Pixel Plot, How to Change Cartesian Coordinates to Pixels
02-06-2018, 02:13 PM
Post: #1
 Eddie W. Shore Senior Member Posts: 927 Joined: Dec 2013
Pixel Plot, How to Change Cartesian Coordinates to Pixels
Blog Post: https://edspi31415.blogspot.com/2018/02/...hange.html

Changing Cartesian Coordinates to Pixels

When running programs on the HP Prime, the screen has a pixel coordinate system of 320 x 220 (to allow room for soft menu keys).

There are two ways to calculate to translate Cartesian coordinates to pixel coordinates on the HP Prime. The easy way is to use the C→PX command.

However, if you are working in custom made apps, C→PX may not work because the command requires that app has the Plot variables Xmin, Xmax, Ymin, and Ymax. This will require a conversion formula.

Given a desired xmin, xmax, ymin, and ymax, the following formulas I use are:

Scaling:
xs = (xmax – xmin)/320
ys = (ymax – ymin)/-220 = (ymin – ymax)/220

Conversion to pixels of coordinates (x,y):
xp = (x – xmin)/xs
yp = (y – ymax)/ys
Code:
 HP Prime Program: PIXELPLOT EXPORT PIXELPLOT() BEGIN // EWS 2014-02-04 LOCAL xm,ym,xp,yp,x,y; LOCAL xn,yn,xs,ys; LOCAL ya,ch,flag; LOCAL fx; // set color scheme LOCAL col1,col2; col1:={#BF00FFh,#7DF9FFh, #00FF00h,#D4AF37h,#FF0000h}; col2:={#4B0082h,#000080h, #228B22h,#C3B091h,#800000h}; // Radians HAngle:=0; INPUT({{fx,[2]}, xm,xn,ym,yn, {ch, {"Purple","Blue","Green", "Gold","Red"}}}, "Pixel Plot Official", {"f(x) string:", "x-min: ","x-max: ", "y-min: ","y-max: ", "Color: "}); RECT_P(0); // calulate the scale xs:=(xn-xm)/320; ys:=(yn-ym)/−220; // drawing // color choice LOCAL c1,c2; c1:=col1[ch]; c2:=col2[ch]; // axis information LOCAL st1,st2; st1:="x:["+xm+","+xn+"]"; st2:="y:["+ym+","+yn+"]"; TEXTOUT_P(st1,0,0,2,#C0C0C0h); TEXTOUT_P(st2,0,20,2,#C0C0C0h); // function FOR x FROM xm TO xn STEP xs DO // skip 0 for now IF x==0 THEN CONTINUE; END; // function y:=EXPR(fx); // point→pixel, plot // only if y is real IF TYPE(y)==0 THEN xp:=(x-xm)/xs; yp:=(y-yn)/ys; PIXON_P(xp,yp,c1); END; END; // freeze screen FREEZE; END;

Notes:

1. You should not have to include the string characters for f(x) as they are included in the input.

2. Use the lowercase x.

3. The program errors if a plot reaches point where f(x) is not defined. I put in a condition when f(x) is complex (for example, the square root of negative number) for the plot to skip that pixel. However, I have not put error skipping conditions when it comes to ln(x) or 1/p(x) where p(x) is a polynomial. However, the pixel at x=0 is skipped to hopefully alieve some problems. Be sure your range is appropriate.

4. The program uses Radians angle mode (HAngle = 0).

5. I chose to have a black background with five color options (purple, electric blue, green, gold, and red) just for fun.
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