[Gif] interpreting GIF files

✓ Http://blog.csdn.net/tongyue/archive/2007/07/30/1716572.aspx

 

How to compress GIF images and compress images

 

We know that bitmap files and Icon files record image information in pixels. Such records generate very large files called uncompressed files, it can be used on a single machine. it is undesirable to spread it online. How can we further compress images? GIF is a common compression technology and a popular image file format on the Internet.

 

GIF compression standard:

As a compression standard, GIF is widely supported by many software companies, including Microsoft. GIF is available in two versions: gif87a and gif89a, and is an early animation format.

How is gif compressed? It first converts an image into an 8-bit 256-color image (or 128-color, 64-color, 32-color, 16-color, 8-color, 4-color, or 2-color ), taking the 256 color as an example, an 8-bit binary information is required for a pixel, that is, one byte. Then, based on the pixel arrangement rules in the image, GIF creates a color-like arrangement table, which uses 9-bit integer values to represent the arrangement rules of pixels, achieve the goal of compression (its name is LZW CompressionAlgorithm). It sounds hard to understand. I will give an example later. Finally, when saving the data to the file, add a nine-digit integer (in the range of 0 ~ 511) convert to an 8-bit byte value and save it to the GIF file.

 

Color Sorting table:

Assume that there is a series of image pixel streams (stored in the byte value type Buf () array) for GIF compression, the first 20 of these data streams, that is, Buf (0 )~ The values of BUF (19) are:

8 8 8 8 99 99 8 8 8 8

8 8 8 99 99 8 99 8 99 8

The first value of the data stream is recorded in the Information Section of the file, and the actual computation starts from the second one:

OperationProgramCode:

(Bint (0) = 256)

SP = right $ ("00" & Buf (0), 3)

Icode = Buf (0)

For I = 1 to 19

...

...

SB = right $ ("00" & Buf (I), 3)

SP = sp & sb

On Error resume next

Whether the required members have been saved in the icode = coltable (SP) 'Set

If err <> 0 then' is not found

N = coltable. Count

Coltable. Add N + 258, SP 'add members and Indexes

J = J + 1

Bint (j) = icode' Save the converted value to the integer set to be saved

SP = sb

Icode = Buf (I)

Err. Clear

End if

Next

The variable changes during the operation are as follows:

Buf () No. coltable bint () Value

Member Index

256

1 258 008008 8

3 259 008008008 258

4 260 008099 8

5 261 099099 99

6 262 099008 99

9 263 008008008008 259

13 264 008008008008099 263

15 265 099099008 261

17 266 008099008 260

260

As shown in the above calculation results, several problems can be explained:

1. the file is compressed, and 20 bytes are compressed into 10 9-bit integers, because the color rules are saved in the set of color arrangement tables, A value can be used to represent the information of several pixels at the same time.

2. the size of the compressed file is related to the size of the image (the length of the pixel stream). The complexity of the image also affects the size of the compressed file, the simpler the image (the larger the block with the same color), the smaller the compressed file.

3. To ensure that the converted integer value cannot exceed 9 digits, the color sorting table can only contain 256 ~ 511, And because 256,257 is retained as a special sign, only 258 ~ is actually available ~ 511 a total of 254, if more than, it is necessary to clear the old table, re-establish (256 is the sign of re-initializing the color arrangement table, 257 is the sign of the end of the pixel Stream)

4. It must be noted that when converting a 256-Color Pixel stream, unlike a bitmap, it starts from the upper left corner of the image and performs a downward scan. If you use an API function to obtain the pixel stream, you must rearrange the stream.

 

GIF compression features:

Before GIF compression, the image must be converted to an image of less than 256 colors. Therefore, the quality of the image is greatly reduced. However, from the preceding compression results, each value in the pixel stream of the original image can be accurately traced and saved. That is to say, when the GIF is decompressed, each pixel on the image can be accurately restored (if the source image is 256 colors, the image is taken as an image ). Therefore, GIF compression is also called lossless compression. This compression can accurately reflect the clear edge of the image. If the image is based on text scanning, GIF compression is definitely the best choice.

 

GIF file composition:

The GIF file consists of the following parts:

 

Version File Header System color palette [secondary information] image header information Image Compression information ending mark

 

(6 bytes) (7 bytes) (8 bytes) (11 bytes) (255 + 255 + + n) bytes) (2 bytes)
 
 

 

 

 

 

 

 

Version: gif89a or gif87a

Auxiliary Information: this information is available only in GIF Transparent Display or GIF animated files. It is also set to implement these features.

System color palette: In bitmap and Icon files, each color is 4 bytes, and in GIF files, each color is 3 bytes.

