目录
- 一、解码显示png图片
- 1、思路分析
- 2、libpng移植
- 3、zlib移植
- 4、参考源码包自带的资料
- 5、学习了解示例代码
- 6、完整移植好的代码
一、解码显示png图片
1、思路分析
(1)png更像是jpg而不像是bmp
(2)png和jpg都是压缩格式的图片，都是二进制文件，不同之处是压缩和解压缩的算法不同。
(3)通过libjpeg来编解码jpg图片，那么同样有一个libpng用来编解码png图片。
(4)工作思路和顺序：找到并移植并部署libpng，然后查readme和其他文档示例代码
等来使用libpng提供的API来对png图片进行解码，并将解码出来的数据丢到framebuffer中去显示。
2、libpng移植
(1)下载源码包：
官方网址：http://www.libpng.org/pub/png/libpng.html
链接：https://pan.baidu.com/s/1Z7sC2X4B78vYy_ob4PGICQ
提取码：puqn
–来自百度网盘超级会员V5的分享
本篇文章使用libpng-1.6.37.tar.gz
(2)解压、配置、修改Makefile、编译、部署
./configure --host=arm-linux --enable-shared --enable-static --prefix=/opt/libdecode(3)配置出错，报错信息：configure: error: zlib not installed
分析问题是因为libpng依赖于zlib库，所以要先移植zlib库才可以。
(4)移植了zlib后再过来配置，还是报错，原因是因为没有导出相关环境变量，所以libpng在配置的时候找不到刚才移植的zlib库的库文件和头文件。
(5)解决方案就是使用epport临时性的导出，
#命令行执行，导出zlib所在的路径，使得编译时可以找到zlib
export LDFLAGS="-L/opt/libdecode/lib"
export CFLAGS="-I/opt/libdecode/include"
export CPPFLAGS="-I/opt/libdecode/include"(6)导出后再次配置就过了，然后编译和安装
(7)make && make install
(8)部署生成的动态链接库：
#修改该项目的主Makefile
LDFLAGS := -ljpeg -lz -lpng -L/opt/libdecode/lib #指定编译器链接库(根据实际项目手动修改)#将动态链接库放置到开发板的根文件系统的/usr/lib目录下
#动态链接库所在的目录：/opt/libdecode/lib/
#开发板根文件系统在Ubuntu中的目录：/home/rootfs
cp /opt/libdecode/lib/*so* /home/rootfs/usr/lib/ -rf3、zlib移植
(1)下载：http://www.zlib.net/，并解压，本篇文章使用zlib-1.2.8.tar.gz
(2)命令行执行：export CC=arm-linux-gcc
进行配置：./configure -shared --prefix=/opt/libdecode
(3)make && make install
(4)make install后/opt/libdecode目录下的lib和include目录下就有了zlib的静态库动态库和头文件了，然后再回去继续libpng的移植。
4、参考源码包自带的资料
(1)README
(2)libpng-manual.txt
(3)example.c 和 pngtest.c
对于例程中所给的函数想知道在哪libpng的那个文件，可在ubuntu中通过grep命令进行搜索，也可以在windows中通过sourceinsight等软件进行查找！
5、学习了解示例代码
//example.c
#include <png.h>
/* The png_jmpbuf() macro, used in error handling, became available in
* libpng version 1.0.6. If you want to be able to run your code with older
* versions of libpng, you must define the macro yourself (but only if it
* is not already defined by libpng!).
*/
#ifndef png_jmpbuf
# define png_jmpbuf(png_ptr) ((png_ptr)->png_jmpbuf)
#endif
/* Check to see if a file is a PNG file using png_sig_cmp(). png_sig_cmp()
* returns zero if the image is a PNG and nonzero if it isn't a PNG.
*
* The function check_if_png() shown here, but not used, returns nonzero (true)
* if the file can be opened and is a PNG, 0 (false) otherwise.
*
* If this call is successful, and you are going to keep the file open,
* you should call png_set_sig_bytes(png_ptr, PNG_BYTES_TO_CHECK); once
* you have created the png_ptr, so that libpng knows your application
* has read that many bytes from the start of the file. Make sure you
* don't call png_set_sig_bytes() with more than 8 bytes read or give it
* an incorrect number of bytes read, or you will either have read too
* many bytes (your fault), or you are telling libpng to read the wrong
* number of magic bytes (also your fault).
