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 D H 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 pngSignature$ 
 Line Input pngLineEnd$ 
 Line Input 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