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Active Forums => Programs => Topic started by: SMcNeill on September 09, 2018, 02:05:02 pm

Title: PNG chunk information
Post by: SMcNeill on September 09, 2018, 02:05:02 pm
Since TerryRichie was asking how to get the background color from a PNG file, I thought I'd go ahead and take a few moments to write up a program to do that for us:

Code: QB64: [Select]
  1. DIM DataLength AS _UNSIGNED LONG
  2. DIM code AS STRING * 4
  5. f$ = "e1.png"
  6.  
  9.     GET #1, , DataLength
  10.     DataLength = ConvertUL(DataLength)
  11.     PRINT DataLength,
  12.     GET #1, , code
  13.     PRINT code
  14.     IF code = "bKGD" THEN 'we found the background color
  15.         junk$ = " "
  16.         GET #1, , red
  17.         GET #1, , junk$ 'PNG stores the data as 2 bytes, so we need to read an extra character to move file pointer
  18.         GET #1, , blue
  19.         GET #1, , junk$ 'same as before
  20.         GET #1, , green
  21.         GET #1, , junk$ 'same as before
  22.         PRINT "BACKGROUND: "; red, blue, green
  23.         junk$ = "    " 'the CRC check, which we don't really care about, but we need to advance the file pointer to the next chunk
  24.         GET #1, , junk$
  25.     ELSE
  26.         junk$ = SPACE$(DataLength + 4)
  27.         GET #1, , junk$
  28.     END IF
  29.     SLEEP
  30. LOOP UNTIL code$ = "IEND"
  31.  
  32. FUNCTION ConvertUL~& (x AS _UNSIGNED LONG)
  33.     ConvertUL = x \ 2 ^ 24 OR x * 2 ^ 24 OR (x AND &HFF0000) \ 2 ^ 8 OR (x AND &HFF00~&) * 2 ^ 8
  35.  

Test image is below, which has a yellow background set as found here: http://www.schaik.com/pngsuite/pngsuite_bck_png.html
Title: Re: PNG chunk information
Post by: TerryRitchie on September 09, 2018, 02:25:19 pm
My goodness! That's awesome. Thank you for taking the time to create this.

I've been reading about your personal struggles too. I'm thinking about you and hoping for the best Steve.
Title: Re: PNG chunk information
Post by: SMcNeill on September 09, 2018, 02:30:22 pm
PNGs are actually quite easy to sort out; everything is stored in set chunks of data.

4 bytes for data length
4 bytes for data type
data of length given above
4 bytes for CRC check

Last chunk is always "IEND", so just DO.... LOOP and read each chunk as you're ready to deal with them.  ;)
Title: Re: PNG chunk information
Post by: TerryRitchie on September 09, 2018, 05:30:01 pm
I'm not sure what is going on here. The image you supplied worked fine with the code, however whenever I try to bring it into my paint program the program freaks with it. It shows no image and complains there is "insufficient memory" when I try to do anything with it. Windows picture viewer shows it no problem though.

I then tried to use one of the PNGs I created with a clear background and the code fails to find it. I then downloaded a random picture from the Internet (dog.png below) and the code fails to find the background as well.

Is there something missing?
Title: Re: PNG chunk information
Post by: Petr on September 09, 2018, 05:40:52 pm
TerryRitchie: Your file contains none bKGD flag, Steve´s file it contains.

maybe try read tRNS chunk. Its also about transparency, if is one color transparent.
Title: Re: PNG chunk information
Post by: SMcNeill on September 09, 2018, 06:00:43 pm
Quote from: Petr on September 09, 2018, 05:40:52 pm
TerryRitchie: Your file contains none bKGD flag, Steve´s file it contains.

Petr's correct; your file has no background color saved.  "bKGD" is an optional chunk and you're not guaranteed that any file will have it.  If it's there, it's easy to detect and extract.  Hopefully Sprite Sheets you're using have it, otherwise you're going to have to manually find it.
Title: Re: PNG chunk information
Post by: TerryRitchie on September 09, 2018, 06:11:30 pm
So there are different versions/types of PNG files out there I take it, sort of like the Gif87/Gif89a.

I don't want a user of the library having to deal with PNG versions to make sure he/she has the correct type. I'll use the crappy detection routine I have for now. I'll need to investigate that document about PNG files for more info on the subject.

Thanks for your help though Steve. I definitely want to eventually use a solution such as yours.
Title: Re: PNG chunk information
Post by: SMcNeill on September 09, 2018, 06:33:02 pm
PNG's selling point is flexibility.  All they *have* to contain is the Header, Pixel Data, End of Data tag; anything else is considered optional.  Some hold text, like creation date/time, image creator, version, ect.  Others contain dithering, palettes, compression, background data, ect... 

