【LittlevGL】看看LVGL8的draw

调用"lv_disp_drv_init"函数初始化一个显示设备驱动，一个显示设备驱动的结构体内容如下：

typedef struct _lv_disp_drv_t {lv_coord_t hor_res;         /**< Horizontal resolution.*/lv_coord_t ver_res;         /**< Vertical resolution.*/lv_coord_tphysical_hor_res;     /**< Horizontal resolution of the full / physical display. Set to -1 for fullscreen mode.*/lv_coord_tphysical_ver_res;     /**< Vertical resolution of the full / physical display. Set to -1 for fullscreen mode.*/lv_coord_toffset_x;             /**< Horizontal offset from the full / physical display. Set to 0 for fullscreen mode.*/lv_coord_t offset_y;             /**< Vertical offset from the full / physical display. Set to 0 for fullscreen mode.*//** Pointer to a buffer initialized with `lv_disp_draw_buf_init()`.* LVGL will use this buffer(s) to draw the screens contents*/lv_disp_draw_buf_t * draw_buf;uint32_t direct_mode : 1;        /**< 1: Use screen-sized buffers and draw to absolute coordinates*/uint32_t full_refresh : 1;       /**< 1: Always make the whole screen redrawn*/uint32_t sw_rotate : 1;          /**< 1: use software rotation (slower)*/uint32_t antialiasing : 1;       /**< 1: anti-aliasing is enabled on this display.*/uint32_t rotated : 2;            /**< 1: turn the display by 90 degree. @warning Does not update coordinates for you!*/uint32_t screen_transp : 1;      /**Handle if the screen doesn't have a solid (opa == LV_OPA_COVER) background.* Use only if required because it's slower.*/uint32_t dpi : 10;              /** DPI (dot per inch) of the display. Default value is `LV_DPI_DEF`.*//** MANDATORY: Write the internal buffer (draw_buf) to the display. 'lv_disp_flush_ready()' has to be* called when finished*/void (*flush_cb)(struct _lv_disp_drv_t * disp_drv, const lv_area_t * area, lv_color_t * color_p);/** OPTIONAL: Extend the invalidated areas to match with the display drivers requirements* E.g. round `y` to, 8, 16 ..) on a monochrome display*/void (*rounder_cb)(struct _lv_disp_drv_t * disp_drv, lv_area_t * area);/** OPTIONAL: Set a pixel in a buffer according to the special requirements of the display* Can be used for color format not supported in LittelvGL. E.g. 2 bit -> 4 gray scales* @note Much slower then drawing with supported color formats.*/void (*set_px_cb)(struct _lv_disp_drv_t * disp_drv, uint8_t * buf, lv_coord_t buf_w, lv_coord_t x, lv_coord_t y,lv_color_t color, lv_opa_t opa);void (*clear_cb)(struct _lv_disp_drv_t * disp_drv, uint8_t * buf, uint32_t size);/** OPTIONAL: Called after every refresh cycle to tell the rendering and flushing time + the* number of flushed pixels*/void (*monitor_cb)(struct _lv_disp_drv_t * disp_drv, uint32_t time, uint32_t px);/** OPTIONAL: Called periodically while lvgl waits for operation to be completed.* For example flushing or GPU* User can execute very simple tasks here or yield the task*/void (*wait_cb)(struct _lv_disp_drv_t * disp_drv);/** OPTIONAL: Called when lvgl needs any CPU cache that affects rendering to be cleaned*/void (*clean_dcache_cb)(struct _lv_disp_drv_t * disp_drv);/** OPTIONAL: called when driver parameters are updated */void (*drv_update_cb)(struct _lv_disp_drv_t * disp_drv);/** On CHROMA_KEYED images this color will be transparent.* `LV_COLOR_CHROMA_KEY` by default. (lv_conf.h)*/lv_color_t color_chroma_key;lv_draw_ctx_t * draw_ctx;void (*draw_ctx_init)(struct _lv_disp_drv_t * disp_drv, lv_draw_ctx_t * draw_ctx);void (*draw_ctx_deinit)(struct _lv_disp_drv_t * disp_drv, lv_draw_ctx_t * draw_ctx);size_t draw_ctx_size;#if LV_USE_USER_DATAvoid * user_data; /**< Custom display driver user data*/
#endif} lv_disp_drv_t;

