PNG chunk information

[SMcNeill]

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
3. DIM red AS _UNSIGNED _BYTE, blue AS _UNSIGNED _BYTE, green AS _UNSIGNED _BYTE
4. _CONTROLCHR OFF
5. f$ = "e1.png"
6.  
7. OPEN f$ FOR BINARY AS #1
8. DO
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
34. END FUNCTION
35.  

Test image is below, which has a yellow background set as found here: http://www.schaik.com/pngsuite/pngsuite_bck_png.html

[TerryRitchie]

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.

[SMcNeill]

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.  ;)

[TerryRitchie]

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?

[Petr]

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.

[SMcNeill]

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.

[TerryRitchie]

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.

[SMcNeill]

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.

[Petr]

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
13. END TYPE
14.  
15. TYPE Block
16.     Data_Size AS _UNSIGNED LONG
17.     Chunk AS STRING * 4
18. END TYPE
19.  
20. TYPE IHDR
21.     width AS _UNSIGNED LONG
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
28. END TYPE
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.  
46. DO WHILE NOT EOF(1)
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
180. LOOP
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
184. END FUNCTION
185.  
186. SUB kontinue
187.     PRINT "Press any key...": SLEEP
188. END SUB
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$)
198. END FUNCTION
199.  
200.