PNG Graphic Dimensions

- JanetTerra Apr 16, 2006

It is possible to load and display graphics in formats other than bitmap in Liberty BASIC programs. This does require the use of the atl DLL or a third party DLL such as lbbrowse3.dll. Finding the dimensions of a Portable Network Graphics (PNG) file is very similar to that of getting the dimensions of a bitmap file. The file must be opened and the first three Lines Inputted. The 7th, 8th, 11th and 12th Bytes of the third Line Input are then parsed for the pertinent information.

' Choose a PNG File
    Filedialog "Finding the PNG Dimensions", "*.*", pngFile$
    If pngFile$ = "" Then End
 
' Obtain pngInfo in 3 Line Inputs
    Dim pngInfo$(3)
    Open pngFile$ for Input as #png
        For i = 1 to 3
            Line Input #png, pngInfo$
            pngInfo$(i) = pngInfo$
        Next i
    Close #png
    Print
 
' 1st Byte is a High Bit to reduce possibility text file is mistaken for png file
    hiBitDec = Asc(Mid$(pngInfo$(1), 1, 1))
    hiBitHex$ = DecHex$(hiBitDec)
    Print hiBitHex$ 'hiBitHex$ = "89"
 
' Next 3 Bytes are the ASCII values of P N G for easy identification of format
    For i = 2 to 4
        Print Mid$(pngInfo$(1), i, 1);" "; ' Prints P N G
    Next i
    Print: Print
 
' Next are Bytes coded to detect the line ending conversion
'    OA = Line Feed
'    OD OA = Carriage Return plus Line Feed
'    1A = Stop Display of File
    Print pngInfo$(2)
    Print
 
' The rest of the information follows in the third Line Input
' Each block of information is referred to as a chunk
' The first chunk in this block is always the header identified by IHDR
' Only a few of the Bytes need to be parsed to find the width and height
    For i = 1 to 4
        Print Mid$(pngInfo$(3), i, 1);" "; ' Prints I H D R
    Next i
    Print: Print
' The PNG Width is stored in the 7th and 8th Bytes of this third Line Input
    pngWidth = Asc(Mid$(pngInfo$(3), 8, 1)) + Asc(Mid$(pngInfo$(3), 7, 1)) * 256
    Print "pngWidth = ";pngWidth
    Print
' The PNG Height is stored in the 11th and 12th Bytes of this third Line Input
    pngHeight = Asc(Mid$(pngInfo$(3), 12, 1)) + Asc(Mid$(pngInfo$(3), 11, 1)) * 256
    Print "pngHeight = ";pngHeight
    Print
 
    End

To use this within your Liberty BASIC program, you only need the these lines

Open pngFile$ for Input as #png
    Line Input #png, pngSignature$
    Line Input #png, pngLineEnd$
    Line Input #png, pngInfo$
Close #png
pngWidth = Asc(Mid$(pngInfo$, 8, 1)) + Asc(Mid$(pngInfo$, 7, 1)) * 256
pngHeight = Asc(Mid$(pngInfo$, 12, 1)) + Asc(Mid$(pngInfo$, 11, 1)) * 256