How to use "png_set_dither"

[Zhang Ping]

My graphic device's driver is vga mode and can only support 8bpp. Also, when I want to show a image, I must give the image's palette to the graphic device's driver.

 

In the circumstances, I can use libpng to show a png image which color type is "PNG_COLOR_TYPE_PALETTE", and it performs well.

 

But When I want to show one which color type is "PNG_COLOR_TYPE_RGB" or "PNG_COLOR_TYPE_RGB_ALPHA" and it do not have palletes, I got some problems.

 

I noticed that this function "png_set_dither" can reduce an RGB file to a paletted file. So I created a pallete "std_color_cube" which had 216 entries. I added  "png_set_dither" into

"png_read_png" function.

 

....

png_read_info(png_ptr, info_ptr);

if (info_ptr->color_type ==PNG_COLOR_TYPE_RGB)
{
        printf("From libpng:It is a color_type_rgb!\n");

        png_set_dither(png_ptr, std_color_cube, 216, 216,
            NULL, 0);

 

}

....

 

But when I tried to show a png image which color type was "PNG_COLOR_TYPE_RGB", things looked bad.

I delete this "png_set_dither(png_ptr, std_color_cube, 216, 216,NULL, 0);", and the image look like the one previous.

 

Is there something I overlook or I wrongly used the function "png_set_dither"?

 

I have this abstacle for a long time, and do not know how to deal with it. Is there anybody can help me? Thank you very very much!

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[Greg Roelofs]

> But when I tried to show a png image which color type was
> "PNG_COLOR_TYPE_RGB", things looked bad.
> I delete this "png_set_dither(png_ptr, std_color_cube, 216, 216,NULL, 0);",
> and the image look like the one previous.

> Is there something I overlook or I wrongly used the function
> "png_set_dither"?

To the best of my knowledge, libpng has never had a high-quality dithering
function (unlike, say, libjpeg).  I think that was more of an early design
goal/hope than a serious feature, though I could be wrong about that.
Virtually every image-decoder I'm aware of has its own ditherer and just
uses image codecs to produce native pixels.

Greg

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[Cosmin Truta]

On Sat, Dec 6, 2008 at 1:41 PM, Greg Roelofs wrote:

> To the best of my knowledge, libpng has never had a high-quality dithering
> function (unlike, say, libjpeg).  I think that was more of an early design
> goal/hope than a serious feature, though I could be wrong about that.
> Virtually every image-decoder I'm aware of has its own ditherer and just
> uses image codecs to produce native pixels.

If libpng dithering is bad, and noone complained about it in years, I
am inclined to believe that noone has used it yet. (Is it being used
in pngquant, BTW?)
And if it is indeed unused, maybe removing it from libpng-1.4 wouldn't
be a bad idea, IMO.

Best regards,
Cosmin

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[Glenn Randers-Pehrson]

On Mon, Dec 8, 2008 at 3:24 PM, Cosmin Truta <cosmin@...> wrote:

> On Sat, Dec 6, 2008 at 1:41 PM, Greg Roelofs wrote:
>
>> To the best of my knowledge, libpng has never had a high-quality dithering
>> function (unlike, say, libjpeg).  I think that was more of an early design
>> goal/hope than a serious feature, though I could be wrong about that.
>> Virtually every image-decoder I'm aware of has its own ditherer and just
>> uses image codecs to produce native pixels.
>
> If libpng dithering is bad, and noone complained about it in years, I
> am inclined to believe that noone has used it yet. (Is it being used
> in pngquant, BTW?)
> And if it is indeed unused, maybe removing it from libpng-1.4 wouldn't
> be a bad idea, IMO.

OK, it's turned off by default in libpng-1.4.0beta47.

Glenn

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