# R7RS-PNM

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.
See the COPYING and COPYING.LESSER files for more details.

## Copyright

Copyright (c) 2024 Masaya Tojo <masaya@tojo.tokyo>