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OPL graphics allows you, for example, to:

You can draw using black, grey and white.

Graphics keywords begin a "G". In this manual a lower case "g" is used for example, "gBOX" but you can type them using upper or lower case letters.

IMPORTANT: Some graphics keywords are mentioned only briefly in this chapter. For more details about them, see the `Alphabetic listing' chapter.

Simple graphics

The Series 3a screen is made up of a regular pattern of 480 points across by 160 down. These points are sometimes referred to as pixels.

Each pixel is identified by two numbers, giving its position across and down from the top left corner of the screen. 0,0 denotes the pixel in the top left corner; 2,1 is the pixel 2 points across and one down, and so on. 479,159 is the pixel in the bottom right corner.

Note that these co-ordinates are very different to the cursor positions set by the AT command.

OPL maintains a current position on the screen. Graphics commands which draw on the screen generally take effect at this position. Initially, the current position is the top left corner, 0,0.

You can draw using black, grey and white although grey is not accessible by default. See the section `Drawing in grey' below for further details.

Screen positions

Drawing lines

Here is a simple procedure to draw a horizontal line in the middle of the screen:

gMOVE moves the current position by the specified amount. In this case, it moves the current position 180 pixels right and 80 down, from 0,0 to 180,80. It does not display anything on the screen.

gLINEBY (g-Line-By) draws a line from the current position (just set to 180,80) to a point at the distance you specify in this case 120 to the right and 0 down, ie 300,80.

When drawing a horizontal line, as in the above example, the line that is drawn includes the pixel with the lower x coordinate and excludes the pixel with the higher x coordinate. Similarly, when drawing a vertical line, the line includes the pixel with the lower y coordinate and excludes the pixel with the higher y coordinate.

On the Series 3a screen the y coordinate decreases as you move toward the top of the screen.

When drawing a diagonal line, the coordinates of the end pixels are turned into a rectangle. The top left pixel lies inside the boundary of this rectangle and the bottom right pixel lies outside it. The line drawing algorithm then fills in those pixels that are intersected by a mathematical line between the corners of the rectangle. Thus the line will be drawn minus one or both end points.

gLINEBY also has the effect of moving the current position to the end of the line it draws.

With both gMOVE and gLINEBY, you specify positions relative to the current position. Most OPL graphics commands do likewise. gMOVE and gLINEBY, however, do have corresponding commands which use absolute pixel positions. gAT moves to the pixel position you specify; gLINETO draws a line from the current position to an absolute position. The horizontal line procedure could instead be written:

gAT and gLINETO may be useful in very short graphics programs, and gAT is always the obvious command for moving to a particular point on the screen, before you start drawing. But once you do start drawing, use gMOVE and gLINEBY. They make it much easier to develop and change programs, and allow you to make useful graphics procedures which can display things anywhere you set the current position. Almost all graphics drawing commands use relative positioning for these reasons.

Drawing dots

You can set the pixel at the current position with "gLINEBY 0,0".

Right and down, left and up

gMOVE and gLINEBY find the position to use by adding the numbers you specify onto the current position. If the numbers are positive, it moves to the right and down the screen. If you use negative numbers, however, you can specify positions to the left of and/or above the current position. For example, this procedure draws the same horizontal line as before, then another one above it:

The first two program lines are the same as before. gLINEBY moves the current position to the end of the line it draws, so after the first gLINEBY the current position is 300,80. The second gMOVE moves the current position up by 20 pixels; the second gLINEBY draws a line to a point 120 pixels to the left.

For horizontal and vertical lines, the right-hand/bottom pixel is not set. For diagonal lines, the right-most and bottom-most pixels are not set; these may be the same pixel.

Going off the screen

No error is reported if you try to draw off the edge of the screen. It is quite possible to leave the current position off the screen for example, "gLINETO 600,80" will draw a line from the current position to some point on the right-hand screen edge, but the current position will finish as 600,80.

