Slight refactoring on drawImage_blit

As for an example from a simple spritesheet of ground tiles

following code
dim picslot[5000]
dim picsx[5000]
dim picsy[5000]
dim picsw[5000]
dim picsh[5000]
dim pice[5000]
sub addPic(picnum, slot, src_x,src_y,src_w, src_h)
if pice[picnum] <> 0 then
fprint("Already a picture at picnum ")
print(picnum)
else
picslot[picnum] = slot
picsx[picnum] = src_x
picsy[picnum] = src_y
picsw[picnum] = src_w
picsh[picnum] = src_h
pice[picnum] = 1
end if
end sub
sub modifiySourceXY(picnum, src_x, src_y)
picsx[picnum] = src_x
picsy[picnum] = src_y
end sub
sub drawblit(picnum, x,y)
if pice[picnum] =0 then
fprint("No picture at ")
print(picnum)
else
DrawImage_Blit ( picslot[picnum], x, y, picsx[picnum], picsy[picnum], picsw[picnum], picsh[picnum])
end if
end sub

WindowOpen ( 0, "TEST WINDOW", 100, 100, 640, 480, 0 )
CanvasOpen ( 0, 640, 480, 0, 0, 640, 480, 0 )
SetColor( RGB ( 255, 255, 255 ) )
loadImage(0,"ground2.png")
addPic(0,0,1,1,15,15)
addPic(1,0,1,17,15,15)
addPic(2,0,17,1,15,15)
addPic(3,0,17,17,15,15)
addPic(4,0,1,33,15,15)

while true
drawblit(0,0,0)
drawblit(1,0,15)
drawblit(2,0,30)
drawblit(3,0,45)
drawblit(4,0,60)
Update ( )
wend
returns a quick lineup of the five grass tiles:

still it does end up as a lot of code to fill a single window this way, therefore next part is to write a subroutine to fill a lot of space in a easy to write manner.

Okay as I implied in my last post, a field has a lot of entries which can obfusciate the readability of the source code, on the other hand tbird reminded me that I had another basic techique available for spritesheets being animations. Thanks for your nice and motivating posts tbird and jhonno by the way.

This time I will separate code from data. For a quick definition code is that what does things, and data is that with which things are done. The definitions are entirely correct but for this post they are enough correct to be understandable.
RCBasic already separates code from data when it comes to images. No one on this forum would think about writing an encoded image of the ufo or car he/she uses in code. Besides png encoding is weird to write with hand.

In RCBasic an image is loaded from a image file and then used. Similar is the reasoning to separate the displaying one or several fields from the place where the dat about the field is stored. I have added an attachment of a complete layout window to this post it will be used later.

For the example later a increased spritesheet based on the one in the second posting is used. As you can measure in a graphic program each tile is 15x15 px large, filling a window required some 20 fields or 2000 datas.

For the start I have to decided in which format I store the data, here I decide to store it in text format and a simple *.txt file.

To load a file in RCBasic the built in function is:

FileOpen ( stream, fileName$, mode )
For the subroutine I build I decide to provide the filename in the call, reading a text and use stream one:

and not forgetting to close the file after using it the first and last commands of the subroutine are therefore

While the file is open a loop will run until the end of the file:

sub buildFieldfromFile(filename$)
fileopen(1,filename,TEXT_INPUT)
truth = true
while(truth)
if not EOF(1) then

'... here the actual loading of data from the file happens

else
truth = false
end if
wend
fileclose(1)
end sub
Now I am at the point that I need to decide how to read in my data, do I highlight it someway so that only specific data is used or that during the reading process that subroutine can decide what data belongs to where. I decide for a field data I highlight it by writing an "F" at the first position.

Second do I write the data one after another or do I put in a delimiter. For the start I decide to put in a delimiter being a ',' now this is nice for writing the file and clearly readable. However in this case I need a function to calculate the delimiter out and think about how to approach each value. Afterall I decide I read from the position of the single data to several characters long and then find a string that is before the first comma:

function findBeforeComma$(substr$)
'... the string is evalutated
return substr
end function
this is the first form of the function that takes in a string with comma and returns a string before a comma.