*
* Many applications already read the first 2 or 4 bytes from the start
* of the image to determine the file type, so it would be easiest just
* to pass the bytes to png_sig_cmp() or even skip that if you know
* you have a PNG file, and call png_set_sig_bytes().
*/
#define PNG_BYTES_TO_CHECK 4
int check_if_png(char *file_name, FILE **fp)
{
char buf[PNG_BYTES_TO_CHECK];
/* Open the prospective PNG file. */
if ((*fp = fopen(file_name, "rb")) == NULL)
return 0;
/* Read in some of the signature bytes */
if (fread(buf, 1, PNG_BYTES_TO_CHECK, *fp) != PNG_BYTES_TO_CHECK)
return 0;
/* Compare the first PNG_BYTES_TO_CHECK bytes of the signature.
Return nonzero (true) if they match */
return(!png_sig_cmp(buf, (png_size_t)0, PNG_BYTES_TO_CHECK));
}
/* Read a PNG file. You may want to return an error code if the read
* fails (depending upon the failure). There are two "prototypes" given
* here - one where we are given the filename, and we need to open the
* file, and the other where we are given an open file (possibly with
* some or all of the magic bytes read - see comments above).
*/
#ifdef open_file /* prototype 1 */
void read_png(char *file_name) /* We need to open the file */
{
png_structp png_ptr;
png_infop info_ptr;
unsigned int sig_read = 0;
png_uint_32 width, height;
int bit_depth, color_type, interlace_type;
FILE *fp;
if ((fp = fopen(file_name, "rb")) == NULL)
return (ERROR);
#else no_open_file /* prototype 2 */
void read_png(FILE *fp, unsigned int sig_read) /* File is already open */
{
png_structp png_ptr;
png_infop info_ptr;
png_uint_32 width, height;
int bit_depth, color_type, interlace_type;
#endif no_open_file /* Only use one prototype! */
/* Create and initialize the png_struct with the desired error handler
* functions. If you want to use the default stderr and longjump method,
* you can supply NULL for the last three parameters. We also supply the
* the compiler header file version, so that we know if the application
* was compiled with a compatible version of the library. REQUIRED
*/
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
png_voidp user_error_ptr, user_error_fn, user_warning_fn);
if (png_ptr == NULL)
{
fclose(fp);
return (ERROR);
}
/* Allocate/initialize the memory for image information. REQUIRED. */
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL)
{
fclose(fp);
png_destroy_read_struct(&png_ptr, NULL, NULL);
return (ERROR);
}
/* Set error handling if you are using the setjmp/longjmp method (this is
* the normal method of doing things with libpng). REQUIRED unless you
* set up your own error handlers in the png_create_read_struct() earlier.
*/
if (setjmp(png_jmpbuf(png_ptr)))
{
/* Free all of the memory associated with the png_ptr and info_ptr */
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
fclose(fp);
/* If we get here, we had a problem reading the file */
return (ERROR);
}
/* One of the following I/O initialization methods is REQUIRED */
#ifdef streams /* PNG file I/O method 1 */
/* Set up the input control if you are using standard C streams */
png_init_io(png_ptr, fp);
#else no_streams /* PNG file I/O method 2 */
/* If you are using replacement read functions, instead of calling
* png_init_io() here you would call:
*/
png_set_read_fn(png_ptr, (void *)user_io_ptr, user_read_fn);
/* where user_io_ptr is a structure you want available to the callbacks */
#endif no_streams /* Use only one I/O method! */
/* If we have already read some of the signature */
png_set_sig_bytes(png_ptr, sig_read);
#ifdef hilevel
/*
* If you have enough memory to read in the entire image at once,
* and you need to specify only transforms that can be controlled
* with one of the PNG_TRANSFORM_* bits (this presently excludes
* quantizing, filling, setting background, and doing gamma
* adjustment), then you can read the entire image (including
* pixels) into the info structure with this call:
*/
png_read_png(png_ptr, info_ptr, png_transforms, NULL);
#else
/* OK, you're doing it the hard way, with the lower-level functions */
/* The call to png_read_info() gives us all of the information from the
* PNG file before the first IDAT (image data chunk). REQUIRED
*/
png_read_info(png_ptr, info_ptr);
png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
&interlace_type, NULL, NULL);
/* Set up the data transformations you want. Note that these are all
* optional. Only call them if you want/need them. Many of the
* transformations only work on specific types of images, and many
* are mutually exclusive.