前面的是一些常规的显示设置和显存写回调函数，后面有一个“draw_ctx”字段，这个是一个”绘制语境“结构体，里面包含了最常用几何图形、图片、字符的绘制函数：

typedef struct _lv_draw_ctx_t  {/***  Pointer to a buffer to draw into*/void * buf;/*** The position and size of `buf` (absolute coordinates)*/lv_area_t * buf_area;/*** The current clip area with absolute coordinates, always the same or smaller than `buf_area`*/const lv_area_t * clip_area;void (*draw_rect)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * dsc, const lv_area_t * coords);void (*draw_arc)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_arc_dsc_t * dsc, const lv_point_t * center,uint16_t radius,  uint16_t start_angle, uint16_t end_angle);void (*draw_img_decoded)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_img_dsc_t * dsc,const lv_area_t * coords, const uint8_t * map_p, lv_img_cf_t color_format);lv_res_t (*draw_img)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_img_dsc_t * draw_dsc,const lv_area_t * coords, const void * src);void (*draw_letter)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_label_dsc_t * dsc,  const lv_point_t * pos_p,uint32_t letter);void (*draw_line)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_line_dsc_t * dsc, const lv_point_t * point1,const lv_point_t * point2);void (*draw_polygon)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * draw_dsc,const lv_point_t * points, uint16_t point_cnt);/*** Replace the buffer with a rect without decoration like radius or borders*/void (*draw_bg)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * draw_dsc, const lv_area_t * coords);/*** Wait until all background operations are finished. (E.g. GPU operations)*/void (*wait_for_finish)(struct _lv_draw_ctx_t * draw_ctx);/*** Copy an area from buffer to an other* @param draw_ctx      pointer to a draw context* @param dest_buf      copy the buffer into this buffer* @param dest_stride   the width of the dest_buf in pixels* @param dest_area     the destination area* @param src_buf       copy from this buffer* @param src_stride    the width of src_buf in pixels* @param src_area      the source area.** @note dest_area and src_area must have the same width and height*       but can have different x and y position.* @note dest_area and src_area must be clipped to the real dimensions of the buffers*/void (*buffer_copy)(struct _lv_draw_ctx_t * draw_ctx, void * dest_buf, lv_coord_t dest_stride,const lv_area_t * dest_area,void * src_buf, lv_coord_t src_stride, const lv_area_t * src_area);
#if LV_USE_USER_DATAvoid * user_data;
#endif} lv_draw_ctx_t;

“draw_ctx”后面还有一个“draw_ctx_init”字段，在"lv_disp_drv_init"函数中，如果没有使用硬件GPU的话，会被默认初始化成“lv_draw_sw_init_ctx”函数，这个函数中会将“draw_ctx”字段中的所有绘制回调函数都初始化成软绘（软件绘制）函数，软绘函数都是使用CPU进行绘制处理，如果有GPU的话，可以使用GPU绘制函数初始化“draw_ctx”。

以“draw_ctx”里的“draw_letter”函数为例，这是字符绘制函数，软绘字符绘制函数为“lv_draw_sw_letter”，在这个函数里面会调用“lv_font_get_glyph_dsc”函数用以获取glyph descriptor（字符描述符），如果获取描述符失败，则会显示一个placeholder（占位符，是一个方框框，如下图），表示这个字符是字库里面没有的字符。否则，就会从字库里面获取字符的点阵数据并绘制。

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【LittlevGL】看看LVGL8的draw_ctx

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