Need help using libpng to read an image

Question

Here is my function... I don't know why it's not working. The resulting image looks nothing like what the .png looks like. But there's no errors either.

bool Bullet::read_png(std::string file_name, int pos)
{
    png_structp png_ptr;
    png_infop info_ptr;
    FILE *fp;

    if ((fp = fopen(file_name.c_str(), "rb")) == NULL) {
        return false;
    }

    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);

    if (png_ptr == NULL) {
        fclose(fp);
        return false;
    }

    info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL) {
        fclose(fp);
        png_destroy_read_struct(&png_ptr, NULL, NULL);
        return false;
    }

    if (setjmp(png_jmpbuf(png_ptr))) {
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        fclose(fp);
        return false;
    }

    png_init_io(png_ptr, fp);

    png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_STRIP_16 | PNG_TRANSFORM_SWAP_ALPHA | PNG_TRANSFORM_EXPAND, NULL);

    png_uint_32 width = png_get_image_width(png_ptr, info_ptr);
    png_uint_32 height = png_get_image_height(png_ptr, info_ptr);

    png_uint_32 bitdepth   = png_get_bit_depth(png_ptr, info_ptr);
    png_uint_32 channels   = png_get_channels(png_ptr, info_ptr);
    png_uint_32 color_type = png_get_color_type(png_ptr, info_ptr);

    AS3_Trace(AS3_String("BitDepth"));
    AS3_Trace(AS3_Int(bitdepth));
    AS3_Trace(AS3_String("Channels"));
    AS3_Trace(AS3_Int(channels));
    AS3_Trace(AS3_String("ColorType"));
    AS3_Trace(AS3_Int(color_type));

    imageData[pos].width = width;
    imageData[pos].height = height;

    png_bytepp row_pointers;
    row_pointers = png_get_rows(png_ptr, info_ptr);

    imageData[pos].data = new unsigned int[width*height];

    for (unsigned int i=0; i < height; ++i) {
        memcpy(&imageData[pos].data[i*width], &row_pointers[i], width*sizeof(unsigned int));
    }

    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
    fclose(fp);

    for (unsigned int i=0; i < height; ++i) {
        for (unsigned int j=0; j < width; ++j) {
            unsigned int val = imageData[pos].data[i*width+j];
            unsigned int a = ((val >> 24));
            unsigned int r = (((val - (a << 24)) >> 16));
            unsigned int g = (((val - (a << 24) - (r << 16)) >> 8));
            unsigned int b = (((val - (a << 24) - (r << 16) - (g << 8))));

            std::string s(AS3_StringValue(AS3_Int(i*width+j)));
            s += " ";
            s += AS3_StringValue(AS3_Int(val));
            s += " ";
            s += AS3_StringValue(AS3_Int(a));
            s += " ";
            s += AS3_StringValue(AS3_Int(r));
            s += " ";
            s += AS3_StringValue(AS3_Int(g));
            s += " ";
            s += AS3_StringValue(AS3_Int(b));
            AS3_Trace(AS3_String(s.c_str()));
        }
    }

    return true;
}

ImageData is just a simple struct to keep x, y, width, and height, and imageData is an array of that struct.

struct ImageData {
    int x;
    int y;
    int width;
    int height;
    unsigned int* data;
};

Here is a side by side screenshot of the input and output graphics (dimensions 8x30) (something I made in a minute for testing), and this was after setting alpha to 255 in order to make it show up (because the alpha I was getting back was 1). Left side is original, right side is what happened after reading it through this function. Scaled up 400% for visibility.

http://img217.imageshack.us/img217/1627/testbullet.png

Here is a log of the traces:

BitDepth
8
Channels
4
ColorType
6  // RGBA

//^ All of my pngs will be of the above type    

0 16855328 1 1 49 32
1 16855424 1 1 49 128
2 16855456 1 1 49 160
3 16855488 1 1 49 192
4 16855520 1 1 49 224
5 16855552 1 1 50 0
6 16855584 1 1 50 32
7 16855616 1 1 50 64
8 16855424 1 1 49 128
9 16855456 1 1 49 160
10 16855488 1 1 49 192
11 16855520 1 1 49 224
12 16855552 1 1 50 0
13 16855584 1 1 50 32
14 16855616 1 1 50 64
15 16855648 1 1 50 96
16 16855456 1 1 49 160
17 16855488 1 1 49 192
18 16855520 1 1 49 224
19 16855552 1 1 50 0
20 16855584 1 1 50 32
21 16855616 1 1 50 64
22 16855648 1 1 50 96
23 16855680 1 1 50 128
24 16855488 1 1 49 192
25 16855520 1 1 49 224
26 16855552 1 1 50 0
27 16855584 1 1 50 32
28 16855616 1 1 50 64
29 16855648 1 1 50 96
30 16855680 1 1 50 128
31 16855712 1 1 50 160
32 16855520 1 1 49 224
33 16855552 1 1 50 0
34 16855584 1 1 50 32
35 16855616 1 1 50 64
36 16855648 1 1 50 96
37 16855680 1 1 50 128
38 16855712 1 1 50 160
39 16855744 1 1 50 192
40 16855552 1 1 50 0
41 16855584 1 1 50 32
42 16855616 1 1 50 64
43 16855648 1 1 50 96
44 16855680 1 1 50 128
45 16855712 1 1 50 160
46 16855744 1 1 50 192
47 16855776 1 1 50 224
48 16855584 1 1 50 32
49 16855616 1 1 50 64
50 16855648 1 1 50 96
51 16855680 1 1 50 128
52 16855712 1 1 50 160
53 16855744 1 1 50 192
54 16855776 1 1 50 224
55 16855808 1 1 51 0
56 16855616 1 1 50 64
57 16855648 1 1 50 96
58 16855680 1 1 50 128
59 16855712 1 1 50 160
60 16855744 1 1 50 192
61 16855776 1 1 50 224
62 16855808 1 1 51 0
63 16855840 1 1 51 32
64 16855648 1 1 50 96
65 16855680 1 1 50 128
66 16855712 1 1 50 160
67 16855744 1 1 50 192
68 16855776 1 1 50 224
69 16855808 1 1 51 0
70 16855840 1 1 51 32
71 16855872 1 1 51 64
72 16855680 1 1 50 128
73 16855712 1 1 50 160
74 16855744 1 1 50 192
75 16855776 1 1 50 224
76 16855808 1 1 51 0
77 16855840 1 1 51 32
78 16855872 1 1 51 64
79 16855904 1 1 51 96
80 16855712 1 1 50 160
81 16855744 1 1 50 192
82 16855776 1 1 50 224
83 16855808 1 1 51 0
84 16855840 1 1 51 32
85 16855872 1 1 51 64
86 16855904 1 1 51 96
87 16855936 1 1 51 128
88 16855744 1 1 50 192
89 16855776 1 1 50 224
90 16855808 1 1 51 0
91 16855840 1 1 51 32
92 16855872 1 1 51 64
93 16855904 1 1 51 96
94 16855936 1 1 51 128
95 16855968 1 1 51 160
96 16855776 1 1 50 224
97 16855808 1 1 51 0
98 16855840 1 1 51 32
99 16855872 1 1 51 64
100 16855904 1 1 51 96
101 16855936 1 1 51 128
102 16855968 1 1 51 160
103 16856000 1 1 51 192
104 16855808 1 1 51 0
105 16855840 1 1 51 32
106 16855872 1 1 51 64
107 16855904 1 1 51 96
108 16855936 1 1 51 128
109 16855968 1 1 51 160
110 16856000 1 1 51 192
111 16856032 1 1 51 224
112 16855840 1 1 51 32
113 16855872 1 1 51 64
114 16855904 1 1 51 96
115 16855936 1 1 51 128
116 16855968 1 1 51 160
117 16856000 1 1 51 192
118 16856032 1 1 51 224
119 16856064 1 1 52 0
120 16855872 1 1 51 64
121 16855904 1 1 51 96
122 16855936 1 1 51 128
123 16855968 1 1 51 160
124 16856000 1 1 51 192
125 16856032 1 1 51 224
126 16856064 1 1 52 0
127 16856096 1 1 52 32
128 16855904 1 1 51 96
129 16855936 1 1 51 128
130 16855968 1 1 51 160
131 16856000 1 1 51 192
132 16856032 1 1 51 224
133 16856064 1 1 52 0
134 16856096 1 1 52 32
135 16856128 1 1 52 64
136 16855936 1 1 51 128
137 16855968 1 1 51 160
138 16856000 1 1 51 192
139 16856032 1 1 51 224
140 16856064 1 1 52 0
141 16856096 1 1 52 32
142 16856128 1 1 52 64
143 16856160 1 1 52 96
144 16855968 1 1 51 160
145 16856000 1 1 51 192
146 16856032 1 1 51 224
147 16856064 1 1 52 0
148 16856096 1 1 52 32
149 16856128 1 1 52 64
150 16856160 1 1 52 96
151 16856192 1 1 52 128
152 16856000 1 1 51 192
153 16856032 1 1 51 224
154 16856064 1 1 52 0
155 16856096 1 1 52 32
156 16856128 1 1 52 64
157 16856160 1 1 52 96
158 16856192 1 1 52 128
159 16856224 1 1 52 160
160 16856032 1 1 51 224
161 16856064 1 1 52 0
162 16856096 1 1 52 32
163 16856128 1 1 52 64
164 16856160 1 1 52 96
165 16856192 1 1 52 128
166 16856224 1 1 52 160
167 16856256 1 1 52 192
168 16856064 1 1 52 0
169 16856096 1 1 52 32
170 16856128 1 1 52 64
171 16856160 1 1 52 96
172 16856192 1 1 52 128
173 16856224 1 1 52 160
174 16856256 1 1 52 192
175 16856288 1 1 52 224
176 16856096 1 1 52 32
177 16856128 1 1 52 64
178 16856160 1 1 52 96
179 16856192 1 1 52 128
180 16856224 1 1 52 160
181 16856256 1 1 52 192
182 16856288 1 1 52 224
183 16856320 1 1 53 0
184 16856128 1 1 52 64
185 16856160 1 1 52 96
186 16856192 1 1 52 128
187 16856224 1 1 52 160
188 16856256 1 1 52 192
189 16856288 1 1 52 224
190 16856320 1 1 53 0
191 0 0 0 0 0
192 16856160 1 1 52 96
193 16856192 1 1 52 128
194 16856224 1 1 52 160
195 16856256 1 1 52 192
196 16856288 1 1 52 224
197 16856320 1 1 53 0
198 0 0 0 0 0
199 0 0 0 0 0
200 16856192 1 1 52 128
201 16856224 1 1 52 160
202 16856256 1 1 52 192
203 16856288 1 1 52 224
204 16856320 1 1 53 0
205 0 0 0 0 0
206 0 0 0 0 0
207 0 0 0 0 0
208 16856224 1 1 52 160
209 16856256 1 1 52 192
210 16856288 1 1 52 224
211 16856320 1 1 53 0
212 0 0 0 0 0
213 0 0 0 0 0
214 0 0 0 0 0
215 0 0 0 0 0
216 16856256 1 1 52 192
217 16856288 1 1 52 224
218 16856320 1 1 53 0
219 0 0 0 0 0
220 0 0 0 0 0
221 0 0 0 0 0
222 0 0 0 0 0
223 0 0 0 0 0
224 16856288 1 1 52 224
225 16856320 1 1 53 0
226 0 0 0 0 0
227 0 0 0 0 0
228 0 0 0 0 0
229 0 0 0 0 0
230 0 0 0 0 0
231 0 0 0 0 0
232 16856320 1 1 53 0
233 0 0 0 0 0
234 0 0 0 0 0
235 0 0 0 0 0
236 0 0 0 0 0
237 0 0 0 0 0
238 0 0 0 0 0
239 0 0 0 0 0