What'd I do for a library like yours is to look for the background code first, and if available, go with it.  If it's not found, then I'd manually look for the background color (or just ask the user to supply it).    If it's there, you *know* it's right; if not, you have to make a "best guess" at the color.
Title: Re: PNG chunk information
Post by: Petr on September 10, 2018, 09:51:24 am
Hello. I tried to write a program for a more detailed PNG statement. It seems that 15 color pallets from 256 in the picture with the dog has some transparency, pallet number 1 is fully transparent, it should be the color _RGBA32 (71, 112, 76, 0) - palette 1 and RGBA32 (74, 65 , 50, 3) - palette 8

Code: QB64: [Select]
  1. 'PNG reader  / read some (not all) information from PNG file. Source informations for this program are from http://www.fileformat.info/format/png/corion.htm
  2.  
  3. file$ = "dog.png"
  4.  
  5.  
  6. TYPE PNG_header
  7.     id AS _UNSIGNED _BYTE ' 89h
  8.     PNG AS STRING * 3 '     PNG
  9.     id1 AS _UNSIGNED _BYTE '13
  10.     id2 AS _UNSIGNED _BYTE '10
  11.     id3 AS _UNSIGNED _BYTE '26
  12.     id4 AS _UNSIGNED _BYTE '10
  14.  
  15. TYPE Block
  16.     Data_Size AS _UNSIGNED LONG
  17.     Chunk AS STRING * 4
  19.  
  20. TYPE IHDR
  22.     height AS _UNSIGNED LONG
  23.     bit_depth AS _UNSIGNED _BYTE
  24.     color_type AS _UNSIGNED _BYTE
  25.     compression_type AS _UNSIGNED _BYTE
  26.     filter_type AS _UNSIGNED _BYTE
  27.     interlacce_type AS _UNSIGNED _BYTE
  29.  
  30.  
  31.  
  32.  
  33.  
  34. DIM Header AS PNG_header, Block AS Block, IHDR AS IHDR
  35.  
  36. OPEN file$ FOR BINARY AS #1
  37. GET #1, , Header
  38.  
  39. PRINT "PNG id (muss be 89H): "; HEX$(Header.id) + LTRIM$("H")
  40. PRINT "PNG id (muss be PNG): "; Header.PNG$
  41. PRINT "Header indentity (muss be 13, 10, 26, 10): "; Header.id1; Header.id2; Header.id3; Header.id4
  42.  
  43.  
  44.  
  45.  
  47.  
  48.     SEEK #1, SEEK(1) - 7
  49.     GET #1, , Block
  50.  
  51.     SELECT CASE Block.Chunk
  52.         CASE "IHDR"
  53.             GET #1, , IHDR
  54.             start = 1
  55.             size = ConvertUL~&(Block.Data_Size)
  56.             PRINT STRING$(45, "-")
  57.             PRINT "Chunk: "; Block.Chunk
  58.             PRINT "Data area for this chunk: "; size ' OK
  59.             PRINT "Image width: "; ConvertUL~&(IHDR.width)
  60.             PRINT "Image height: "; ConvertUL~&(IHDR.height)
  61.             PRINT "Image bit depth: "; IHDR.bit_depth
  62.             PRINT "Image color type: "; IHDR.color_type
  63.             PRINT "Image compression type: "; IHDR.compression_type
  64.             PRINT "Image filter type: "; IHDR.filter_type
  65.             PRINT "Image interlacce type: "; IHDR.interlacce_type
  66.             PRINT STRING$(45, "-")
  67.             kontinue
  68.  
  69.         CASE "PLTE"
  70.             REDIM RED AS _UNSIGNED _BYTE, GREEN AS _UNSIGNED _BYTE, BLUE AS _UNSIGNED _BYTE
  71.  
  72.             size = ConvertUL~&(Block.Data_Size)
  73.             PRINT STRING$(45, "-")
  74.             PRINT "Chunk: "; Block.Chunk
  75.             PRINT "Data area for this chunk: "; size ' OK
  76.             PRINT "Color palette: "
  77.  
  78.             IF size MOD 3 <> 0 THEN BEEP ' error, muss be dividible by 3
  79.             FOR R = 1 TO size / 3
  80.                 GET #1, , RED: GET #1, , GREEN: GET #1, , BLUE
  81.                 PRINT "Palette color: "; R; "RED: "; RED; "GREEN: "; GREEN; "BLUE:"; BLUE
  82.                 IF R MOD 15 = 0 THEN kontinue
  83.             NEXT
  84.             PRINT STRING$(45, "-")
  85.             kontinue
  86.         CASE "IDAT"
  87.             PRINT "IDAT: Image data block found..."
  88.             PRINT STRING$(45, "-")
  89.             PRINT "Chunk: "; Block.Chunk
  90.             size = ConvertUL~&(Block.Data_Size)
  91.             PRINT "Data area for this chunk: "; size ' OK
  92.             PRINT STRING$(45, "-")
  93.  
  94.             kontinue
  95.         CASE "IEND"
  96.             PRINT "IEND: End of image found..."
  97.             kontinue
  98.  
  99.  
  100.         CASE "bKGD" 'this next chunks are optional, muss not be contained in file
  101.             PRINT STRING$(45, "-")
  102.             PRINT "Chunk: "; Block.Chunk
  103.             PRINT "Data area for this chunk: "; size ' OK