There's no harm in the current position being off the screen. It allows you to write procedures to display a certain pattern at the current position, and not have to worry whether that position is too close to the screen edge for the whole pattern to fit on.

Clearing the screen

"gCLS" clears the screen.

Drawing in grey

Initialising for the use of grey

To draw in grey you need to use "DEFAULTWIN 1" at the start of your program. (Note that this clears the screen.) Grey is not automatically available because it requires twice the memory (and takes longer to scroll or move) compared to having just black. So programs that do not need to use grey are not unnecessarily penalised.

"DEFAULTWIN 0" disables the use of grey again, also clearing the screen.

Note : It is not possible to have a screen using grey only.
"DEFAULTWIN 1" does not cause PRINT to print in grey it applies only to graphics and graphics text (see gPRINT later).

When you use "DEFAULTWIN 1" the existing black-only screen is cleared and replaced by one which contains a black plane and also a grey plane. The black plane is also sometimes called the the normal plane. These are referred to as `planes' because intuitively it is simplest to think of there being a plane of black pixels in front of (or on top of) a plane of grey pixels, with any grey only ever visible if the black pixel in front of it is clear.

If you draw a pixel using both black and grey, it will appear black. If you then clear the black plane only, the same pixel will appear grey. If you draw a pixel using grey only it will appear grey unless it is already black, in which case it is effectively hidden behind the black plane.

If you need to use grey, you are recommended to use "DEFAULTWIN 1" once and for all at the start of your program. One reason is because DEFAULTWIN can fail with a `No system memory' error and it is unlikely that you would want to continue without grey after trying to enable it.

Note : Note that gXBORDER, gBUTTON and gDRAWOBJECT all use grey and therefore can only be used when grey in enabled. If grey is not enabled, they raise a `General failure' error.

Using grey

Once you have used "DEFAULTWIN 1" you can use the gGREY command to set which plane should be used for all subsequent graphics drawing (until the next use of gGREY).

gGREY 0 	draws to the black plane only.
gGREY 1		draws to the grey plane only.
gGREY 2 	draws to both planes.
"gGREY 1" and "gGREY 2" raise an error if the current window does not have a grey plane.

As mentioned earlier, when you set a pixel using both black and grey, the pixel appears black because the black plane is effectively in front of the grey plane. So drawing to both planes is generally only used for clearing pixels. For example, if your screen has both black and grey pixels, gCLS will clear the pixels only in the plane selected by gGREY. To clear the whole screen with gCLS, you therefore need "gGREY 2".

To draw in grey when the pixels to which you are drawing are currently black, you first need to clear the black.

A pixel will appear white only if it is clear in both planes.


The following procedure initialises the screen to allow grey, draws a horizontal line in grey, another below it in black only and a third below it in both black and grey. Pressing a key clears the black plane only, revealing the grey behind the black in the bottom line and clearing the middle line altogether.

Overwriting pixels

Drawing rectangles

The gBOX command draws a box outline. For example, "gBOX 100,20" draws a box from the current position to a point 100 pixels to the right and 20 down. If the current position were 200,40, the four corners of this box would be at 200,40, 300,40, 300,60 and 200,60.

If you have used "DEFAULTWIN 1" and gGREY as described earlier, the box is drawn to the black and/or grey plane as selected.

gBOX does not change the current position.

gFILL draws a filled box in the same way as gBOX draws a box outline, but it has a third argument to say which pixels to set. If set to 0, the pixels which make up the box would be set. If set to 1, pixels are cleared; if set to 2, they are inverted, that is, pixels already set on the screen become cleared, and vice versa. The values 1 and 2 are used when overwriting areas of the screen which already have pixels set.

If you have used "DEFAULTWIN 1" and gGREY as described earlier, the filled box will be set, cleared or inverted in the black and/or grey plane as selected. Once again, it helps to think of the pixels being set or clear in each plane independently: so clearing the pixel in the black plane reveals the grey plane behind it where the pixel may be set or clear.

