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[fix] DjVu gamma correction #921

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merged 3 commits into from
Jun 10, 2019
Merged

[fix] DjVu gamma correction #921

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dd170ab
[fix] DjVu gamma correction
Frenzie Jun 10, 2019
b082d1c
whitespace typo
Frenzie Jun 10, 2019
553189a
cap values before passing 'em on
Frenzie Jun 10, 2019
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33 changes: 16 additions & 17 deletions djvu.c
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,8 @@
#include "koptcrop.h"
#include "djvu.h"

#define ABS(x) ((x<0)?(-x):(x))

#define MIN(a, b) ((a) < (b) ? (a) : (b))
#define MAX(a, b) ((a) > (b) ? (a) : (b))

Expand Down Expand Up @@ -654,6 +656,20 @@ static int drawPage(lua_State *L) {
BlitBuffer *bb = (BlitBuffer*) lua_topointer(L, 3);
ddjvu_render_mode_t djvu_render_mode = (int) luaL_checkint(L, 6);
ddjvu_rect_t pagerect, renderrect;
// map KOReader gamma to djvulibre gamma
// djvulibre goes from 0.5 to 5.0
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What happens when you give it < 0.5 or > 5.0 ? which can happen if one hacked koptoptions.lua to extend boundary values.

Which it seems I once did, I have that in my own patch:

                 name_text = S.CONTRAST,
                 buttonprogress = true,
-                values = {1/0.8, 1/1.0, 1/1.5, 1/2.0, 1/3.0, 1/4.0, 1/6.0, 1/9.0},
+                -- for pdf reflowing mode (kopt_contrast):
+                values = {1/0.8, 1/1.0, 1/1.5, 1/2.0, 1/4.0, 1/6.0, 1/10.0, 1/50.0},
                 default_pos = 2,
                 default_value = DKOPTREADER_CONFIG_CONTRAST,
                 event = "GammaUpdate",
-                args = {0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 9.0},
-                labels = {0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 9.0},
+                -- for pdf non-reflowing mode (mupdf):
+                args =   {0.8, 1.0, 1.5, 2.0, 4.0, 6.0, 10.0, 50.0},
+                labels = {0.8, 1.0, 1.5, 2.0, 4.0, 6.0, 10.0, 50.0},
+                -- see https://github.com/koreader/koreader/issues/1299#issuecomment-65183895
                 name_text_hold_callback = optionsutil.showValues,

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(I mean: I don't mind having my higher values limited and causing no change from previous values - just want to be sure there could be no crash or strange things.)

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djvulibre ignores out of bound values.

https://gitlab.com/koreader/djvulibre/blob/9658b01431cd7ff6344d7787f855179e73fe81a7/libdjvu/ddjvuapi.cpp#L2166

(One wonders if it wouldn't make more sense to cap them instead of to ignore them completely, but that aside.)

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it wouldn't make more sense to cap them instead of to ignore them

I think you should cap them, after your gamma = A - B*gamma s, as they could give out of bound values, so better to have them capped that suddenly getting back with no gamma change.

double gamma = ABS(dc->gamma); // not sure why, but 1 is given as -1?
if (gamma == 2) {
// default
gamma = 2.2;
} else if (gamma < 2) {
// with this function, 0.8 = 5, 2 = 2.2
gamma = 6.86666 - 2.33333 * gamma;
} else if (gamma > 2) {
// with this function, 9 = 0.5, 2 = 2.2
gamma = 2.68571 - 0.242856 * gamma;
}
ddjvu_format_set_gamma(page->doc->pixelformat, gamma);
int bbsize = (bb->w)*(bb->h)*page->doc->pixelsize;
uint8_t *imagebuffer = bb->data;

Expand Down Expand Up @@ -692,23 +708,6 @@ static int drawPage(lua_State *L) {
if (!ddjvu_page_render(page->page_ref, djvu_render_mode, &pagerect, &renderrect, page->doc->pixelformat, bb->w*page->doc->pixelsize, imagebuffer))
memset(imagebuffer, 0xFF, bbsize);

/* Gamma correction of the blitbuffer, do we need to build this into BlitBuffer? */
unsigned char gamma_map[256];
unsigned char *s = imagebuffer;
int k, x, y;

if (dc->gamma != -1.0) {
for (k = 0; k < 256; k++)
gamma_map[k] = pow(k / 255.0f, dc->gamma) * 255;

for (y = 0; y < bb->h; y++) {
for (x = 0; x < bb->w; x++) {
k = y*bb->pitch + x;
s[k] = gamma_map[s[k]];
}
}
}

return 0;
}

Expand Down