为Evince-PDF添加灰色背景

Ubuntu系统的默认PDF阅读软件

ubuntu操作系统自带了Evince-PDF阅读器，其优点是简单便捷，不过与其他常用的PDF阅读器相比，缺少很多定制化的选项，例如为避免眼睛疲劳的替换阅读界面背景颜色的功能。使用GNU/Linux操作系统的一个优势是，可以获取从内核至应用各个层面的软件源码，并加以修改后重新编译，可以方便地实现所需的新功能。笔者在该文章中记录了为Evince阅读器添加灰色背景的操作过程。作为对比，在添加该功能之前，笔者打开SystemTap教程文档的背景如下：

修改之后，在Evince的选项中使能Night Mode，阅读的背景会被修改为：

获取Evince的源代码

笔者使用的Ubuntu系统版本为20.04。获取Evince源码并能够编译，需要做一些准备工作。首先，将/etc/apt/sources.list中的# deb-src前面的#号删除：

sudo sed -e 's/# deb-src/deb-src/g' -i /etc/apt/sources.list
sudo apt-get update

之后，安装重新编译Evince所需的依赖和系统软件：

sudo apt-get install build-essential libtool autoconf make pbuilder
sudo apt-get build-dep evince

最后，下载Evince的源码包：

mkdir ~/evince-rebuild
cd ~/evince-rebuild
apt-get source evince

修改Evince源码以加载动态库

Evince有一个Night Mode的选项，可以将界面的亮度反转。其实现比较简单，是使用到了“差异算子”，将界面与最大亮度值求差：

/* libdocument/ev-document-misc.c */
void ev_document_misc_invert_surface (cairo_surface_t *surface) {    cairo_t *cr;    cr = cairo_create (surface);    /* white + DIFFERENCE -> invert */    cairo_set_operator (cr, CAIRO_OPERATOR_DIFFERENCE);    cairo_set_source_rgb (cr, 1., 1., 1.);    cairo_paint(cr);    cairo_destroy (cr);    
} 

笔者在实现该功能时，考虑到替换该函数的实现，就可以修改阅读界面的背景。不过，笔者没有在该函数中直接操作cairo_surface_t对象，而是通过加载动态库实现的，这样Evince只需被重新编译一次。笔者对该代码的修改如下：

diff --git a/libdocument/ev-document-misc.c b/libdocument/ev-document-misc.c
index 1d0acc5..da6c6ad 100644
--- a/libdocument/ev-document-misc.c
+++ b/libdocument/ev-document-misc.c
@@ -27,6 +27,9 @@#include "ev-document-misc.h"+#include 
+#include 
+/* Returns a new GdkPixbuf that is suitable for placing in the thumbnail view.* It is four pixels wider and taller than the source.  If source_pixbuf is not* NULL, then it will fill the return pixbuf with the contents of
@@ -463,10 +466,32 @@ ev_document_misc_surface_rotate_and_scale (cairo_surface_t *surface,return new_surface;}+typedef void (* ext_invert_func)(cairo_surface_t *);
+voidev_document_misc_invert_surface (cairo_surface_t *surface) {cairo_t *cr;
-
+       void * invhdl;
+       struct stat invst;
+       ext_invert_func extfunc;
+       const char * invlib = "/usr/lib/evince_invert.so";
+
+       if (stat(invlib, &invst) == -1)
+               goto next;
+       if (S_ISREG(invst.st_mode) == 0 || invst.st_size == 0)
+               goto next;
+       invhdl = dlopen(invlib, RTLD_NODELETE | RTLD_NOW);
+       if (invhdl == NULL)
+               goto next;
+       extfunc = (ext_invert_func) dlsym(invhdl, "external_evince_invert");
+       if (extfunc == NULL) {
+               dlclose(invhdl);
+               goto next;
+       }
+       extfunc(surface);
+       dlclose(invhdl);
+       return;
+next:cr = cairo_create (surface);/* white + DIFFERENCE -> invert */

重新编译Evince之前，需要提交以上修改：

dpkg-source --commit

dpkg-source --commit需要交互操作（可带一些命令行参数，非交互），过程如下：

~/evince-rebuild/evince-3.36.10$ dpkg-source --commit
dpkg-source: info: using patch list from debian/patches/series
dpkg-source: info: local changes detected, the modified files are:evince-3.36.10/libdocument/ev-document-misc.c
Enter the desired patch name: load-external-invert-library
dpkg-source: info: local changes have been recorded in a new patch: evince-3.36.10/debian/patches/load-external-invert-library

接下来，执行编译操作：

debuild -S # 请忽略 debsign相关的错误：debsign: gpg error occurred
debuild # 编译链接过程会出错

上面的debuild会链接出错，原因是我们加入了dlopen相关的调用：

./libdocument/ev-document-misc.c:483: undefined reference to `dlopen'
/usr/bin/ld: ./libdocument/ev-document-misc.c:486: undefined reference to `dlsym'
/usr/bin/ld: ./libdocument/ev-document-misc.c:492: undefined reference to `dlclose'
/usr/bin/ld: ./libdocument/ev-document-misc.c:488: undefined reference to `dlclose'
collect2: error: ld returned 1 exit status
make[4]: *** [Makefile:787: libevdocument3.la] Error 1