So with "gGREY 1" set for drawing to the grey plane only, inverting the pixels in the filled box will change the grey plane only black pixels are left alone but clear or grey pixels are inverted to grey and clear pixels respectively. Similarly, inverting the black plane changes clear pixels to black, but "clearing" black pixels displays grey if the pixel is set in the grey plane.

This procedure displays a "robot" face, using gFILL to draw set and cleared boxes:

Before calling such a procedure, you would set the current position to be where you wanted the top left corner of the head.

You could make the robot wink with the following procedure, which inverts part of one eye:

Again, you would set the current position before calling this.

The gPATT command can be used to draw a shaded filled rectangle. To do this, use "-1" as its first argument, then the same three arguments as for gFILL width, height, and overwrite method. Overwrite methods 0, 1 and 2 apply only to the pixels which are `on' in the shading pattern. Whatever was on the screen may still show through, as those pixels which are `clear' in the shading pattern are left as they were.

To completely overwrite what was on the screen with the shaded pattern, gPATT has an extra overwrite method of 3. So, for example, "gPATT -1,120,120,3" in the first procedure would have displayed a shaded robot head, whatever may have been on the screen.

Again, the shaded pattern will be drawn in grey if you have selected the grey plane only using "gGREY 1". And again, if you are writing to the black plane only, any pixels set in the grey plane can be seen if the corresponding pixels in the black plane are clear.

Overwriting with any drawing command

By using the gGMODE command, any drawing command such as gLINEBY or gBOX can be made to clear or invert pixels, instead of setting them. gGMODE determines the effect of all subsequent drawing commands.

The values are the same as for gFILL: "gGMODE 1" for clearing pixels, "gGMODE 2" for inverting pixels, and "gGMODE 0" for setting pixels again. (0 is the initial setting.)

For example, some white lines can give the robot a furrowed brow:

The setting for gGMODE applies to the planes selected by gGREY. With "gGREY 1" for instance, "gGMODE 1" would cause gLINEBY to clear pixels in the grey plane and "gGMODE 0" to set pixels in the grey plane.

Other drawing keywords

Note : Note that commands such as gSCROLL,, which move existing pixels, affect both black and grey planes. gGREY only restricts drawing and clearing of pixels.

Graphical text

Displaying text with gPRINT

The PRINT command displays text in one font, in a screen area controlled by the FONT or SCREEN commands. You can, however, display text in a variety of fonts and styles, at any pixel position, with gPRINT. gPRINT also lets you draw text to the grey plane, if you have used DEFAULTWIN and gGREY (discussed earlier).

Note : You can (to a lesser degree) control the font and style used by OPL's other text-drawing keywords, such as PRINT and EDIT. See "The text and graphics windows" at the end of this chapter.
gPRINT is a graphical version of PRINT, and displays a list of expressions in a similar way. Some examples:
"gPRINT "Hello",name$"
"gPRINT a$"
"gPRINT "Sin(PI/3) is",sin(pi/3)"
Unlike PRINT, gPRINT does not end by moving to a new line. A comma between expressions is still displayed as a space, but a semi-colon has no effect. "gPRINT" used on its own does nothing.

The first character displayed has its left side and baseline at the current position. The baseline is like a line on lined notepaper graphically, this is the horizontal line which includes the lowest pixels of upper case characters. Some characters, such as `g', `j', `p', `q' and `y', set pixels below the baseline.

After using gPRINT, the current position is at the end of the text so that you can print something else immediately beyond it. As with other graphics keywords, no error is reported if you try to display text off the edge of the screen.

While "CURSOR ON" displays a flashing cursor for ordinary text displayed with PRINT, "CURSOR 1" switches on a cursor for graphical text which is displayed at the current position. "CURSOR OFF" removes either cursor.


The gFONT command sets the font to be used by subsequent gPRINT commands.

