IDL Reference Guide: Procedures and Functions

READ_BMP

The READ_BMP function reads a Microsoft Windows Version 3 device independent bitmap file (.BMP) and returns the image.

READ_BMP does not handle 1-bit-deep images or compressed images, and is not fast for 4-bit images. The algorithm works best on images where the number of bytes in each scan-line is evenly divisible by 4.

This routine is written in the IDL language. Its source code can be found in the file read_bmp.pro in the lib subdirectory of the IDL distribution.

Syntax

Result = READ_BMP( Filename, [, R, G, B] [, Ihdr] [, /RGB] )

Return Value

Returns a byte array containing the image. Dimensions are taken from the BITMAPINFOHEADER of the file. In the case of 4-bit or 8-bit images, the dimensions of the resulting array are (biWidth, biHeight).

For 24-bit images, the dimensions are (3, biWidth, biHeight). Color interleaving is blue, green, red; i.e., Result[0,i,j] = blue, Result[1,i,j] = green, etc.

Arguments

Filename

A scalar string specifying the full path name of the bitmap file to read.

R, G, B

Named variables that will contain the color tables from the file. There 16 elements each for 4 bit images, 256 elements each for 8 bit images. Color tables are not defined or used for 24 bit images.

Ihdr

A named variable that will contain a structure holding the BITMAPINFOHEADER from the file. Tag names are as defined in the MS Windows Programmer's Reference Manual.

Keywords

RGB

If this keyword is set, color interleaving of 16- and 24-bit images will be R, G, B, i.e., Result[0,i,j] = red, Result[1,i,j] = green, Result[2,i,j] = blue.

Examples

To open, read, and display the BMP file named foo.bmp in the current directory and store the color vectors in the variables R, G, and B, enter:

; Read and display an image:  
TV, READ_BMP('foo.bmp', R, G, B)  
  
; Load its colors:  
TVLCT, R, G, B  

Many applications that use 24-bit BMP files outside IDL expect BMP files to be stored as BGR. For example, enter the following commands.

; Make a red square image:  
a = BYTARR(3, 200, 200, /NOZERO)  
a[0, *, *] = 255  
  
;View the image:  
TV, a, /TRUE  
WRITE_BMP, 'foo.bmp', a  

If you open the .bmp file in certain bitmap editors, you may find that the square is blue.

image = READ_BMP('foo.bmp')  
  
; IDL reads the image back in and interprets it as red:  
TV, image, /TRUE  
  
; Flip the order of the indices by adding the RGB keyword:  
image = READ_BMP('foo.bmp', /RGB)  
  
; Displays the image in blue:  
TV, image, /TRUE