15.3.4  Creating or recreating images: writergb

The writergb command writes images to png files; the image can be given by the Xcas image structure (see Section 15.3.1, this is what is read in with readrgb) or a simplified version of this structure.

To write an image given by the Xcas image structure to a file:

• writergb takes two arguments:
  • filename, a file name.
  • image, an image in Xcas format.
• writergb(filename,image) writes the image image to the file filename.

Examples.

• Assume the the following image is stored in file image1234.jpg.
  
  After reading it into a variable name with readrgb:
  Input:
  a:= readrgb("image1234.jpg")
  the variable a will contain a list,
  • a[0] will be [4,250,500], the number of channels, the height and the width of the image.
  • a[1], the red channel,
  • a[2], the green channel,
  • a[3], the transparency channel,
  • a[4], the blue channel.
  Then:
  Input:
  writergb("image2134.png",[a[0],a[2],a[1],a[3],a[4]])
  Output:
  
  and the image file image2134.png will be created. This image is simply image1234.png with the green and red colors switched.
• For simple cases, you can type the Xcas image format in by hand.
  Input:

  writergb("image1.png",[[4,2,2],[[255,0],[0,0]],[[0,255],[0,0]],

  [[255,125],[255,255]],[[0,0],[255,0]]])

  Output:
  
  The transparency value of 125 for the upper right point makes it partially transparent and mutes the color.
• For larger images, in some cases the matrix operations of Xcas can be used to create the channels.
  Input:

  writergb("image2.png",[[4,300,300],makemat(0,300,300),makemat(0,300,300),

  makemat(255,300,300),makemat(0,300,300)+idn(300)*255])

  Output:
  

The simplified version of the Xcas image description doesn’t involve stating the number of channels, the size of the image, or the transparency. There is a full color version of this simplified form and a grayscale version.

To create a full color image using the simple description:

• writergb command takes four arguments:
  • filename, the name of the file to store the image.
  • R, a matrix for the red channel.
  • G, a matrix for the green channel.
  • B, a matrix for the blue channel.
• writergb(filename,R,G,B) draws the image given by the matrices to the file filename.

Example.
Input:

Output:

This image will be in the file image2.png.

To create a grayscale image using the simple description:

• writergb command takes two arguments:
  • filename, the name of the file to store the image.
  • M, a matrix representing how dark each point is (where 0 is black and 255 is white).
• writergb(filename,M) draws the image given by M to the file filename.

Example.
Input:

Output:

This image will be in the file image3.png.