A large set of fonts which can be used with gFONT is provided in the Series 3a ROM. In the following list, Swiss fonts refer to fonts without serifs while Roman fonts either have serifs (eg font 6) or are in a style designed for serifs but are too small to show them (eg font 5). Mono-spaced fonts have characters which all have the same width (and have their `pixel size' listed as width "x" height); in proportional fonts each character can have a different width.

font number	Description		pixel size
1		Series 3 normal		8
2		Series 3 bold		8
3		Series 3 digits		6x6
4		Mono			8x8
5		Roman			8
6		Roman			11
7		Roman			13
8		Roman			16
9		Swiss			8
10		Swiss			11
11		Swiss			13
12		Swiss			16
13		Mono			6x6
The special font number "$9a" is set aside to give a machine's default graphics font; this is the font used initially for graphics text. The actual font may vary from machine to machine eg it is font 1 on the Series 3 and font 11 on the Series 3a. So "gFONT 11" or "gFONT $9a" both set the Series 3a standard font, which gPRINT normally uses.

Note : Fonts 1,2 and 3 are the Series 3 fonts, used when running in compatibility mode.
See gINFO in the alphabetic listing if you need to find out more information about fonts.

The following program shows you examples of the fonts. ("!!!" is displayed to emphasise the mono-spaced fonts):

Text style

The gSTYLE command sets the text style to be used by subsequent gPRINT commands.

Choose from these styles:

"gSTYLE 1"	bold
"gSTYLE 2"	underlined
"gSTYLE 4"	inverse
"gSTYLE 8"	double height
"gSTYLE 16"	mono
"gSTYLE 32"	italic
The `mono' style is not proportionally spaced each character is displayed with the same width, in the same way that PRINT displays characters. A proportional font can be displayed as a mono-spaced font by setting the `mono' style. See the previous section for the list of mono-spaced and proportional fonts.

Note : It is inefficient to use the `mono' style to display a font which is already mono-spaced.
You can combine these styles by adding the relevant numbers together. "gSTYLE 12" sets the text style to inverse and double-height (4+8=12). Here's an example of this style:

Use "gSTYLE 0" to reset to normal style.

The bold style provides a way to make any font appear bolded. Except for the smaller fonts, most Series 3a fonts look reasonably bold already. Note that using the bold style sometimes causes a change of font; if you use gINFO you may see the font name change.

Overwriting with gPRINT

gPRINT normally displays text as if writing it with a pen the pixels that make up each letter are set, and that is all. If you're using areas of the screen which already have some pixels set, or even have all the pixels set, use gTMODE to change the way gPRINT displays the text.

gTMODE controls the display of text in the same way as gGMODE controls the display of lines and boxes. The values you use with gTMODE are similar to those for gGMODE: "gTMODE 1" for clearing pixels, "gTMODE 2" for inverting pixels, and "gTMODE 0" for setting pixels again. There is also "gTMODE 3" which sets the pixels of each character while clearing the character's background. This is very useful as it guarantees that the text is readable (as far as the current plane is concerned).

As for gGMODE, the setting for gTMODE applies to the planes selected by gGREY. With "gGREY 1" for instance, "gTMODE 1" would cause gLINEBY to clear pixels in the grey plane and "gTMODE 0" to set pixels in the grey plane.

This procedure shows the various effects possible via gTMODE:

Other graphical text keywords

All of these keywords take the current font and style into account, and work on a single string. They display the text in black or grey according to the current setting of gGREY.


So far, you've used the whole of the screen for displaying graphics. You can, however, use windows rectangular areas of the screen.

Note : Sprites (described in the `Advanced topics' chapter) can display non-rectangular shapes.
OPL allows a program to use up to eight windows at any one time.

Window IDs and the default window

Each window has an ID number, allowing you to specify which window you want to work with at any time.

When a program first runs, it has one window called the default window. Its ID is 1, it is the full size of the screen, and initially all graphics commands operate on it. (This is why `0,0' has so far referred to the top left of the screen: it is true for the default window.)

