Digitizer And Plotter

[Mike Fowler]

Plotdigitizeris an online data extraction tool that allows users to extract data from images in numerical format. In short, it reverse-engineers your visual graphs into numbers. The software comes with plenty of useful and time-saving features.

Upload the graph image to PlotDigitizer, select the graph type, calibrate the axis/axes, and start marking points and data values of the points that are automatically generated. You can also export these data to other formats. For more, read our official documentation.

Online PlotDigitizer's app is a free tool available for online use only. The tool is free and allows users to extract data from various graphs though it comes with limited features. For full access to additional features, like auto-tracing, dataset storage, you have to purchase a pro license

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At the time of the HP 9845, computer graphics were mostly defined through vector design. Devices producing raster images like digital cameras (which were not yet invented) were uncommon and scanners were highly expensive. So graphics in general went all the way from digitizing sketches and diagrams over on-screen viewing and editing towards finally plotting the results with flatbed or large format plotters. This was pretty well suited for engineering applications like CAD, and even today flatbed plotters like the HP 7475A are still quite popular, e.g. as cutting plotter.

Consequently, the HP 9845 had a broad support for digitizing and plotting hardware. This included drivers and interfaces for the 9874A digitizer and the 9111A graphics tablet for digitizing input, and for more than ten different plotter types for output. HP also developed vector CRT display controllers like the 1350A/1351A, which could be programmed very similar like a plotter. Digitizer were not only used for capturing coordinates from existing drawings, but also played an important role as human interface, i.e. as hand-eye control device (a role later occupied by computer mice, see the 9845 mouse project for details).

The secret behind the support for such a large number of graphics devices is easily explained. HP early decided to design its own device independent graphics language and simply called it "HP Graphics Language" or just HPGL. This language could be used both as graphics input and output language, and as such could be used both for transferring graphics data from a digitizer to the computer, and for sending it from the computer to a plotter for a hardcopy.

This project now is dedicated for all those, who want to use the graphics input and output features of the HP 9845, but don't own a digitizer or a plotter, or both. Similar to other projects like the HPDrive project, this project aims on providing an emulator running on a standard PC, which can be used from the vintage host via a HP-IB connection. And it uses HPGL for both input and output, thereby simulating several digitizer and plotter types, including the popular 9111A graphics tablet and the 9872A flatbed plotter.

Another subject of this project is sound. Yes, real sound. Not the built-in 9845 speaker which is limited to a non-modulated beep, but real sound. Not much known, the HPGL command set included an instruction for generating tones with control of frequency, duration and amplitude, a feature which is at least implemented in the 9111A graphics tablet and the 9874A digitizer. The emulator connects these commands directly to the MIDI device of a standard sound card, enabling the 9845 to play MIDI through the HP-IB on the PC.

Actually, use of this digitizer/plotter/sound (DPS) emulator is not limited to an HP 9845. Due to the widely device-independent nature of HPGL, it should work in principle with many other HPGL capable systems. It has been roughly tested (successfully) both with an HP-86 and an HP9836. However, though the execution of HPGL commands is not a science, the HP-IB part of the emulator is still quite sensitive to the not completely standardized, system dependent use of the HP-IB bus.

Just like with the HPDrive utility for mass storage, an HPGL emulator will of course never really replace the feeling with a real graphics tablet and real plotters with their pens flying across the paper. But it can be quite useful if you like to explore the vector graphics capabilities of the HP 9845 without owning the appropriate input/output devices.

The creation of the HP Graphics Language (HPGL, or HP-GL) is closely related to the introduction of the 9872A plotter in 1977. The 9872A, as a successor of the 9862A pen plotter, should be equipped with a programmable pen changer, first with support for four pens (9872A) and later eight pens (9872C). The complex design with stepper motors, a BPC processor and HP-IB instead of a device specific control interface finally resulted in twice the price of the 9862A. Which again caused the Loveland division not to support this plotter within their firmware. For the console based control, a simple two-letter command language was created, and HPGL was born. During some meetings of an HP internal "committee" at San Diego, HPGL was agreed as the standard HP graphics language be used by all of HP for graphics hardware and software. According to Larry Brunetti, the project manager for the 9872A plotter, HPGL took the San Diego division "from a sleepy $30M per year to over $400M".

Fact is, that the HPGL was one of the first and certainly still is the most important plotter language, and the 9872A plotter can be considered as the grandfather of all HPGL devices. HPGL is a page description language similar to PCL, Postscript or PDF. Actually, PCL, as also developed by HP for its printers, inherits HPGL as a subset, and many printer drivers are still capable of printing HPGL instructions. Although originally designed as a plotter control language and later extended for digitizing input, HPGL can also be used as an interchange format for vector graphics data. Actually, CAD applications still use HPGL as a basic interchange format. There is no special file meta data required, the HPGL commands can be directly 'printed' into a file instead of sending them to an output device to obtain a valid HPGL data file.

The success of HPGL probably goes back to several aspects. First of all, HPGL is a quite simple and easy to implement format. It uses plain ASCII text with two-letter command mnemonics and optional command parameters, with one command per line. Next, the HP plotters (all supporting HPGL) as primary hardware base were and still are standard. And finally, HPGL, although lean designed, covers almost all requirements for 2D vector graphics, except including raster images. With the development of new devices HPGL was stepwise extended. Finally, with HPGL/2, the original HPGL specification was completely modernized.

With vintage computers, HPGL plays a key role in control of graphics devices and storage of graphics data. Using HPGL as base technology, operating systems like that of the HP 9845 could implement vector graphics support with a universal, unified approach. Adding another graphics device basically required just adding another HPGL driver for this device. Input and output devices could be simply connected by just using the first as HPGL source and the latter as HPGL sink. Some devices even support direct communication between e.g. a digitizer and a plotting device.

The HP 9845 (as most systems developed by HP) already implements support for a basic set of HPGL commands. Actually, the whole HP 9845 graphics subsystem, including the screen graphics, can work as either input or output device being controlled by commands very similar to HPGL commands. The standard input device is controlled in BASIC by the GRAPHICS INPUT IS, and the standard output device is specified by the PLOTTER IS commands. For example, using PLOTTER IS "GRAPHICS" directs all vector output the the CRT, whereas PLOTTER IS "9872A" generally guides all graphics output to a 9872A plotter connected via HP-IB. Since all 9845 graphics commands (sometimes referred to as 'AGL') can be translated into proper HPGL commands, developing a driver for a 9845 graphics device is a comparably easy task.

The HPGL syntax is extremely basic. HPGL devices are controlled via sending ASCII strings to them, and results are received from HPGL devices as ASCII strings, too. Each control string can hold a (theoretically) arbitrary number of HPGL commands, separated by semicolons. It is recommended to terminate each string with a linefeed character. Whitespace characters and characters which can't be interpreted are generally ignored.

The command syntax consists of a two-byte mnemonic in either upper or lower case designating the requested operation, plus a variable number of comma separated parameters, depending on the command. Once defined, parameters in general can be skipped by leaving the space between two commas empty. In this case, the last specified value for the omitted parameter (either from initial defaults or from a previous command) will be substituted.

In addition to the standard HPGL syntax there is a special mechanism implemented in some HP-IB interfaced devices like the 9111A, which provide a specially efficient way of requesting status information, which is called binary transfer. Normally, data is sent from an HPGL device to the host only as reply to a previous HPGL output command. Once a 9111A is addressed by the host via HP-IB as a talker without sending an HPGL command before, it immediately sends a compact binary status summary to the host in six defined bytes, holding the current status word plus the current stylus coordinates.

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