As several commas might be in a string I decide to go through a while loop until no comma is in the string anynore and each iteration of the loop the length of the string is shortened:

Now I can return to the first subroutine and work on it some more so far I have:

sub buildFieldfromFile(filename$)
fileopen(1,filename,TEXT_INPUT)
truth = true
while(truth)
if not EOF(1) then

'... here the actual loading of data from the file happens

else
truth = false
end if
wend
fileclose(1)
end sub
a single field is an array of 102 elements, an additional element in the fielde array and for the highlight purpose a "F" in the beginning. That is 104 entries separated by commas in each line.

The subroutine reads a line checks wether there is a "F" in the beginning of the line and if the line starts with "F"  it procedes with reading the number of the field described in the line, then follow the x and y position of a field and last the 100 elements that name the pic which is displayed at a location within the field. If the line starts with something else then "F" the values are discarded.

First the position from reading the fieldnumber is set, then two string variables named fieldpos to make it understandable that the number stored is a position value and fieldval for other values are created:

Now its time to read in the rest of the line and put it into specific arrays. After reading in a entry in the line as a string through the in built Mid$ function and shortening it with the

findBeforeComma$ function written above the position counter has to be increased by the length of the outcome string +1.

Now we arrived at the point where we can put together the source code with the one from my last positng:

If everything is without typos then following window should appear:

As you will have noticed when compiling and executig the final source code of the last posting, Tzir does walk around within then window.

However if the blue fields are water then Tzir also walks over water and that though cool is not intended. In so far it is time to add a simple collision detection. To do this a new array is itroduced for each tile which holds a block value, later this block value is scanned for at a point on each side of Tzir's sprite and if it has a value that the program interpretates as blocked field then no movement is done by Tzir.

So far the plan. On to modify the sourcecode.

The first thing is to add the array to hold the block values for a pic, just below the other subpicture related arrays therefore in line 7 following code is added:

the addPic subroutine also gets an additional line an a additional call value:

The other subroutines stay the same as they are, what needs to be done also is to add a blockvalue on each line a picture is added. For the moment 0 means no block and 1 means a block

Now a function is needed which queries the block value of a singel field element on the screen, this depends on Tzir's position and returns the value of block of the specific fieldelement, by quering what number the fieldelement has:

To achieve this loop through each field and each fieldelement,  it is reasonable to not loop through fieldelements which are in a field far away from the point entered into the function. Basically only fieldelements of the field in which the point entered into the function sits are looked at, therefore the coordinates of the field are calculated after checking if it exists:

function fieldvalue(px,py)
dim fieldelem
dim fieldnum
for i = 0 to 99
if fielde[i] <> 0 then
fieldx = field[i,0]
fieldxs = field[i,0]+150
fieldy = field[i,1]
fieldys = field[i,1] + 150
if px > fieldx and px < fieldxs and py > fieldy and py < fieldys then
'... more here
end if
end if
next
return picb[field[fieldnum,fieldelem]]
end function
Additional with the position values calculated before looking at field elements and some math a bit of magic can be done requireing no further loop is necessary, as the coodinate given minus the upper left corner of the field provides us with the coordinate inside the fieldelement we want to know about, we need to divide this through the length of a fieldelement. For the tilesheet used above 15 and turn this into an integer value by a modulo 10 division. Then we have to return it to its number in the array that sets a field by inverting the calculation used for the place it is painted on:

dx = px - fieldx
dy = py - fieldy
xfield = (dx / 15) mod 10
yfield = (dy / 15) mod 10
fieldelem = yfield*10 + xfield +2
Last we need to store the value of the loop over all fields and voila we have the block value:

The next step is to query four points each at one side of Tzir, a block array is introduced for this above the fieldvalue function:

And above the lines setting the movement done by Tzir they are first cleared to 0 and then filled by calling the function fieldvalue with the positions of the query points being in the middle of a side and 7 away from the border. Tzir's sprite is 30 px on each side therefore the values to be entered into the fieldvalue function are:

for i = 0 to 3
block[i] =0
next
block[0] = fieldvalue(fx +37,fy + 15)
block[1] = fieldvalue(fx+ 15,fy-7)
block[2] = fieldvalue(fx -7,fy + 15)
block[3] = fieldvalue(fx +15,fy+37)
block[0] sits to the right side of the sprite

block[1] on top of the sprite

block[2] to the left side of the sprite

block[3] below the sprite.

Finally the conditions for moving are modified:

Now all is ready to be tested:

I hope you enjoyed this thread about using tilesheets in games with RCBasic. If you have suggestions for improvements in my writing style let me know. This was my first coding tutorial.