Other windows you create will have IDs from 2 to 8. When you make another window it becomes the current window, and all subsequent graphics commands operate on it.

The first half of this chapter used only the default window. However, everything actually applies to the current window. For example, if you make a small window current and try to draw a very long line, the current position moves off past the window edge, and only that part of the line which fits in the window is displayed.

Graphics keywords and windows

For OPL graphics keywords, positions apply to the window you are using at any given time. The point 0,0 means the top left corner of the current window, not the top left corner of the screen.

Each window can be created with a grey plane if required, in which case gGREY is used to specify whether the black plane, the grey plane or both should be used for all subsequent graphics commands until the next call to gGREY, exactly as described in the first half of this chapter.

For the default window, the special command DEFAULTWIN is required to enable grey because that window is automatically created for you with only a black plane; "DEFAULTWIN 1" closes the default window and creates a new one which has a grey plane. All other windows must be created with a grey plane if grey is required.

Once a window has been created with a grey plane, grey is used in precisely the same way as in the default window with grey enabled: "gGREY 0" directs all drawing to the black plane only, "gGREY 1" to the grey plane only and "gGREY 2" to both planes. "gGREY 1" and "gGREY 2" raise an error if the current window does not have a grey plane.

gGREY, gGMODE, gTMODE, gFONT and gSTYLE can all be used with created windows in exactly the same way as with the default window, as desribed earlier. They change the settings for the current window only; all the settings are remembered for each window.

Creating new windows

The gCREATE function sets up a new window on the screen. It returns an ID number for the window. Whenever you want to change to this window, use gUSE with this ID.

You can create a window with only a black plane or with both a black and a grey plane. You cannot create a window with just a grey plane.

Here is an example using gCREATE and gUSE, contrasting the point 20,20 in the created window with 20,20 in the default window.

The line "id%=gCREATE(60,40,180,30,1,1)" creates a window with its top left corner at 60,40 on the screen. The window is set to be 180 pixels wide and 30 pixels deep. (You can use any integer values for these arguments, even if it creates the window partially or even totally off the screen.) The fifth argument to gCREATE specifies whether the window should immediately be visible or not; 0 means invisible, 1 (as here) means visible. The sixth argument specifies whether the window should have a grey plane or not; 0 means black only, 1 (as here) means black and grey. If the sixth argument is not supplied at all (eg. "id%=gCREATE(60,40,180,30,1)") the window will not have a grey plane.

gCREATE automatically makes the created window the current window, and sets the current position in it to 0,0. It returns an ID number for this window, which in this example is saved in the variable "id%".

The "gBORDER 0" command draws a border one pixel wide around the current window. Here this helps show the position and size of the window. (gBORDER can draw a variety of borders. You can even display the Series 3a 3-D style borders seen in menus and dialogs, with the gXBORDER keyword.)

The program then sets the pixel at 20,20 in this new window, using "gLINEBY 0,0".

"gUSE 1" goes back to using the default window. The program then shows 20,20 in this window.

Finally, "gUSE id%" goes back to the created window again, and a final message is displayed, in grey and black.

Note that each window has its own current position. The current position in the created window is remembered while the program goes back to the default window. All the other settings, such as the font, style and grey setting are also remembered.

Closing windows

When you've finished with a particular window, close it with gCLOSE followed by its ID for example, "gCLOSE 2". You can create and close as many windows as you like, as long as there are only eight or fewer open at any one time.

If you close the current window, the default window (ID=1) becomes current.

An error is raised if you try to close the default window.

When windows overlap

Windows can overlap on the screen, or even hide each other entirely. Use the gORDER command to control the foreground/background positions of overlapping windows.

"gORDER 3,1" sets the window whose ID is 3 to be in the foreground. This guarantees that it will be wholly visible. "gORDER 3,2" makes it second in the list; unless the foreground window overlaps it, it too will be visible.

