Re: [SeaBIOS] [PATCH] vgabios: Support emulated text in gfx_read_char()

[Kevin O'Connor]

On Tue, Jan 06, 2015 at 08:59:09PM +0100, Paolo Bonzini wrote:
> On 06/01/2015 16:49, Kevin O'Connor wrote:
> > if (vga_emulate_text()) {
> >    // Read bottom right pixel of the cell to guess bg color
> >    op.y += cheight-1;
> >    handle_gfx_op(&op);
> >    op.y -= cheight-1;
> >    bgattr = op.pixels[7];
> >    fgattr = bgattr ^ 0x7;
> >    car = 1;
> > }
>
> It's better to use the top right pixel.  The bottom right has a false
> positive for characters 23 and 95.

Hrmm.  How did you test?  The script I wrote shows the bottom right as
the least number of false positives.  With the 8x16 font, there are
six false positives, but all six are mirror images of other characters
so there is no way to avoid them.

-Kevin

$ python ./scripts/countbits.py < vgasrc/vgafonts.c 
256
 49  83 107 129 113  82  51  16
 58 101 110  98 100 103  61  19
 78 146 139 132 146 132  72  29
 92 136 122 140 154 122  63  17
108 159 142 139 154 147  88  44
 81 135 124 116 134 132  59  14
 66 123 132 152 134 101  66  13
 22  31  42  57  44  30  28  11

256
  6   8  21  41  23  21  19   8
 14  24  40  57  41  32  26   7
 44  78 102 125 110 102  65  13
 48  93  92  92  93 110  76  12
 51  90  92  99  95  99  81  15
 80 146 129 132 138 141  89  26
 85 133 109 134 133 140  95  18
 99 151 121 134 138 149 114  39
 86 129  88 111 120 130  92  13
 80 116  84 105 113 134  90  10
 47 109 126 148 122 136  83  10
 12  16  25  39  30  31  26   6
  9  16  28  47  30  27  22   9
  6   7  20  33  20  20  20   6

256
  6   8  21  41  23  21  19   8
 15  24  40  57  41  33  27   7
 41  72  95 118 100  89  59  20
 43  88  90  85  82  93  75  20
 51  83  79  92  81  91  81  21
 81 140 126 134 138 134  94  31
 79 130  98 122 131 131  89  25
110 153 119 140 146 142 120  52
 82 130  91 107 111 120  92  24
 85 127  87 103 112 125  95  17
 87 117  71 100 105 130  96  18
 49 115 134 154 122 136  90  15
  9  13  23  40  28  31  23   8
  8  13  25  38  29  30  25   8
  8  11  23  41  25  22  19   8
  6   7  20  33  20  20  20   6

==================================== countbits.py script

import sys

count = 0
counts = []
for line in sys.stdin:
    if 'vgafont' in line:
        if counts:
            print count
            for i in range(len(counts)):
                print "%3d" % counts[i],
                if i % 8 == 7:
                    print
            print
        count = 0
        counts = []
    try:
        parts = [int(p.strip(), 0) for p in line.split(',') if p.strip()]
    except ValueError:
        continue
    if not parts:
        continue
    count += 1
    out = [0] * (len(parts) * 8)
    for i in range(len(parts)):
        p = parts[i]
        for j in range(8):
            if (1<<(7-j)) & p:
                out[i*8 + j] += 1
    if not counts:
        counts = out
    else:
        for i in range(len(out)):
            counts[i] += out[i]