void ScreenRecoveryUI::Init()
{
    gr_init();

    gr_font_size(&char_width, &char_height);

    text_col = text_row = 0;
    text_rows = gr_fb_height() / char_height;
    if (text_rows > kMaxRows) text_rows = kMaxRows;
    text_top = 1;

    text_cols = gr_fb_width() / char_width;
    if (text_cols > kMaxCols - 1) text_cols = kMaxCols - 1;

    backgroundIcon[NONE] = NULL;
    LoadBitmapArray("icon_installing", &installing_frames, &installation);
    backgroundIcon[INSTALLING_UPDATE] = installing_frames ? installation[0] : NULL;
    backgroundIcon[ERASING] = backgroundIcon[INSTALLING_UPDATE];
    LoadBitmap("icon_error", &backgroundIcon[ERROR]);
    backgroundIcon[NO_COMMAND] = backgroundIcon[ERROR];

    LoadBitmap("progress_empty", &progressBarEmpty);
    LoadBitmap("progress_fill", &progressBarFill);
    LoadBitmap("stage_empty", &stageMarkerEmpty);
    LoadBitmap("stage_fill", &stageMarkerFill);

    LoadLocalizedBitmap("installing_text", &backgroundText[INSTALLING_UPDATE]);
    LoadLocalizedBitmap("erasing_text", &backgroundText[ERASING]);
    LoadLocalizedBitmap("no_command_text", &backgroundText[NO_COMMAND]);
    LoadLocalizedBitmap("error_text", &backgroundText[ERROR]);

    pthread_create(&progress_t, NULL, progress_thread, NULL);

    RecoveryUI::Init();
}

efault images are available in different densities and are located inbootable/recovery/res$DENSITY/images 
(e.g., bootable/recovery/res-hdpi/images). To use a static image during installation, 
you need only provide the icon_installing.png image and set the number of frames in the animation to 0 
(the error icon is not animated; it is always a static image).
// 以上文档意思就是说，Android5.x以上的版本，机器人的动画是PNG图片和帧动画组成的，
//我们可以使用recovery目录下的interlace-frames.py这个python脚本来进行合成，
//具体的合成方法需要使用recovery代码中的一个python合成工具。
//我就可以把android原生态的动画给换了，因为这个机器人实在是丑。

源码如下：

# Copyright (C) 2014 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Script to take a set of frames (PNG files) for a recovery animation
and turn it into a single output image which contains the input frames
interlaced by row.  Run with the names of all the input frames on the
command line, in order, followed by the name of the output file."""
import sys
try:
  import Image
  import PngImagePlugin
except ImportError:
  print "This script requires the Python Imaging Library to be installed."
  sys.exit(1)
frames = [Image.open(fn).convert("RGB") for fn in sys.argv[1:-1]]
assert len(frames) > 0, "Must have at least one input frame."
sizes = set()
for fr in frames:
  sizes.add(fr.size)
assert len(sizes) == 1, "All input images must have the same size."
w, h = sizes.pop()
N = len(frames)
out = Image.new("RGB", (w, h*N))
for j in range(h):
  for i in range(w):
    for fn, f in enumerate(frames):
      out.putpixel((i, j*N+fn), f.getpixel((i, j)))
# When loading this image, the graphics library expects to find a text
# chunk that specifies how many frames this animation represents.  If
# you post-process the output of this script with some kind of
# optimizer tool (eg pngcrush or zopflipng) make sure that your
# optimizer preserves this text chunk.
meta = PngImagePlugin.PngInfo()
meta.add_text("Frames", str(N))
out.save(sys.argv[-1], pnginfo=meta)

这也就是为什么，调用这张图片需要用到LoadBitmapArray这个函数的原因。

void ScreenRecoveryUI::LoadBitmapArray(const char* filename, int* frames, gr_surface** surface) {
    int result = res_create_multi_display_surface(filename, frames, surface);
    if (result < 0) {
        LOGE("missing bitmap %s\n(Code %d)\n", filename, result);
    }
}

调完这个函数后会调用resources.cpp中的res_create_multi_display_surface函数用于显示，源码如下：

int res_create_multi_display_surface(const char* name, int* frames, GRSurface*** pSurface) {
    GRSurface** surface = NULL;
    int result = 0;
    png_structp png_ptr = NULL;
    png_infop info_ptr = NULL;
    png_uint_32 width, height;
    png_byte channels;
    int i;
    png_textp text;
    int num_text;
    unsigned char* p_row;
    unsigned int y;

    *pSurface = NULL;
    *frames = -1;

    result = open_png(name, &png_ptr, &info_ptr, &width, &height, &channels);
    if (result < 0) return result;