*/
/* Tell libpng to strip 16 bit/color files down to 8 bits/color.
* Use accurate scaling if it's available, otherwise just chop off the
* low byte.
*/
#ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED
png_set_scale_16(png_ptr);
#else
png_set_strip_16(png_ptr);
#endif
/* Strip alpha bytes from the input data without combining with the
* background (not recommended).
*/
png_set_strip_alpha(png_ptr);
/* Extract multiple pixels with bit depths of 1, 2, and 4 from a single
* byte into separate bytes (useful for paletted and grayscale images).
*/
png_set_packing(png_ptr);
/* Change the order of packed pixels to least significant bit first
* (not useful if you are using png_set_packing). */
png_set_packswap(png_ptr);
/* Expand paletted colors into true RGB triplets */
if (color_type == PNG_COLOR_TYPE_PALETTE)
png_set_palette_to_rgb(png_ptr);
/* Expand grayscale images to the full 8 bits from 1, 2, or 4 bits/pixel */
if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
png_set_expand_gray_1_2_4_to_8(png_ptr);
/* Expand paletted or RGB images with transparency to full alpha channels
* so the data will be available as RGBA quartets.
*/
if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
png_set_tRNS_to_alpha(png_ptr);
/* Set the background color to draw transparent and alpha images over.
* It is possible to set the red, green, and blue components directly
* for paletted images instead of supplying a palette index. Note that
* even if the PNG file supplies a background, you are not required to
* use it - you should use the (solid) application background if it has one.
*/
png_color_16 my_background, *image_background;
if (png_get_bKGD(png_ptr, info_ptr, &image_background))
png_set_background(png_ptr, image_background,
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
/* Some suggestions as to how to get a screen gamma value
*
* Note that screen gamma is the display_exponent, which includes
* the CRT_exponent and any correction for viewing conditions
*/
if (/* We have a user-defined screen gamma value */)
{
screen_gamma = user-defined screen_gamma;
}
/* This is one way that applications share the same screen gamma value */
else if ((gamma_str = getenv("SCREEN_GAMMA")) != NULL)
{
screen_gamma = atof(gamma_str);
}
/* If we don't have another value */
else
{
screen_gamma = PNG_DEFAULT_sRGB; /* A good guess for a PC monitor
in a dimly lit room */
screen_gamma = PNG_GAMMA_MAC_18 or 1.0; /* Good guesses for Mac systems */
}
/* Tell libpng to handle the gamma conversion for you. The final call
* is a good guess for PC generated images, but it should be configurable
* by the user at run time by the user. It is strongly suggested that
* your application support gamma correction.
*/
int intent;
if (png_get_sRGB(png_ptr, info_ptr, &intent))
png_set_gamma(png_ptr, screen_gamma, PNG_DEFAULT_sRGB);
else
{
double image_gamma;
if (png_get_gAMA(png_ptr, info_ptr, &image_gamma))
png_set_gamma(png_ptr, screen_gamma, image_gamma);
else
png_set_gamma(png_ptr, screen_gamma, 0.45455);
}
#ifdef PNG_READ_QUANTIZE_SUPPORTED
/* Quantize RGB files down to 8 bit palette or reduce palettes
* to the number of colors available on your screen.
