Re: [dev] Looking for simple, alpha supporting image format

From: koneu <koneu93_AT_googlemail.com>
Date: Wed, 16 Jul 2014 15:48:52 +0200

On July 16, 2014 3:28:44 PM CEST, Markus Teich <markus.teich_AT_stusta.mhn.de> wrote:
>koneu wrote:
>> If anything, I would store the numbers as unsigned 64 bit LSB and
>change the
>> read/write functions for MSB architectures.
>
>Heyho,
>
>max(uint32_t) = 4.294.967.296 is already way more than the proposed max
>of
>999.999.999. I would even suggest uint16_t is enough. To store images
>larger
>than 65536^2 (=16GiB with 4 bytes per pixel) is far enough in the
>future and
>also the argument with the need for greater color depth applies. We
>could just
>use htons and ntohs then:
>
>spec
>----
>
>Bytes Description
>9 ASCII string: "imagefile"
>2 Width of the image stored in network byte order (big-endian)
>2 Height of the image stored in network byte order (big-endian)
>
>Then, (width*height) pixels arranged in height scanlines, where each
>pixel is
>four bytes. Each byte represents red, green, blue, and alpha
>respectively.
>
>This function reads an image:
>
> char *
> readimage(int fd, uint16_t *w, uint16_t *h)
> {
> char hdr[13];
> char *data;
> int len;
>
> if (read(fd, hdr, 13) != 13 || strcmp(hdr, "imagefile"))
> return NULL;
>
> *w = ntohs(hdr[9]);
> *h = ntohs(hdr[11]);
> len = (*w) * (*h) * 4;
>
> if (!(data = malloc(len)) || read(fd, data, len) != len) {
> free(data);
> return NULL;
> }
> return data;
> }
>
>This function writes an image:
>
> int
> writeimage(int fd, uint16_t w, uint16_t h, char *data)
> {
> uint16_t nw = htons(w);
> uint16_t nh = htons(h);
> return (write(fd, "imagefile", 9) == 9 &&
> write(fd, &nw, 2) == 2 &&
> write(fd, &nh, 2) == 2 &&
> write(fd, data, w*h*4) == w*h*4);
> }
>
>We also avoid *printf and stdio.h.
>
>--Markus

This will pass a char to ntohs; expanding it to a uint16_t, instead of passing ntohs two bytes from the header.
The proper way would involve some pointer casting or a header struct.

struct Header {
uint8_t magic[9];
uint16_t width, height;
}

Then read() and write() into/out of the header struct.
Received on Wed Jul 16 2014 - 15:48:52 CEST