How to scale a bmp image?

oldersu · June 17, 2019, 7:26am

Hi,
Now i want to scale a bmp image with littlevgl , do you have any suggest for me？Thanks.

xiongyu · June 17, 2019, 8:02am

I also want to know if there is a dynamic way to change the size of the picture.

embeddedt · June 17, 2019, 12:49pm

You might be able to use the lv_canvas object with lv_canvas_rotate(img, canvas_obj, 0, 0, 0, 0). That could scale the image without rotating.

kisvegabor · June 18, 2019, 3:45am

How do you mean it? rotate just rotates not scales.

embeddedt · June 18, 2019, 2:45pm

Well, maybe I’m missing something, but it looks like it is prepared for differing widths and heights. In addition, upscaling is mentioned:

github.com

littlevgl/lvgl/blob/b9081cacef369feac1327c65aa61bad99f694a7e/src/lv_objx/lv_canvas.c#L275-L290


int32_t img_width   = img->header.w;
int32_t img_height  = img->header.h;
int32_t dest_width  = ext_dst->dsc.header.w;
int32_t dest_height = ext_dst->dsc.header.h;


int32_t x;
int32_t y;
for(x = -offset_x; x < dest_width - offset_x; x++) {
    for(y = -offset_y; y < dest_height - offset_y; y++) {
        /*Get the target point relative coordinates to the pivot*/
        int32_t xt = x - pivot_x;
        int32_t yt = y - pivot_y;


        /*Get the source pixel from the upscaled image*/
        int32_t xs = ((cosma * xt - sinma * yt) >> (LV_TRIGO_SHIFT - 8)) + pivot_x * 256;
        int32_t ys = ((sinma * xt + cosma * yt) >> (LV_TRIGO_SHIFT - 8)) + pivot_y * 256;

kisvegabor · June 20, 2019, 11:50am

@embeddedt
Sorry, it seems I forget to answer it.

Upscale in this context means to virtually upscale the image to deal with 256x256 subpixels / pixel. When getting the source pixel related to a destination pixel the “upscaled” image with subpixels is rotated. The rotation tells which subpixel of the source image corresponds to the destination pixels. It gives the opportunity for interpolation. E.g. if the very edge of the source pixel needs to be used in the destination pixel then its color is mixed with the neighbor pixel.

embeddedt · June 20, 2019, 1:19pm

@kisvegabor Thanks! I learned something new today.

embeddedt · July 27, 2019, 8:34pm

For any future readers, the current situation (as of v6.0) is that you can’t scale a picture. This might have changed in a future version, however.

embeddedt · November 8, 2019, 11:09pm

dev-7.0 has added an experimental lv_img_set_zoom function.

incity · July 4, 2020, 6:59am

@kisvegabor @embeddedt how to scale image just in one direction?

embeddedt · July 4, 2020, 11:32am

Only proportional scaling is supported at the moment. You can’t scale the image in one direction.