hello,

I don't have arduino or BT but without these, the code below
seems to work.

Remarks:

• it would be easier for us if you use English in the Code (var names, func names, comments)
• use ctrl-t to format your code (indent), better to read
• insetad of   text("X =", 15, 280);     text(x, 55, 280);   you can write:   text("X =" + x, 15, 280);

I also commented out these lines:

1.    BTser.write(PMbyte);
2.    delay(20);
3.    BTser.write(LMbyte);
4.    delay(20);

It's never wise to use delay. Maybe you can work without it. Or if you need a pause, work with millis (see here).

Greetings, Chrisir   

1. PFont pismo;
2. // import knihoven
3. // import processing.serial.*;
4. // deklarace promennych - vypocty
5. float x;
6. float otaceni;
7. float y;
8. float plyn;
9. // deklarace promennych - motory
10. float LM;
11. float PM;
12. int LMbyte;
13. int PMbyte;
14. // deklarace promennych - posuvniky
15. int Lposuvnik;
16. int Pposuvnik;
17. //Serial BTser;
18. ////////////////////////////////////////////////////
19. void setup()
20. {
21.   size(400, 440);
22.   // background(102);
23.   // nacteni pisma
24.   pismo = createFont("Font.vlw", 14);
25.   textFont(pismo, 20);
26.   // background(102);
27.   // inicializace serioveho portu
28.   //BTser = new Serial(this, "COM19", 9600);
29. }
30. ////////////////////////////////////////////////////
31. void souradnice_vypocet()
32. {
33.   background(102);
34.   //prepocet souradnic
35.   x = mouseX-120;
36.   y = -mouseY+145;
37.   plyn = y/100;
38.   otaceni = x/100;
39. }
40. void souradnice_zobrazeni()
41. {
42.   background(102);
43.   //Vypis souradnic
44.   text("X =", 15, 280);
45.   text(x, 55, 280);
46.   text("Y =", 15, 300);
47.   text(y, 65, 300);
48.   //Ctverec joysticku
49.   fill(255, 255, 255);
50.   text("Joystick", 80, 25);
51.   rectMode(CORNERS);
52.   rect(15, 40, 225, 250);
53.   line(120, 40, 120, 250);
54.   line(15, 145, 225, 145);
55.   //Kurzor
56.   ellipseMode(CENTER);
57.   fill(255, 0, 0);
58.   ellipse(x+120, -y+145, 10, 10);
59. }
60. ////////////////////////////////////////////////////
61. void kvadranty()
62. {
63.   // vypis kvadrantu + vypocet vykonu motoru
64.   fill(255, 255, 255);
65.   if (x > 0 && y > 0) // 1.kvadrant
66.   {
67.     text("1.kvadrant", 15, 325);
68.     LM = plyn;
69.     PM = plyn*(1-otaceni);
70.   }
71.   if (x < 0 && y > 0) // 2.kvadrant
72.   {
73.     text("2.kvadrant", 15, 325);
74.     PM = plyn;
75.     LM = plyn*(1+otaceni);
76.   }
77.   if (x < 0 && y < 0) // 3.kvadrant
78.   {
79.     text("3.kvadrant", 15, 325);
80.     PM = plyn;
81.     LM = plyn*(1+otaceni);
82.   }
83.   if (x > 0 && y < 0) // 4.kvadrant
84.   {
85.     text("4.kvadrant", 15, 325);
86.     LM = plyn;
87.     PM = plyn*(1-otaceni);
88.   }
89.   if (x == 0)
90.   {
91.     LM = plyn;
92.     PM = plyn;
93.   }
94. }
95. ////////////////////////////////////////////////////
96. void indikace()
97. {
98.   //Vypis plyn a otaceni
99.   text("Plyn", 15, 350);
100.   text(plyn, 105, 350);
101.   text("Otaceni", 15, 370);
102.   text(otaceni, 105, 370);
103.   //Vypis motoru
104.   text("L motor", 15, 395);
105.   text(LM, 105, 395);
106.   text("P motor", 15, 415);
107.   text(PM, 105, 415);
108.   // indikatory motoru
109.   // levy motor
110.   fill(255, 255, 255);
111.   text("LM", 265, 25);
112.   rectMode(CORNERS);
113.   rect(255, 40, 305, 250);
114.   fill(0);
115.   line(255, 145, 305, 145);
116.   //posuvnik levy motor
117.   Lposuvnik = 250 - (round(LM*100) + 105);
118.   fill(255, 0, 0);
119.   rectMode(CENTER);
120.   rect(280, Lposuvnik, 50, 10);
121.   //pravy motor
122.   fill(255, 255, 255);
123.   text("PM", 345, 25);
124.   rectMode(CORNERS);
125.   rect(335, 40, 385, 250);
126.   fill(0);
127.   line(335, 145, 385, 145);
128.   //posuvnik pravy motor
129.   Pposuvnik = 250 - (round(PM*100) + 105);
130.   fill(255, 0, 0);
131.   rectMode(CENTER);
132.   rect(360, Pposuvnik, 50, 10);
133. }
134. ////////////////////////////////////////////////////
135. void prepocet_ST()
136. {
137.   //prepocet na hodnotu 1-255
138.   fill(255, 255, 255);
139.   text("Hodnoty pro", 255, 280);
140.   text("Sabertooth:", 255, 300);
141.   // Levy motor
142.   if (LM < 0) {
143.     LMbyte = round((1+LM)*63);
144.   }   // hodnota 1 - 63
145.   if (LM == 0) {
146.     LMbyte = 64;
147.   }                  // hodnota 64
148.   if (LM > 0) {
149.     LMbyte = round(LM*63 + 64);
150.   }     // hodnota 65 - 127
151.   if (LMbyte == 0) {
152.     LMbyte = 1;
153.   } // hodnota 0 vypina oba motory
154.   text("L motor", 255, 325);
155.   text(LMbyte, 335, 325);
156.   // Pravy motor
157.   if (PM < 0) {
158.     PMbyte = round((1+PM)*63 + 128);
159.   }    // hodnota 128 - 191
160.   if (PM == 0) {
161.     PMbyte = 192;
162.   }                   // hodnota 192
163.   if (PM > 0) {
164.     PMbyte = round(PM*63 + 192);
165.   }  // hodnota 128-191
166.   text("P motor", 255, 345);
167.   text(PMbyte, 335, 345);
168. }
169. ////////////////////////////////////////////////////
170. void odeslani()
171. {
172.   //odeslani pres seriovy port
173.   /*
174.   BTser.write(PMbyte);
175.    delay(20);
176.    BTser.write(LMbyte);
177.    delay(20);
178.    */
179. }
180. void draw()
181. {
182.   if (mousePressed && mouseX >= 20 && mouseX <= 220 && mouseY >= 45 && mouseY <= 245)
183.   {
184.     souradnice_vypocet();
185.   }
186.   souradnice_zobrazeni();
187.   kvadranty();
188.   indikace();
189.   prepocet_ST();
190.   odeslani();
191. }

I wrote you my thoughts (delay etc.)

more I can not say.