Image compression information: because the size of the information after image compression is related to the complexity of the image itself, it is not like a bitmap. As long as you know the size of the image, you can calculate the file size, the file size must be known until all the pixel streams are compressed. Therefore, the image compression information in a GIF file is in the unit of 254 bytes and applied backward (each unit is preceded by a byte to indicate the length of the unit, the actual size of each unit is 255 bytes), and the last unit is applied for in actual size.

End mark: 0 59 is used as the end mark of the file.

 

Programming example:

To Compress GIF files, use VB6 to create a project named "bnptogif" and scalemode = 3-pixel. A text box is used to enter the file name to be opened or the file name to be saved; two buttons, three labels, and two image boxes. picture2 is used to display the File compression progress, width: 400, height: 13. Picture1 is used to display open bitmap. Options include visible = false, autoredraw = true, autosize = true, scalemode = 3-pixel, and borderstyle = 0-none.

The "save" button is Enabled = false at the beginning. After you press "open" to open the bitmap file entered in text1, it becomes available, click to convert the original bitmap to a 256-color GIF file (automatically change the extension name or save it as the re-input file name in text1)

 
 

 

 

 

 

 

 

 

 

 

 

 

 

Program code

Option explicit

 

Private type rgbtriple 'color palette

Rgbred as byte

Rgbgreen as byte

Rgbblue as byte

End type

 

Private type gifscreendescriptor 'file Header

Logical_screen_width as integer 'image width

Logical_screen_height as integer 'image height

Flags as byte indicates the quality of the file (the color in the color palette ).

Background_color_index as byte 'saves the value of the first pixel in the pixel stream

Pixel_aspect_ratio as byte 'is 0

End type

'In the file header, the flags value is 256 in 231 colors, 128 in 198 colors, 165 in 64 colors, 132 in 32 colors, 227 in 16 colors, 194 in 8 colors, and 161 in 4 colors.

 

 

Private type gifimagedescriptor 'image information Header

Imageseparator as byte 'is 44

Left as integer 'left

Top as integer 'Right

Width as integer 'width

Height as integer 'height

Format as byte 'is 0

Data as byte is the number of digits occupied by each pixel.

End type

 

Private type gifimageend' end flag

Dat1 as byte is 0

Dat2 as byte is 59

End type

 

Private const gif89a = "gif89a"

 

Dim coltable as new collection 'is used to sort colors during LZW compression.

 

Dim bitpos and bitval as byte are used for bit location during bit operation and converted byte values, and the value is written to gfbyt (p255 ).

Dim p255 as long 'total number of bytes written to the file (positioning)

Dim gfbyt () as byte 'refers to the byte stream written to the file

Dim Buf () as byte 'image pixel stream (256 colors or less)

Dim BPOs (12) as long 'is used to compare the numbers of 1, 2, 4, 8 ....

Dim fname as string

 

'Segment gfbyt () (254 for segments) and write the file

Private sub putbyte ()

Dim I, K as integer

Dim J as long

K = int (p255/254)

Dim SZ () as byte

Redim SZ (k)

SZ (K) = p255 mod 254

For I = k-1 to 0 step-1: SZ (I) = 254: Next

 

K = 0

For J = 0 to p255

If J mod 254 = 0 then put #1, SZ (k): K = k + 1

Put #1, gfbyt (j)

Next

End sub

 

'Write the compressed Table value to gfbyt ()

'When the color is 256, the table value is an integer with a variable length (9-12 digits), 256 is used to clear the old table, 257 is used to end the image, and the table items are 3837 in

'In the case of four colors, the table value is an integer with a variable length (3-12 digits), 4 is to clear the old table, 5 is to end the image, and the table items are 6-4089 in total

Private sub addbyte (byval bval as long, byval vlen as byte)

Dim I as byte

For I = 0 to vlen-1

If (bval and BPOs (I) Then bitval = bitval + BPOs (bitpos)

Bitpos = bitpos + 1

If bitpos = 8 then

Bitpos = 0

Gfbyt (p255) = bitval

Bitval = 0

P255 = p255 + 1

Redim preserve gfbyt (p255) 'apply for another byte and keep the original data

Gfbyt (p255) = 0

End if

Next

End sub

 

Private sub commandementclick ()

Fname = text1.text

If right (fname, 3) <> "BMP" then label3.caption = "the file is not a. BMP file": Exit sub

If Dir (fname) = "" Then label3.caption = "the file does not exist": Exit sub

Picture1.picture = loadpicture (fname)

Command2.enabled = true

End sub

 

 

Private sub command2_click ()

'Check whether the file name is correct

Fname = text1.text

Dim I, j as integer

Dim PW, pH as integer

I = instr (1, fname ,"/")

Do while I> 0: J = I: I = instr (1, fname, "/"): loop

If Dir (mid (fname, 1, J), vbdirectory) = "" Then label3.caption = "the file path does not exist": Exit sub

If right (fname, 4) <> ". GIF" then fname = mid (fname, 1, Len (fname)-4) & ". GIF"

 

Doevents

Jindu 10

'Convert the image to 256 colors and store it in the Buf () pixel stream.

