Screen-Shader

Prepares color screens for printing by replacing the paper and ink colors with pixel patterns, so that a COPY dump shows the picture in shades instead of solid black and white. The HELP text says this works on any printer that uses COPY. The eight shade patterns are the UDGs A–H, one per color 0–7, and are shown on the menu as “PRESENT PATTERNS”. They can be set to two shades (Black/White) or to a graded set (Shaded), loaded from a file of your own UDGs, or set with Isolate so that only the colors you choose are shaded, which is useful for merging screens. The menu also offers Shade, Show Screen, Get Original, Save Screen, Load Screen and Print. A separate HELP program explains each option and loads when you type HELP at any of the filename prompts.

The menu reads “SCREEN-SHADER By Ruud Muilwijk”. The program loads a 112-byte machine code routine, shade, at 60000. The download also has three sample 6912-byte screens, JC, @WIT.S and @MARTYM. @WIT.S is a digitized photograph. To try one, choose 8, type its name at the prompt, choose 5 to copy it to the working screen, and then choose 0 to shade it.

Program Analysis

Structure

Line 20 sets CLEAR 29999 and line 25 loads the shading code. Lines 30–40 poke a 12-byte block copy routine at 60200: LD HL, LD DE, LD BC,6912, LDIR, RET. The subroutines at 9000, 9100 and 9200 poke source and destination addresses into it before each call. 9000 copies the working buffer at 30000 to the screen, 9100 copies the screen back to the buffer, and 9200 copies the original at 40000 to the working buffer. The menu at lines 50–90 is printed over the working picture, and line 100 dispatches with GO TO 500+500*(VAL C$+1), so keys 0–9 go to lines 1000–5500.

Options

• 0 Shade (1000): shows the working picture, calls the machine code at 60000 and stores the shaded screen back in the buffer.
• 1 Black/White (1500): fills UDGs A–D solid and clears E–H.
• 2 Shaded (2000): sets A solid, H blank and B–G from the 48 bytes of DATA in line 2000.
• 3 Isolate (2500): clears all eight UDGs, then fills solid the UDG for each color number entered until a blank line is entered.
• 4 Show Screen (3000): shows the working picture without the menu until a key is pressed.
• 5 Get Original (3500): copies the loaded original back to the working buffer.
• 6 Load UDG, 7 Save Screen and 8 Load Screen (4000–5010): load a pattern file to USR "A", save the working buffer as 6912 bytes of CODE, or load a screen to 40000.
• 9 Print (5500): shows the picture and dumps it with COPY.

Notable Techniques

• One small poked LDIR routine, re-addressed by POKE before each call, handles all three screen-sized block moves.
• The shade patterns live in the UDG area, so they can be changed from BASIC, saved and reloaded as ordinary CODE files.
• The computed GO TO in line 100 replaces a chain of IF tests for the ten menu keys.

Potential Issues

• Load Screen puts the file in the original buffer at 40000, so it does not appear until Get Original (5) is chosen. Choosing 5 before any screen is loaded copies whatever is in memory at 40000.
• The filename prompts for Load UDG and Load Screen accept an empty name, which loads the next CODE file on the tape.

This copy came from an Oliger (JLO SAFE) disk. For tape, the Oliger / was removed from every LOAD and SAVE in both programs, and the three “cat” filename shortcuts, which listed the disk catalog, were replaced by REMs. It was tested in ZEsarUX by loading the @WIT.S sample by name and showing it.