*/
if (color_type & PNG_COLOR_MASK_COLOR)
{
int num_palette;
png_colorp palette;
/* This reduces the image to the application supplied palette */
if (/* We have our own palette */)
{
/* An array of colors to which the image should be quantized */
png_color std_color_cube[MAX_SCREEN_COLORS];
png_set_quantize(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
MAX_SCREEN_COLORS, NULL, 0);
}
/* This reduces the image to the palette supplied in the file */
else if (png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette))
{
png_uint_16p histogram = NULL;
png_get_hIST(png_ptr, info_ptr, &histogram);
png_set_quantize(png_ptr, palette, num_palette,
max_screen_colors, histogram, 0);
}
}
#endif /* PNG_READ_QUANTIZE_SUPPORTED */
/* Invert monochrome files to have 0 as white and 1 as black */
png_set_invert_mono(png_ptr);
/* If you want to shift the pixel values from the range [0,255] or
* [0,65535] to the original [0,7] or [0,31], or whatever range the
* colors were originally in:
*/
if (png_get_valid(png_ptr, info_ptr, PNG_INFO_sBIT))
{
png_color_8p sig_bit_p;
png_get_sBIT(png_ptr, info_ptr, &sig_bit_p);
png_set_shift(png_ptr, sig_bit_p);
}
/* Flip the RGB pixels to BGR (or RGBA to BGRA) */
if (color_type & PNG_COLOR_MASK_COLOR)
png_set_bgr(png_ptr);
/* Swap the RGBA or GA data to ARGB or AG (or BGRA to ABGR) */
png_set_swap_alpha(png_ptr);
/* Swap bytes of 16 bit files to least significant byte first */
png_set_swap(png_ptr);
/* Add filler (or alpha) byte (before/after each RGB triplet) */
png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);
#ifdef PNG_READ_INTERLACING_SUPPORTED
/* Turn on interlace handling. REQUIRED if you are not using
* png_read_image(). To see how to handle interlacing passes,
* see the png_read_row() method below:
*/
number_passes = png_set_interlace_handling(png_ptr);
#else
number_passes = 1;
#endif /* PNG_READ_INTERLACING_SUPPORTED */
/* Optional call to gamma correct and add the background to the palette
* and update info structure. REQUIRED if you are expecting libpng to
* update the palette for you (ie you selected such a transform above).
*/
png_read_update_info(png_ptr, info_ptr);
/* Allocate the memory to hold the image using the fields of info_ptr. */
/* The easiest way to read the image: */
png_bytep row_pointers[height];
/* Clear the pointer array */
for (row = 0; row < height; row++)
row_pointers[row] = NULL;
for (row = 0; row < height; row++)
row_pointers[row] = png_malloc(png_ptr, png_get_rowbytes(png_ptr,
info_ptr));
/* Now it's time to read the image. One of these methods is REQUIRED */
#ifdef entire /* Read the entire image in one go */
png_read_image(png_ptr, row_pointers);
#else no_entire /* Read the image one or more scanlines at a time */
/* The other way to read images - deal with interlacing: */
for (pass = 0; pass < number_passes; pass++)
{
#ifdef single /* Read the image a single row at a time */
for (y = 0; y < height; y++)
{
png_read_rows(png_ptr, &row_pointers[y], NULL, 1);
}
#else no_single /* Read the image several rows at a time */
for (y = 0; y < height; y += number_of_rows)
{
#ifdef sparkle /* Read the image using the "sparkle" effect. */
png_read_rows(png_ptr, &row_pointers[y], NULL,
number_of_rows);
#else no_sparkle /* Read the image using the "rectangle" effect */
png_read_rows(png_ptr, NULL, &row_pointers[y],
number_of_rows);
#endif no_sparkle /* Use only one of these two methods */
}
/* If you want to display the image after every pass, do so here */
#endif no_single /* Use only one of these two methods */
}
#endif no_entire /* Use only one of these two methods */
/* Read rest of file, and get additional chunks in info_ptr - REQUIRED */
png_read_end(png_ptr, info_ptr);
#endif hilevel
/* At this point you have read the entire image */
/* Clean up after the read, and free any memory allocated - REQUIRED */
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
/* Close the file */
fclose(fp);
/* That's it */
return (OK);
}6、完整移植好的代码
创建fb_png.c,将其放在display目录下，修改该目录的子Makefile：
/************************************************************
FileName: fb_png.c
Author:Mr.Zhang Version:1.0 Date:2021/11/10
Description: This file is used to parse the data of PNG
images and load them into FB.
***********************************************************/
#include <stdio.h>
#include <string.h>
#include <config.h>
#include <fb.h>
#include <png.h>
#include <pngstruct.h>//需要去源码包中得到该文件
#include <pnginfo.h>//同上，这几个头文件顺序很重要
#define PNG_BYTES_TO_CHECK 8
//参数列表：path:要解析的图片的文件名
//函数功能 :判断一个图片文件是否为一个合法的png文件
//返回值 ：若是则返回0, 不是返回非0
static int is_png(const char *path)
{
char buf[PNG_BYTES_TO_CHECK];
FILE *fp = NULL;
/* Open the prospective PNG file. */
if ((fp = fopen(path, "rb")) == NULL)
return -1;
/* Read in some of the signature bytes */
if (fread(buf, 1, PNG_BYTES_TO_CHECK, fp) != PNG_BYTES_TO_CHECK)
return -1;
/* Compare the first PNG_BYTES_TO_CHECK bytes of the signature.
