palette.

Third-party HAM implementations

A device produced by Black Belt known as HAM-E was able to produce images with HAM8 color depth at low horizontal resolution from an Amiga with an Original Chipset.^[3]

The Amiga would be set up to produce high resolution images (640 pixels wide, 720 with overscan). This required the use of four bitplanes at 70 ns per pixel. The first few lines of the image encoded information to configure the HAM-E unit. Then each pair of pixels was encoded with information for the HAM-E unit, which converted the information into one 140 ns pixel (generating an image 320 pixels wide, or 360 with overscan, at a color depth of eight bitplanes). The quality of HAM-E was thus comparable to a low-resolution HAM8 image. The HAM-E technique exploited the fact that a high resolution image with four bitplanes delivers a third more memory bandwidth, and therefore a third more data, than a low resolution image with six bitplanes.

References

1. 1 2 3 Commodore-Amiga, Inc. (1991). Amiga Hardware Reference Manual. Amiga Technical Reference Series (Third ed.). Addison-Wesley. ISBN 0-201-56776-8. 
2. ↑ Standards and specs: The Interchange File Format (IFF) (See HAM and Cheese)
3. ↑ Black Belt HAM-E

• Specification for the Advanced Amiga (AA) Chip Set, Commodore-Amiga

External links

• An animated demo rendered in HAM mode: HAM-6 version, and HAM-8 version (Java required).