    *frames = 1;
    if (png_get_text(png_ptr, info_ptr, &text, &num_text)) {
        for (i = 0; i < num_text; ++i) {
            if (text[i].key && strcmp(text[i].key, "Frames") == 0 && text[i].text) {
                *frames = atoi(text[i].text);
                break;
            }
        }
        printf("  found frames = %d\n", *frames);
    }

    if (height % *frames != 0) {
        printf("bad height (%d) for frame count (%d)\n", height, *frames);
        result = -9;
        goto exit;
    }

    surface = reinterpret_cast<GRSurface**>(malloc(*frames * sizeof(GRSurface*)));
    if (surface == NULL) {
        result = -8;
        goto exit;
    }
    for (i = 0; i < *frames; ++i) {
        surface[i] = init_display_surface(width, height / *frames);
        if (surface[i] == NULL) {
            result = -8;
            goto exit;
        }
    }

#if defined(RECOVERY_ABGR) || defined(RECOVERY_BGRA)
    png_set_bgr(png_ptr);
#endif

    p_row = reinterpret_cast<unsigned char*>(malloc(width * 4));
    for (y = 0; y < height; ++y) {
        png_read_row(png_ptr, p_row, NULL);
        int frame = y % *frames;
        unsigned char* out_row = surface[frame]->data +
            (y / *frames) * surface[frame]->row_bytes;
        transform_rgb_to_draw(p_row, out_row, channels, width);
    }
    free(p_row);

    *pSurface = reinterpret_cast<GRSurface**>(surface);

exit:
    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);

    if (result < 0) {
        if (surface) {
            for (i = 0; i < *frames; ++i) {
                if (surface[i]) free(surface[i]);
            }
            free(surface);
        }
    }
    return result;
}

其余的和text无关图片，会用到LoadBitmap这个函数:

void ScreenRecoveryUI::LoadBitmapArray(const char* filename, int* frames, gr_surface** surface) {
    int result = res_create_multi_display_surface(filename, frames, surface);
    if (result < 0) {
        LOGE("missing bitmap %s\n(Code %d)\n", filename, result);
    }
}

同样调用到以下函数:

int res_create_multi_display_surface(const char* name, int* frames, GRSurface*** pSurface) {
    GRSurface** surface = NULL;
    int result = 0;
    png_structp png_ptr = NULL;
    png_infop info_ptr = NULL;
    png_uint_32 width, height;
    png_byte channels;
    int i;
    png_textp text;
    int num_text;
    unsigned char* p_row;
    unsigned int y;

    *pSurface = NULL;
    *frames = -1;

    result = open_png(name, &png_ptr, &info_ptr, &width, &height, &channels);
    if (result < 0) return result;

    *frames = 1;
    if (png_get_text(png_ptr, info_ptr, &text, &num_text)) {
        for (i = 0; i < num_text; ++i) {
            if (text[i].key && strcmp(text[i].key, "Frames") == 0 && text[i].text) {
                *frames = atoi(text[i].text);
                break;
            }
        }
        printf("  found frames = %d\n", *frames);
    }

    if (height % *frames != 0) {
        printf("bad height (%d) for frame count (%d)\n", height, *frames);
        result = -9;
        goto exit;
    }

    surface = reinterpret_cast<GRSurface**>(malloc(*frames * sizeof(GRSurface*)));
    if (surface == NULL) {
        result = -8;
        goto exit;
    }
    for (i = 0; i < *frames; ++i) {
        surface[i] = init_display_surface(width, height / *frames);
        if (surface[i] == NULL) {
            result = -8;
            goto exit;
        }
    }

#if defined(RECOVERY_ABGR) || defined(RECOVERY_BGRA)
    png_set_bgr(png_ptr);
#endif

    p_row = reinterpret_cast<unsigned char*>(malloc(width * 4));
    for (y = 0; y < height; ++y) {
        png_read_row(png_ptr, p_row, NULL);
        int frame = y % *frames;
        unsigned char* out_row = surface[frame]->data +
            (y / *frames) * surface[frame]->row_bytes;
        transform_rgb_to_draw(p_row, out_row, channels, width);
    }
    free(p_row);

    *pSurface = reinterpret_cast<GRSurface**>(surface);

exit:
    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);

    if (result < 0) {
        if (surface) {
            for (i = 0; i < *frames; ++i) {
                if (surface[i]) free(surface[i]);
            }
            free(surface);
        }
    }
    return result;
}

