From 544f0d337220ff1745fbb03dcd59398b91270875 Mon Sep 17 00:00:00 2001
From: Masaya Tojo <masaya@tojo.tokyo>
Date: Sun, 4 Aug 2024 20:33:10 +0900
Subject: Add 'Usage' section to README.md

---
 README.md | 123 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)

diff --git a/README.md b/README.md
index 928f8fa..331f9db 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,129 @@ R7RS-PNM is a R7RS library for handling PNM (Portable Any Map) files,
 providing functionality for reading and writing PNM files,as well as
 accessing and modifying pixel data.
 
+## Usage
+
+### Import `(pnm)` library
+
+```scheme
+(import (pnm))
+```
+
+### Image
+
+#### `(image? x) → boolean`
+
+Returns `#t` if `x` is a valid image object; otherwise, it returns
+`#f`.
+
+#### `(image-type image) → 'pbm | 'pgm | 'ppm`
+
+Returns the type of the given image.
+
+- `pbm`: Portable Bit Map
+- `pgm`: Portable Gray Map
+- `ppm`: Portable Pixel Map
+
+#### `(image-width image) → integer`
+
+Returns the width of `image`.
+
+#### `(image-height image) → integer`
+
+Returns the height of `image`.
+
+### PBM (Portable Bit Map)
+
+#### `(make-pbm-image width height) → <image>`
+
+Returns a new Portable Bit Map (PBM) image with the specified width
+and height.
+
+#### `(image-maxval pbm-image) → #t`
+
+Returns the maximum value for pixel intensity in a PBM image, which is
+always `#t` for PBM images, indicating binary values (black and
+white).
+
+#### `(image-ref pbm-image x y) → boolean`
+
+Returns the value of the pixel at coordinates `(x, y)` in the
+specified `image`. The return value is `#t` for a white pixel and `#f`
+for a black pixel.
+
+#### `(image-set! pbm-image x y b)`
+
+Sets the pixel at coordinates `(x, y)` in the specified `image` to
+the boolean value `b`. If `b` is `#t`, the pixel is set to white; if
+`#f`, it is set to black.
+
+### PGM (Portable PGM Map)
+
+#### `(make-pgm-image width height [maxval]) → <image>`
+
+Creates a new Portable Gray Map (PGM) image with the specified width
+and height. The optional `maxval` parameter defines the maximum pixel
+intensity, defaulting to 255 if not provided.
+
+#### `(image-maxval pgm-image) → integer`
+
+Returns the maximum pixel intensity value for the specified PGM image.
+
+#### `(image-ref image x y) → integer`
+
+Returns the gray value of the pixel at coordinates `(x, y)` in the
+specified PGM image. The value is an integer between 0 and `maxval`.
+
+#### `(image-set! image x y v)`
+
+Sets the pixel at coordinates `(x, y)` in the specified PGM image to
+the given integer value `v`, which must be between 0 and `maxval`.
+
+### PPM (Portable PPM Map)
+
+#### `(make-ppm-image width height [maxval]) → <image>`
+
+Creates a new Portable Pixel Map (PPM) image with the specified width
+and height. The optional `maxval` parameter defines the maximum color
+intensity, defaulting to 255 if not provided.
+
+#### `(image-maxval pgm-image) → integer`
+
+Returns the maximum color intensity value for the specified PPM image.
+
+#### `(image-ref ppm-image x y) → (values integer integer integer)`
+
+Returns the RGB values of the pixel at coordinates `(x, y)` in the
+specified PPM image as three integers, representing the red, green,
+and blue components, respectively.
+
+### `(image-set! ppm-image x y r g b)`
+
+Sets the pixel at coordinates `(x, y)` in the specified PPM image to
+the given RGB values `r`, `g`, and `b`, which must be integers between
+0 and `maxval`.
+
+
+### Reading
+
+#### `(image-read port) → <image>`
+
+Reads a PNM image from the specified input `port` and returns the
+corresponding image object (PBM, PGM, or PPM).
+
+### `(pnm-parse-error? exn) → boolean`
+
+Returns `#t` if the exception `exn` is a parse error that occurred
+during the execution of the `image-read` procedure; otherwise, it
+returns `#f`.
+
+### Writing
+
+### `(image-write image port)`
+
+Writes the specified `image` (PBM, PGM, or PPM) to the given output
+`port`.
+
 ## license
 
 This library is released under the LGPLv3+ License.
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