Any position greater than the number of windows you have is interpreted as the end of the list. "gORDER 3,9" will therefore always force the window whose ID is 3 to the background, behind all others.

Note in particular that making a window the current window with gUSE does not bring it to the foreground. You can make a background window current and draw all kinds of things to it, but nothing will happen on the screen until you bring it to the foreground with gORDER.

When a window is first created with gCREATE it always becomes the foreground window as well as the current window.

Hiding windows

If you are going to use several drawing commands on a particular window, you may like to make it invisible while doing so. When you then make it visible again, having completed the drawing commands, the whole pattern appears on the screen in one go, instead of being built up piece by piece.

Use "gVISIBLE ON" and "gVISIBLE OFF" to perform this function on the current window. You can also make new windows invisible as you create them, by using 0 as the fifth argument to the gCREATE command, and you can hide windows behind other windows.

The graphics cursor in windows

To make the graphics cursor appear in a particular window, use the CURSOR command with the ID of the window. It will appear flashing at the current position in that window, provided it is not obscured by some other window.

The window you specify does not have to be the current window, and does not become current; you can have the cursor in one window while displaying graphical text in another. If you want to move to a different window and put the graphics cursor in it, you must use both gUSE and CURSOR.

Since the default window always has an ID of 1, "CURSOR 1" will, as mentioned earlier, put the graphics cursor in it.

CURSOR OFF turns off the cursor, wherever it is.

Information about your windows

You don't have to keep a complete copy of all the information pertaining to each window you use. These functions return information about the current window:

Other window keywords

You can use this command on the default window, if you wish, but you must also use the SCREEN command to ensure that the text window the area for PRINT commands to use is wholly contained within the default window. See `The text and graphics windows', later in this chapter.

Copying grey between windows

The commands gCOPY and gPATT can use two windows and therefore special rules are needed for the cases when one window has a grey plane and the other does not.

With "gGREY 0" in the destination window, only the black plane of the source is copied.

With "gGREY 1" in the destination window, only the grey plane of the source is copied, unless the source has only one plane in which case that plane is used as the source.

With "gGREY 2" in the destination window, if the source has both planes, they are copied to the appropriate planes in the destination window (black to black, grey to grey); if the source has only one plane, it is copied to both planes of the destination.

Advanced graphics

This section should provide a taste of some of the more exotic things you can do with OPL graphics.


A bitmap is an area in memory which acts just like an off-screen window, except that it does not have two planes so that gGREY cannot be used. You can create bitmaps with gCREATEBIT. They have the following uses:

OPL treats a bitmap as the equivalent of a window in most cases:

Together, windows and bitmaps are known as drawables places you can draw to.

Most graphics keywords can be used with bitmaps in the same way as with windows, but remember that a bitmap corresponds to only one plane in a window. Once you have drawn to it, you might copy it to the appropriate plane of a window.

The keywords that can be used with bitmaps include: gUSE, gBORDER, gCLOSE, gCLS, gCOPY, gGMODE, gFONT, gIDENTITY, gPATT, gPEEKLINE, gSAVEBIT, gSCROLL, gTMODE, gWIDTH, gHEIGHT and gINFO. These keywords are described earlier in this chapter.

Speed improvements

The Series 3a screen is usually updated whenever you display anything on it. gUPDATE OFF switches off this feature. The screen will be updated as few times as possible, although you can force an update by using the gUPDATE command on its own. (An update is also forced by GET, KEY and by all graphics keywords which return a value, other than gX, gY, gWIDTH and gHEIGHT).

This can result in a considerable speed improvement in some cases. You might, for example, use "gUPDATE OFF", then a sequence of graphics commands, followed by "gUPDATE". You should certainly use gUPDATE OFF if you are about to write exclusively to bitmaps.

gUPDATE ON returns to normal screen updating.

As mentioned previously, a window with both black and grey planes takes longer to move or scroll than a window with only a black plane. So avoid creating windows with unnecessary grey planes.