Was stuck on this for a couple of days, so I would appreciate any help I can get.

Edit: Here's another 2 sets of before and after comparisons... once again, in all of the traces the alpha was 1, and was changed to 255 before the screenshots were taken.

http://img707.imageshack.us/img707/6953/82597335.png

Thanks Roger! Is there no other way except adding the indents? Is there a way to add the indents automatically not by hand? — jonathanasdf, Apr 24 '10 at 04:55
@jonathan: You're welcome. Ctrl-k or there's a toolbar button, either after selecting text. The [editing help](http://stackoverflow.com/editing-help) may be useful (even though it doesn't cover shortcuts or the toolbar). — , Apr 24 '10 at 04:59
Ahh. Thanks again. And that's exactly why the editing help wasn't useful - I tried using
but that didn't work out very well. Never thought of looking at the toolbar, because I never use these kind of rich text toolbars :) — jonathanasdf, Apr 24 '10 at 05:00
Yeah, I feel the same way about those kind of toolbars, except that shortcuts need to be discoverable (so display them on menu items! but menus are hard to do on web pages..). BTW, I'm not familiar with libpng and it would be a bit much just now to go read all the docs to give an adequate answer, but it appears you're flipping channels (green becomes blue) and rows/columns. (Are you getting the byte order right?) — , Apr 24 '10 at 05:13
yeah, I don't expect someone completely unfamiliar with it to be able to go through the documentation to give an answer.... unless my problem was something that had nothing to do with the library itself and was as you said messing up the channels/rows/columns haha. I'll check again. The only channel i'm swapping is the Alpha channel, so switching from RGBA, default of .png, to ARGB, default of flash, but that transform is done through a function built into the library so I don't expect that to go wrong — jonathanasdf, Apr 24 '10 at 05:20
I'm basing this on the appearance of the before and after images, and figured you'd probably already considered it (but mentioned just in case). I've not looked at the code. — , Apr 24 '10 at 05:27
added 2 more sets of before and afters... you can obviously tell from this that it is definitely not a problem of messing up the channels or messing up the rows/columns... overall it's just... bizarre. My brain is fried, I'm going to sleep. Maybe it'll help if I look at it with a fresh set of eyes tomorrow.... — jonathanasdf, Apr 24 '10 at 05:52
@Roger, thanks for your help too. Turns out it was a horribly stupid mistake. But at least I now know how to format code :) — jonathanasdf, Apr 24 '10 at 16:05

flownt · Accepted Answer · 2010-04-24T21:49:30.513

5

I believe you have an error at this line: memcpy(&imageData[pos].data[i*width], &row_pointers[i], width*sizeof(unsigned int)); the type of row pointers seems to be char**. that would mean you are copying pointers to char to your image data!

the compiler reports no error because all pointers are implicitly convertible to void* and this ìs a pointer.

edited Apr 24 '10 at 21:49

answered Apr 24 '10 at 09:59

flownt

760
5
11

yeah, the type of row pointers is unsigned char** and that ... would make sense. hahaha i was really tired, i'll try it out. – jonathanasdf Apr 24 '10 at 15:59
wow.. I can't believe it.. that was the problem after all. How could I have overlooked that. – jonathanasdf Apr 24 '10 at 16:04

Need help using libpng to read an image

1 Answers1