PNG decoding - Why red and blue are swapped?

Bug (archived)
1

Hi!
Looking at png_decoder.c, I can see how a PNG file is decoded by a call to lodepng_decode32.

What I don’t understand, is why png_decoder then swaps red and blue (when using 32bit color depth):

static void convert_color_depth(uint8_t * img, uint32_t px_cnt)
{
#if LV_COLOR_DEPTH == 32
    lv_color32_t * img_argb = (lv_color32_t*)img;
    lv_color_t c;
    lv_color_t * img_c = (lv_color_t *) img;
    uint32_t i;
    for(i = 0; i < px_cnt; i++) {
        c = LV_COLOR_MAKE(img_argb[i].red, img_argb[i].green, img_argb[i].blue);
        img_c[i].red = c.blue;
        img_c[i].blue = c.red;
    }

How is Littlevgl raw image format different from “RGBA” that lodepng generates? And why did you choose using this different raw image format on Littlevgl?

Thanks!

Amir

2

Hi,

See this comment.

LittlevGL has ARGB format which is the most common as I saw so far. PNG has RGBA in big endian. When it is interpreted as little-endian it becomes ABGR (compared LittlevGL’s ARGB).

To make it clearer:


    lv_color_t c = LV_COLOR_MAKE(0x11,0x22,0x33);
    uint8_t * b = (uint8_t *)&c;
    printf("0x%02x, 0x%02x, 0x%02x, 0x%02x\n", b[0], b[1], b[2], b[3]);
    printf("0x%08x\n", c.full);

0x33, 0x22, 0x11, 0xff
0xff112233

So if interpreted as uint32_t is looks like ARGB but because of the little endian interpretation, it is stored as B, G, R, A.

The current color format was compatible with Linux frame buffer (/dev/fb0) so I thought it’s should be a good choice. (It might be different on other machines but there were complains so far)

3

I see, thanks to the explanation @kisvegabor!

I tried taking the data produced by lodepng_decode32 without any manipulations. So as expected, blue and red are swapped. But what about the alpha?

See the image below - I would expect a transparent background instead of a black background.

image

Any idea?

4

I’ve tested in on dev-6.0 at it’s working for me. Here is the v6.0 compatible code:
png_decoder_test.c (125.1 KB)

I’ve didn’t tried it v5.3 for a while but it should the same way.

5

Oh - it was my mistake.
I used LV_IMG_CF_TRUE_COLOR instead of LV_IMG_CF_TRUE_COLOR_ALPHA.
Now it work just fine. Thanks!

6

Glad to hear that! :slight_smile:

7

When I was compiler this demo , I got a error LV_IMG_FORMAT_TRUE_COLOR_ALPHA was undeclared, I can’t search this macro in the lvgl.

8

It’s defined here: https://github.com/littlevgl/lvgl/blob/7c90b84560aa8cea64c1f8f08b4398d3fb29e7da/src/lv_draw/lv_img_decoder.h#L83

9

WOW, Thanks for your reply.
But LV_IMG_FORMAT_TRUE_COLOR_ALPHA is equal to the LV_IMG_CF_TRUE_COLOR_ALPHA ？anyway I’ll give a try.

10

I think CF stands for “Color Format”, all that enum members start with LV_IMG_CF_