Return nonzero (true) if they match */
return(png_sig_cmp(buf, (png_size_t)0, PNG_BYTES_TO_CHECK));
}
//参数列表：pPic:描述图片相关信息的结构体指针
//函数功能 :解析一张png格式的图片并进行显示
//返回值 ：若失败则返回-1，若成功解码则返回0
static int png_analyze(pic_info *pPic)
{
FILE *fp = NULL;
png_structp png_ptr;
png_infop info_ptr;
int color_type;
png_bytep *row_pointers;
unsigned long len = 0;
int pos = 0;
int i = 0, j = 0;
//第一步：打开文件
if ((fp = fopen(pPic->pathname, "rb")) == NULL)
{
fprintf(stderr, "fopen %s error.\n", pPic->pathname);
return -1;
}
//第二步：相关数据结构实例化
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, 0, 0, 0);
if (png_ptr == NULL)
{
fclose(fp);
return -1;
}
/* Allocate/initialize the memory for image information. REQUIRED. */
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL)
{
fclose(fp);
png_destroy_read_struct(&png_ptr, NULL, NULL);
return -1;
}
//第三步：设置错误处理函数
if (setjmp(png_jmpbuf(png_ptr)))
{
/* Free all of the memory associated with the png_ptr and info_ptr */
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
fclose(fp);
/* If we get here, we had a problem reading the file */
return -1;
}
//第四步：将要解码的png图片的文件指针和png解码器绑定起来
png_init_io(png_ptr, fp);
//第五步：读取png图片的信息
png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_EXPAND | PNG_TRANSFORM_STRIP_ALPHA, NULL);
color_type = info_ptr->color_type;
DBG("color_type = %d.\n", color_type);
pPic->width = info_ptr->width;
pPic->height = info_ptr->height;
pPic->bpp = info_ptr->pixel_depth;
DBG("width = %u,height = %u, bpp = %u", pPic->width, pPic->height, pPic->bpp);
// 第六步: 读取真正的图像信息
row_pointers = png_get_rows(png_ptr,info_ptr);
// 只处理RGB24位真彩色图片，其他格式的图片不管
//图像数据移动到我们自己的buf中
if(color_type == PNG_COLOR_TYPE_RGB)
{
for(i=0; i<pPic->height; i++)
{
for(j=0; j<3*pPic->width; j+=3)
{
pPic->pData[pos++] = row_pointers[i][j+0];
pPic->pData[pos++] = row_pointers[i][j+1];
pPic->pData[pos++] = row_pointers[i][j+2];
}
}
}
//第七步：进行清理工作
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
//第八步：关闭打开的png文件
fclose(fp);
return 0;
}
//封装一个对外使用的jpg显示函数,本函数对外只需要一个jpg图片的
//pathname即可,那些复杂的显示数据处理过程在显示模块内部处理，
//正确显示图片返回0，错误返回-1
int display_png(const char*pathname)
{
int ret = -1;
pic_info png;
//第一步：检测给的图片是不是png图片
ret = is_png(pathname);
if (ret != 0)
{
return -1;
}
//第二步：解析jpg图片
png.pathname = pathname;
png.pData = rgb_buf;
png_analyze(&png);
//第三步：显示该图片
lcd_display_jpeg_png(0, 0, &png);
return 0;
}void lcd_display_jpeg_png(int x0, int y0, pic_info *pPic)
{
const unsigned char *pData = pPic->pData;//指针指向图像数据数组
unsigned int x, y, color, p = 0;
if ((pPic->bpp != 32) && (pPic->bpp != 24))
{
fprintf(stderr, "BPP %d is not support.\n", pPic->bpp);
}
for(y = y0; y < (pPic->height+y0); y++)
{
if(y > HEIGHT)
break;
for(x = x0; x < (pPic->width+x0); x++)
{
if(x > WIDTH)
{
p += 3;
continue;
}
color = ((pData[p+2] << 0)|(pData[p+1] << 8)|(pData[p+0] << 16));//得到图片一个像素点的颜色
*(pfb + y * WIDTH + x) = color;//将图片像素点的颜色填充到LCD屏幕的某个像素点上
p += 3;
}
}
DBG("lcd_display_picture ending.\n");
}