关于图片我们大概都知道怎么来显示的了，所以，现在我们可以替换Android原生态中的图片，换成我们自己的图片，

当然，也不是什么图都可以的，在recovery中，所有的png图片必须是RGB且不带且不能带alhpa通道信息。
关于这一点，我们可以看open_png这个函数:

static int open_png(const char* name, png_structp* png_ptr, png_infop* info_ptr,
                    png_uint_32* width, png_uint_32* height, png_byte* channels) {
    char resPath[256];
    unsigned char header[8];
    int result = 0;
    int color_type, bit_depth;
    size_t bytesRead;

    snprintf(resPath, sizeof(resPath)-1, "/res/images/%s.png", name);
    resPath[sizeof(resPath)-1] = '\0';
    FILE* fp = fopen(resPath, "rb");
    if (fp == NULL) {
        result = -1;
        goto exit;
    }

    bytesRead = fread(header, 1, sizeof(header), fp);
    if (bytesRead != sizeof(header)) {
        result = -2;
        goto exit;
    }

    if (png_sig_cmp(header, 0, sizeof(header))) {
        result = -3;
        goto exit;
    }

    *png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!*png_ptr) {
        result = -4;
        goto exit;
    }

    *info_ptr = png_create_info_struct(*png_ptr);
    if (!*info_ptr) {
        result = -5;
        goto exit;
    }

    if (setjmp(png_jmpbuf(*png_ptr))) {
        result = -6;
        goto exit;
    }

    png_init_io(*png_ptr, fp);
    png_set_sig_bytes(*png_ptr, sizeof(header));
    png_read_info(*png_ptr, *info_ptr);

    png_get_IHDR(*png_ptr, *info_ptr, width, height, &bit_depth,
            &color_type, NULL, NULL, NULL);

    *channels = png_get_channels(*png_ptr, *info_ptr);

    if (bit_depth == 8 && *channels == 3 && color_type == PNG_COLOR_TYPE_RGB) {
        // 8-bit RGB images: great, nothing to do.
    } else if (bit_depth <= 8 && *channels == 1 && color_type == PNG_COLOR_TYPE_GRAY) {
        // 1-, 2-, 4-, or 8-bit gray images: expand to 8-bit gray.
        png_set_expand_gray_1_2_4_to_8(*png_ptr);
    } else if (bit_depth <= 8 && *channels == 1 && color_type == PNG_COLOR_TYPE_PALETTE) {
        // paletted images: expand to 8-bit RGB.  Note that we DON'T
        // currently expand the tRNS chunk (if any) to an alpha
        // channel, because minui doesn't support alpha channels in
        // general.
        png_set_palette_to_rgb(*png_ptr);
        *channels = 3;
    } else {
        fprintf(stderr, "minui doesn't support PNG depth %d channels %d color_type %d\n",
                bit_depth, *channels, color_type);
        result = -7;
        goto exit;
    }

    return result;

  exit:
    if (result < 0) {
        png_destroy_read_struct(png_ptr, info_ptr, NULL);
    }
    if (fp != NULL) {
        fclose(fp);
    }

    return result;
}

在代码中，我们可以看到如下：

 if (bit_depth == 8 && *channels == 3 && color_type == PNG_COLOR_TYPE_RGB) {
        // 8-bit RGB images: great, nothing to do.
    } else if (bit_depth <= 8 && *channels == 1 && color_type == PNG_COLOR_TYPE_GRAY) {
        // 1-, 2-, 4-, or 8-bit gray images: expand to 8-bit gray.
        png_set_expand_gray_1_2_4_to_8(*png_ptr);
    } else if (bit_depth <= 8 && *channels == 1 && color_type == PNG_COLOR_TYPE_PALETTE) {
        // paletted images: expand to 8-bit RGB.  Note that we DON'T
        // currently expand the tRNS chunk (if any) to an alpha
        // channel, because minui doesn't support alpha channels in
        // general.
        png_set_palette_to_rgb(*png_ptr);
        *channels = 3;
    } else {
        fprintf(stderr, "minui doesn't support PNG depth %d channels %d color_type %d\n",
                bit_depth, *channels, color_type);
        result = -7;
        goto exit;
    }

以下参考一位网友给出的答案。
这个函数将图片文件的数据读取到内存，我在其中输出了一些调试信息，输出图片的 color_type, channels 等信息。
查看LOG发现，android原生的图片 channels == 3，channels 即色彩通道个数，等于 3 的话，意味着只有 R,G,B 三个通道的信息，
没有 ALPHA 通道信息！这段代码的逻辑是如果channels 不等于3， 则按channels = 1 来处理，即灰度图。
美工给的图片是带 alpha通道信息的，即channels = 4，被当成灰度图像来处理了，怪不得显示的效果是灰度图像。
我一直以为 png 图像就只有一种格式，都是带有 alpha通道的。。。
使用图像处理工具(photoshop 或者 gimp)，将美工给的图片去掉 alpha 通道信息，再替换recovery 的图片，编译，替换recovery.img ，
reboot -r 。图片终于正常显示啦。

在下一节里，我们将继续分析UI显示....