Dim R, G, B, Ra, Ga, Ba, lindex as integer

Dim Col as long

Dim K, KK as long

 

PH = picture1.height-1

PW = picture1.width-1

Kk = (Ph + 1) * (PW + 1)-1

Redim Buf (kk) as byte

'The calculation method used for conversion must be consistent with the algorithm used to create a color palette.

For I = 0 to pH

For J = 0 to pw

Col = picture1.point (J, I)

If cola = 16777215 then

Buf (K) = 215

Else

B = int (COL/65536)

Col = col-B * 65536

G = int (COL/256)

R = Col mod 256.

Ra = CINT (R/51)

GA = CINT (G/51)

BA = CINT (B/51)

Buf (K) = ba * 36 + GA * 6 + RA

End if

K = k + 1

Next

Next

Doevents

Jindu 50

'Write a GIF File

Dim SCR as gifscreendescriptor

Dim IM as gifimagedescriptor

Dim gifpalette (0 to 255) as rgbtriple

Dim Gend as gifimageend

Dim sprefix as string

Dim sbyte as string

Dim intcode as integer

Dim ncount as byte

'Create up to 216 colors in the system color palette. The calculation method for converting 256 colors in front must be consistent with the method used here!

For B = 0 to 255 step 51

For G = 0 to 255 step 51

For r = 0 to 255 step 51

Gifpalette (lindex). rgbblue = B

Gifpalette (lindex). rgbgreen = G

Gifpalette (lindex). rgbred = r

Lindex = lindex + 1

Next

Next

Next

SCR. Back ground_color_index = 215

SCR. Flags = 231

SCR. pixel_aspect_ratio = 0

SCR. logical_screen_width = picture1.width

SCR. logical_screen_height = picture1.height

Im. imageseparator = 44

Im. Data = 8

Im. format = 0

Im. Height = picture1.height

Im. width = picture1.width

'Write the GIF File Header

Open fname for binary as #1

Put #1, gif89a

Put #1, SCR

Put #1, gifpalette

Put #1, Im

'Perform GIF compression, from Buf () to gfbyt ()

Dim L as integer

Redim gfbyt (0)

Gfbyt (0) = 0

L = 9

Addbyte 256, l

Sprefix = right $ ("00" & Buf (0), 3)

Intcode = Buf (0)

R = 0

For k = 1 to bufsize-1

Sbyte = right $ ("00" & Buf (K), 3)

Sprefix = sprefix & sbyte

On Error resume next

Intcode = coltable (sprefix)

If err <> 0 then

Ncount = coltable. Count

If ncount = 3837 then

Addbyte intcode, l

Set coltable = nothing

Set coltable = new collection

Addbyte 256, l

L = 9

Sprefix = sbyte

Intcode = Buf (k)

Goto 10

End if

Coltable. Add ncount + 258, sprefix

If getlen (ncount + 257)> L then l = L + 1' increase the number of bytes as needed

Addbyte intcode, l

Sprefix = sbyte

Intcode = Buf (k)

Err. Clear

End if

10:

Next

Addbyte intcode, l' write the last compressed Value

Ncount = coltable. Count

If ncount> 3837 then addbyte 256, 9

Addbyte 257, 9' write end flag

Putbyte' Write File

Gend. dat1 = 0

Gend. dat2 = 59

Put #1, Gend

Close #1

Erase Buf

Erase gfbyt

Label3.caption = "successfully written" & fname & "file"

Jindu 100.

End sub

 

Private sub jindu (byval vol as integer) 'is used to display the progress

Picture2.cls

Label2.caption = vol & "%"

Picture2.line (0, 0)-step (Vol * 4, 13), BF

Doevents

End sub

 

 

Private function getlen (byval bval as long) as byte

Select case bval

Case 0, 1

Getlen = 1

Case is <4

Getlen = 2

Case is <8

Getlen = 3

Case is <16

Getlen = 4

Case is <32

Getlen = 5

Case is <64

Getlen = 6

Case is <128

Getlen = 7

Case is <256

Getlen = 8

Case is <512

Getlen = 9

Case is <1024

Getlen = 10

Case is <2048

Getlen = 11

Case else

Getlen = 12

End select

 

End Function

 

Private sub form_load ()

Text1.text = app. Path & "\ yeye.bmp"

Dim I as integer

For I = 0 to 12: BPOs (I) = 2 ^ I: Next 'initialize the number of bit operations

End sub

 

 

Programming example: e-book format (4-color GIF ):

The color of the system color palette directly affects the quality of the converted image and the size of the compressed file. If the image is based on text scanning, I can use two colors (black and white) for compression to obtain the maximum compression ratio without affecting image reading. In the file header, flags = 161 (4 colors). In the color palette, set only one color (255,255,255). The remaining three colors are not assigned (, 0 ), when converting the pixel stream of an image, convert it to 2 colors to achieve the highest compression rate. Many e-books now use this format (4-color GIF) compression.