Also, remember that scrolling and moving windows require every pixel in a window to be redrawn.

The gPOLY command draws a sequence of lines, as if by gLINEBY and gMOVE commands. If you have to draw a lot of lines (or dots, with "gLINEBY 0,0"), gPOLY can greatly reduce the time taken to do so.

Displaying a running clock

gCLOCK displays or removes a running clock showing the system time. The clock can be digital or conventional, and can use many different formats.

User-defined fonts and cursors

If you have a user-defined font you can load it into memory with gLOADFONT. This returns an ID for the font; use this with gFONT to make the font current. The gUNLOADFONT command removes a user-defined font from memory when you have finished using it.

You can use four extra arguments with the CURSOR command. Three of these specify the ascent, width and height of the cursor. The ascent is the number of pixels (-128 to 127) by which the top of the cursor should be above the baseline of the current font. The height and width arguments should both be between 0 and 255. For example, "CURSOR 1,12,4,14" sets a cursor 4 pixels wide by 14 high in the default window (ID=1), with the cursor top at 12 pixels above the font baseline.

If you do not use these arguments, the cursor is 2 pixels wide, and has the same height and ascent as the current font.

By default the cursor has square corners, is black and is flashing. Supply the fifth argument as 1 for a rounded cursor, 2 for non-flashing or 4 for grey. You can add these together eg use 5 for a grey, rounded cursor.

Note that the gINFO command returns information about the cursor and font.

The text and graphics windows

PRINT displays mono-spaced text in the text window. You can change the text window font (ie that used by PRINT) using the FONT keyword. You can use any of those listed earlier in the chapter in the description of gFONT; initially font 4 is used.

The text window is in fact part of the default graphics window. If you have other graphics windows in front of the default window, they may therefore hide any text you display with PRINT.

Initially the text window is very slightly smaller than the default graphics window which is full-screen size. They are not the same because the text window height and width always fits a whole number of characters of the current text window font. If you use the FONT command to change the font of the text window, this first sets the default graphics window to the maximum size that will fit in the screen (excluding any status window) and then resizes the text window to be as large as possible inside it.

You can also use the STYLE keyword to set the style for all characters subsequently written to the text window. This allows the mixing of different styles in the text window. You can only use those styles which do not change the size of the characters ie inverse video and underline. (Any other styles will be ignored.) Use the same values as listed for gSTYLE, earlier in the chapter.

To find out exactly where the text window is positioned, use "SCREENINFO info%()". This sets "info%(1)/info%(2)" to the number of pixels from the left/top of the default window to the left/top of the text window. (These are called the margins.) "info%(7)" and "info%(8)" are the text window's character width and height respectively.

Note : The margins are fully determined by the font being used and therefore change from their initial value only when FONT is used. You cannot choose your own margins. gSETWIN and SCREEN do not change the margins, so you can use FONT to select a font (also clearing the screen), followed by SCREENINFO to find out the size of the margins with that font, and finally gSETWIN and SCREEN to change the sizes and positions of the default window and text window taking the margins into account (see example below). The margins will be the same after calling gSETWIN and SCREEN as they were after FONT.
It is not generally recommended to use both the text and graphics windows. Graphics commands provide much finer control over the screen display than is possible in the text window, so it is not easy to mix the two.

If you do need to use the text window, for example to use keywords like EDIT, it's easy to use SCREEN to place it out of the way of your graphics windows. You can, however, use it on top of a graphics window for example, you might want to use EDIT to simulate an edit box in the graphics window. Use gSETWIN to change the default window to about the size and position of the desired edit box. The text window moves with it you must then make it the same size, or slightly smaller, with the SCREEN command. Use 1,1 as the last two arguments to SCREEN, to keep its top left corner fixed. "gORDER 1,1" will then bring the default window to the front, and with it the text window. EDIT can then be used.

Here is an example program which uses this technique moving an `edit box', hiding it while you edit, then finally letting you move it around.

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