  104.             SELECT CASE IHDR.color_type
  105.                 CASE 3
  106.                     REDIM palete AS _UNSIGNED _BYTE 'return PALETTE color value used as background, this palette is returned in PLTE block
  107.                     GET #1, , palete
  108.                     PRINT "Image type 3, palette background: "; palete
  109.                 CASE 4, 0
  110.                     REDIM palete2 AS _UNSIGNED INTEGER
  111.                     GET #1, , palete2
  112.                     PRINT "Image type 4 or 0, palette background: "; palete2
  113.                 CASE 2, 6
  114.                     REDIM RedPal AS STRING * 2, GreenPal AS STRING * 2, BluePal AS STRING * 2
  115.                     GET #1, , RedPal
  116.                     GET #1, , GreenPal
  117.                     GET #1, , BluePal
  118.                     PRINT "Image type 2 or 6, backgroud palette: Red 0 to"; re(RedPal); " Green 0 to "; re(GreenPal); " Blue 0 to "; re(BluePal)
  119.             END SELECT
  120.             kontinue
  121.  
  122.         CASE "cHRM" 'chromizace... co to je....
  123.             PRINT STRING$(45, "-")
  124.             PRINT "Chunk: "; Block.Chunk
  125.             PRINT "Data area for this chunk: "; size ' OK
  126.             PRINT "chromatization chunk.... not writed"
  127.  
  128.         CASE "tRNS"
  129.             PRINT STRING$(45, "-")
  130.             REDIM ALFA AS _UNSIGNED _BYTE
  131.  
  132.             size = ConvertUL~&(Block.Data_Size)
  133.  
  134.             PRINT "Chunk: "; Block.Chunk
  135.             PRINT "Data area for this chunk: "; size ' OK
  136.             SELECT CASE IHDR.color_type
  137.                 CASE 3 '                                                                                your DOG.PNG is this case
  138.  
  139.                     FOR R = 1 TO size
  140.                         GET #1, , ALFA
  141.                         PRINT "ALFA setting for colors from palette: Palette color: "; R; "ALFA: "; ALFA
  142.                         IF R MOD 15 = 0 THEN kontinue
  143.                     NEXT
  144.                     PRINT STRING$(45, "-")
  145.                     '255 is full visible, 0 is full transparent, palette values not listed here are all with alpha 255.
  146.  
  147.                 CASE 0
  148.                     REDIM ALFA0 AS STRING * 2
  149.                     GET #1, , ALFA0
  150.                     PRINT "Image type 0, alfa: "; re(ALFA0)
  151.                 CASE 2
  152.                     REDIM alfaR AS STRING * 2, alfaG AS STRING * 2, alfaB AS STRING * 2
  153.                     GET #1, , alfaR
  154.                     GET #1, , alfaG
  155.                     GET #1, , alfaB
  156.                     PRINT "image type 2 alfa for RED is "; re(alfaR); "for GREEN is "; re(alfaG); "for BLUE is "; re(alfaB)
  157.             END SELECT
  158.  
  159.         CASE "tIME"
  160.             TYPE intdat
  161.                 year AS STRING * 2
  162.                 month AS _UNSIGNED _BYTE
  163.                 day AS _UNSIGNED _BYTE
  164.                 hour AS _UNSIGNED _BYTE
  165.                 min AS _UNSIGNED _BYTE
  166.                 sec AS _UNSIGNED _BYTE
  167.             END TYPE
  168.             REDIM intdat AS intdat
  169.             GET #1, , intdat
  170.  
  171.             PRINT STRING$(45, "-")
  172.             size = ConvertUL~&(Block.Data_Size)
  173.  
  174.             PRINT "Chunk: "; Block.Chunk
  175.             PRINT "Data area for this chunk: "; size ' OK
  176.             PRINT "Image last edit: "; re(intdat.year); "-"; intdat.month; "-"; intdat.month; "-"; intdat.day; "-"; intdat.hour; "-"; intdat.min; "-"; intdat.min; "-"; intdat.sec
  177.             PRINT STRING$(45, "-")
  178.             kontinue
  179.     END SELECT
  181.  
  182. FUNCTION ConvertUL~& (x AS _UNSIGNED LONG)
  183.     ConvertUL = x \ 2 ^ 24 OR x * 2 ^ 24 OR (x AND &HFF0000) \ 2 ^ 8 OR (x AND &HFF00~&) * 2 ^ 8
  185.  
  186. SUB kontinue
  187.     PRINT "Press any key...": SLEEP
  189.  
  190.  
  191. FUNCTION re (num AS STRING)
  192.     n$ = ""
  193.     FOR f = LEN(num) TO 1 STEP -1
  194.         n$ = n$ + MID$(num, f, 1)
  195.     NEXT f
  196.     IF LEN(num) = 4 THEN re = CVL(n$)
  197.     IF LEN(num) = 2 THEN re = CVI(n$)
  199.  
  200.  
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