# Reading and Writing Images

#### 2020-07-24

## Reading TIFF files

Check out the following video:

As you can see, it’s a colour video of a banana dancing in front of the R logo. Hence, it has colour channel (red, green and blue) and frame (a video is comprised of several frames) information inside. I have this video saved in a TIFF file.

    path_dancing_banana <- system.file("img", "Rlogo-banana.tif",
                                       package = "ijtiff")
    print(path_dancing_banana)
    #> [1] "/private/var/folders/l_/2mwm03p55zg7zjykv084hhvr0000gn/T/RtmpQp6rpI/Rinst92363f12626a/ijtiff/img/Rlogo-banana.tif"

To read it in, you just need `read_tif()` and the path to the image.

    pacman::p_load(ijtiff, magrittr)
    img_dancing_banana <- read_tif(path_dancing_banana)
    #> Reading Rlogo-banana.tif: an 8-bit, 78x100 pixel image of
    #> unsigned integer type. Reading 3 channels and 8 frames . . .
    #>  Done.

Let’s take a peek inside of `img_dancing_banana`.

    print(img_dancing_banana)
    #> 78x100 pixel ijtiff_img with 3 channels and 8 frames.
    #> Preview (top left of first channel of first frame):
    #>      [,1] [,2] [,3] [,4] [,5] [,6]
    #> [1,]  255  255  255  255  255  255
    #> [2,]  255  255  255  255  255  255
    #> [3,]  255  255  255  255  255  255
    #> [4,]  255  255  255  255  255  255
    #> [5,]  255  255  255  255  255  255
    #> [6,]  255  255  255  255  255  255
    #> ── TIFF tags ─────────────────────────────────────────────────────────────────────────────────────────────────────────
    #> ● bits_per_sample: 8
    #> ● samples_per_pixel: 3
    #> ● sample_format: uint
    #> ● planar_config: contiguous
    #> ● rows_per_strip: 78
    #> ● compression: deflate
    #> ● software: ijtiff package, R 4.0.0
    #> ● color_space: black is zero

You can see it’s a 4-dimensional array. The last two dimensions are 3 and 8; this is because these are the channel and frame slots respectively: the image has 3 channels (red, green and blue) and 8 frames. The first two dimensions tell us that the images in the video are 78 pixels tall and 100 pixels wide. The image object is of class `ijtiff_img`. This guarantees that it is a 4-dimensional array with this structure. The attributes of the `ijtiff_img` give information on the various TIFF tags that were part of the TIFF image. You can read more about various TIFF tags at https://www.awaresystems.be/imaging/tiff/tifftags.html. To read just the tags and not the image, use the `read_tags()` function.

Let’s visualize the constituent parts of that 8-frame, colour TIFF.

There you go: 8 frames in 3 colours.

### Reading only certain frames

It’s possible to read only certain frames. This can be a massive time and memory saver when working with large images.

Suppose we only want frames 3, 5 and 7 from the image above.

    img_dancing_banana357 <- read_tif(path_dancing_banana, frames = c(3, 5, 7))
    #> Reading Rlogo-banana.tif: an 8-bit, 78x100 pixel image of
    #> unsigned integer type. Reading 3 channels and 3 frames . . .
    #>  Done.

Let’s visualize again.

Just in case you’re wondering, it’s not currently possible to read only certain channels. This would be more difficult to implement, but if you need this, ask me and I’ll see what I can do.

### More examples

If you read an image with only one frame, the frame slot (4) will still be there:

    path_rlogo <- system.file("img", "Rlogo.tif", package = "ijtiff")
    img_rlogo <- read_tif(path_rlogo)
    #> Reading Rlogo.tif: an 8-bit, 76x100 pixel image of unsigned
    #> integer type. Reading 4 channels and 1 frame . . .
    #>  Done.
    dim(img_rlogo)  # 4 channels, 1 frame
    #> [1]  76 100   4   1
    class(img_rlogo)
    #> [1] "ijtiff_img" "array"
    display(img_rlogo)

You can also have an image with only 1 channel:

    path_rlogo_grey <- system.file("img", "Rlogo-grey.tif", package = "ijtiff")
    img_rlogo_grey <- read_tif(path_rlogo_grey)
    #> Reading Rlogo-grey.tif: a 32-bit, 76x100 pixel image of
    #> floating point type. Reading 1 channel and 1 frame . . .
    #>  Done.
    dim(img_rlogo_grey)  # 1 channel, 1 frame
    #> [1]  76 100   1   1
    display(img_rlogo_grey)

## Writing TIFF files

To write an image, you need an object in the style of an `ijtiff_img` object (see `help("ijtiff_img", package = "ijtiff")`). The basic idea is to have your image in a 4-dimensional array with the structure `img[y, x, channel, frame]`. Then, to write this image to the location `path`, you just type `write_tif(img, path)`.

    path <- tempfile(pattern = "dancing-banana", fileext = ".tif")
    print(path)
    #> [1] "/var/folders/l_/2mwm03p55zg7zjykv084hhvr0000gn/T//RtmpKDz1AT/dancing-banana93941d0c7730.tif"
    write_tif(img_dancing_banana, path)
    #> Writing dancing-banana93941d0c7730.tif: an 8-bit, 78x100 pixel
    #> image of unsigned integer type with 3 channels and 8 frames .
    #> . .
    #>  Done.

## Reading text images

Note: if you don’t know what text images are, see `vignette("text-images", package = "ijtiff")`.

You may have a text image that you want to read (but realistically, you might never).

    path_txt_img <- system.file("img", "Rlogo-grey.txt", package = "ijtiff")
    txt_img <- read_txt_img(path_txt_img)
    #> Reading 76x100 pixel text image 'Rlogo-grey.txt' . . .
    #>  Done.

## Writing text images

Writing a text image works as you’d expect.

    write_txt_img(txt_img, path = tempfile(pattern = "txtimg", fileext = ".txt"))
    #> Writing txtimg939448e85020.txt: a 76x100 pixel text image with 1 channel and 1 frame . . .
    #>  Done.