I used a B4 (1150*800 pixels) image with around 1000 characters for 4-color GIF compression. Let's look at the following group of numbers: the 24-Bit Bitmap is about 2630 K, the 256-color GIF is about 45 K, and the 4-color GIF is about 24 K. It can be seen that the maximum compression ratio of a 4-color GIF image can be obtained, which is about 100: 1, and the image quality is very good. It is still clear after the image viewing tool is scaled up several times. This is also the reason why GIF became the exclusive e-book kingdom. Here is another example to show how to compress a 4-color GIF image.

Add a command3 in the previous program and write the code:

 

Private sub command3_click ()

'Check whether the file name is correct

Fname = text1.text

Dim I, j as integer

Dim PW, pH as integer

I = instr (1, fname ,"/")

Do while I> 0: J = I: I = instr (1, fname, "/"): loop

If Dir (mid (fname, 1, J), vbdirectory) = "" Then label3.caption = "the file path does not exist": Exit sub

If right (fname, 4) <> ". GIF" then fname = mid (fname, 1, Len (fname)-4) & ". GIF"

 

Doevents

Jindu 10

'Convert the image to 256 colors and store it in the Buf () pixel stream.

Dim Col as long

Dim K, KK as long

 

PH = picture1.height-1

PW = picture1.width-1

Kk = (Ph + 1) * (PW + 1)-1

Redim Buf (kk) as byte

'The calculation method used for conversion must be consistent with the algorithm used to create a color palette.

For I = 0 to pH

For J = 0 to pw

Col = picture1.point (J, I)

If cola = 16777215 then

Buf (K) = 1' only two colors, black and white

Else

Buf (K) = 0

End if

K = k + 1

Next

Next

Doevents

Jindu 50

 

'Write a GIF File

Dim SCR as gifscreendescriptor

Dim IM as gifimagedescriptor

Dim gifpalette (3) as rgbtriple

Dim Gend as gifimageend

Dim sprefix as string

Dim sbyte as string

Dim intcode as long

Dim ncount as long

Gifpalette (0). rgbblue = 0

Gifpalette (0). rgbgreen = 0

Gifpalette (0). rgbred = 0

Gifpalette (1). rgbblue = 255

Gifpalette (1). rgbgreen = 255

Gifpalette (1). rgbred = 255

SCR. background_color_index = 0

SCR. Flags = 161

SCR. pixel_aspect_ratio = 0

SCR. logical_screen_width = pw

SCR. logical_screen_height = Ph

Im. imageseparator = 44

Im. Data = 2

Im. format = 0

Im. Height = Ph

Im. width = pw

'Write the GIF File Header

Open fname for binary as #1

Put #1, gif89a

Put #1, SCR

Put #1, gifpalette

Put #1, Im

Dim L as integer

'Perform GIF compression, from Buf () to gfbyt ()

Redim gfbyt (0)

Gfbyt (0) = 0

L = 3

Addbyte 4, L

Sprefix = CSTR (BUF (0 ))

Intcode = Buf (0)

For k = 1 to bufsize-1

Sbyte = Buf (k)

Sprefix = sprefix & sbyte

On Error resume next

Intcode = coltable (sprefix)

If err <> 0 then

Ncount = coltable. Count

If ncount = 4089 then' the table value is nine characters long and the old table is cleared.

Addbyte intcode, l

'Note! The "4" sign added when the table is re-colored should be written into 12 bits, instead of three bits.

'Is 000000000100

Addbyte 4, L

Set coltable = nothing

Set coltable = new collection

L = 3

Ncount = 0

Sprefix = Buf (k)

Intcode = Buf (k)

Goto 10

End if

Coltable. Add ncount + 6, sprefix

If getlen (ncount + 5)> L then l = L + 1' increase the number of bytes as needed

Addbyte intcode, l

Sprefix = sbyte

Intcode = Buf (k)

Err. Clear

End if

10:

Next

Addbyte intcode, l' write the last compressed Value

'Write files

Addbyte 5, L

Putbyte

Gend. dat1 = 0

Gend. dat2 = 59

Put #1, Gend

Close #1

Erase Buf

Erase gfbyt

Label3.caption = "successfully written" & fname & "file"

Jindu 100.

End sub

 

 

Tong Yue, Ji tou Elementary School, Hua 'an County, Fujian Province

Tongyue2007@126.com

May 1, 2007

 

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Http://zh.wikipedia.org/zh-tw/GIF