修改libdocument/Makefile的第787行，加入链接选项-ldl，之后执行make继续编译，将编译得到的动态库libevdocument3.so.4.0.0替换到系统路径下：

sed -e '787s/$/ -ldl/' -i libdocument/Makefile
make
sudo cp -v ./libdocument/.libs/libevdocument3.so.4.0.0 /usr/lib/x86_64-linux-gnu/

编写evince_invert.so动态库修改界面背景

笔者编写了一个简单的动态库，用于将Evince阅读界面的白色背景替换为灰色。笔者实现的灰度转换曲线如下：

其默认最大亮度为208，不过可以通过环境变量EVINCE_MAXVAL指定：

/* evince_invert.c* gcc -shared -o evince_invert.so evince_invert.c -Wall -O2 -fPIC -D_GNU_SOURCE -std=c99 -lcairo -lm*/
#include 
#include 
#include 
#include 
#include extern void external_evince_invert(cairo_surface_t * surf);
extern void evince_invert_init(void) __attribute__((constructor));#define EVINCE_GRAY_NUM 256
static unsigned char invert_map[EVINCE_GRAY_NUM];
static unsigned char rgb_map[EVINCE_GRAY_NUM][EVINCE_GRAY_NUM];void external_evince_invert(cairo_surface_t * surf)
{int pixelsize;cairo_format_t cfmt;unsigned char * pdat;int iwid, ihei, istr, idx, jdx;cfmt = cairo_image_surface_get_format(surf);if (cfmt == CAIRO_FORMAT_ARGB32)pixelsize = 0x4;else if (cfmt == CAIRO_FORMAT_RGB24)pixelsize = 0x3;else {fprintf(stderr, "Unknown cairo surface format: %#x\n",(unsigned int) cfmt);fflush(stderr);return;}pdat = cairo_image_surface_get_data(surf);if (pdat == NULL)return;iwid = cairo_image_surface_get_width(surf);if (iwid <= 0)return;ihei = cairo_image_surface_get_height(surf);if (ihei <= 0)return;istr = cairo_image_surface_get_stride(surf);if (istr < (pixelsize * iwid))return;for (jdx = 0; jdx < ihei; ++jdx) {unsigned char * pd = pdat;for (idx = 0; idx < iwid; ++idx) {unsigned int newgray, gray;unsigned int red, blue, green;blue  = (unsigned int) pd[0];green = (unsigned int) pd[1];red   = (unsigned int) pd[2];/** Y = 0.2126 * R + 0.7152 * G + 0.0722 * B** 0.2126 * 65536 = 13932.9536 * 0.7152 * 65536 = 46871.3472* 0.0722 * 65536 = 4731.6992*/gray  = (red * 13933 + green * 46871 + blue * 4732) >> 16;
#if 0 /* impossible, as 13933 + 46871 + 4732 = 65536 */if (gray >= 0x100)gray = 0xff;
#endifnewgray = (unsigned int) invert_map[gray];if (newgray >= gray) {pd += pixelsize;continue;}/* important: `newgray is less than `gray: */
#if 1const unsigned char * rgbmap;rgbmap = rgb_map[gray];pd[0] = rgbmap[blue];pd[1] = rgbmap[green];pd[2] = rgbmap[red];
#elsepd[0] = (unsigned char) (blue * newgray / gray);pd[1] = (unsigned char) (green * newgray / gray);pd[2] = (unsigned char) (red * newgray / gray);
#endifpd += pixelsize;}pdat += istr;}
}#define EVINCE_MAXVAL_DEFAULT 208
void evince_invert_init(void)
{const char * evmax;int idx, jdx, maxval, step;const int half = EVINCE_GRAY_NUM / 2;maxval = EVINCE_MAXVAL_DEFAULT;evmax = getenv("EVINCE_MAXVAL");if (evmax != NULL) {maxval = (int) strtol(evmax, NULL, 0);if (maxval <= half || maxval >= EVINCE_GRAY_NUM)maxval = EVINCE_MAXVAL_DEFAULT;}for (idx = 0; idx < EVINCE_GRAY_NUM; ++idx)invert_map[idx] = (unsigned char) idx;/* NOTE: step is a positive integer: */step = maxval - half;for (idx = half; idx < EVINCE_GRAY_NUM; ++idx) {double dval = (double) half;double didx = (double) (idx - half);dval += step * log(1.0 + didx * (M_E - 1.0) / half);invert_map[idx] = (unsigned char) ((long) (dval + 0.5));}for (jdx = 0; jdx < EVINCE_GRAY_NUM; ++jdx) {int ngray;unsigned char * pgray;pgray = rgb_map[jdx];ngray = (int) invert_map[jdx];ngray &= 0xff;if (ngray >= jdx) {for (idx = 0; idx < EVINCE_GRAY_NUM; ++idx)*pgray++ = (unsigned char) idx;continue;}for (idx = 0; idx < EVINCE_GRAY_NUM; ++idx)*pgray++ = (unsigned char) (idx * ngray / jdx);}
}

按照注释的命令编译完成后，将evince_invert.so复制到/usr/lib/目录下，就可以通过Evince的Night Mode转换PDF的阅读界面为灰度背景了；该方法不会改变颜色的显示。不过该功能也有其缺陷，不建议反复多次设置Night Mode选项，否则界面会越来越暗。若有需要而不要重新编译Evince，可发邮件给笔者提供这两个动态库文件：xiaoqzye@qq.com。

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