[ft-devel] ftgrays - exact pixel coverage?

[address@hidden]

[Applies to: FreeType 2.4.8, file "ftgrays.c"]

I believe there is discrepancy between what's written in ftgrays.c and its actual functioning.
It is said it "computes the _exact_ coverage of the outline on each pixel cell".
But the following program shows this is not true.
I used quadratic Bezier arc P0(0,1)-P1(1,1)-P2(1,0) (in integer pixel coords), that is, the whole 
outline lies inside one pixel.
And I calculated coverage using equations (=5/6). So grey level should be 255*(5/6)=213,

but ftgrays returns 192.

This is not just because outline is too small, the problem persists for larger outlines too.

I don't test larger outlines here to keep example as simple as possible.

The program uses ftgrays in standalone mode (see ftgrays.c).

// file: test.c

#define _STANDALONE_

#define FT_ABS(a) ( (a) < 0 ? -(a) : (a) )
#include "ftgrays.c"

void main()
{
    FT_Raster_New_Func raster_new = ft_grays_raster.raster_new;
    FT_Raster_Reset_Func raster_reset = ft_grays_raster.raster_reset;
    FT_Raster_Render_Func raster_render = ft_grays_raster.raster_render; 

    FT_Raster raster = 0;
    char pool[16*1024] = {0}; // raster pool

    //////////////////////////////////////////////////////////////////////////
    FT_Raster_Params params = {0};

    FT_Outline outline = {0};
    FT_Bitmap bitmap = {0};

    // init source outline
    {
        enum{ n = 4 }; // number of points
        FT_Vector points[n] = {{0,0},{0,1},{1,1},{1,0}}; // in integer pixel coords
        char tags[n] = {1,1,0,1}; // P1(1,1) is off the curve
        short endpoint = n-1;
        int i;

        outline.n_contours = 1;
        outline.n_points = n;
        // convert points to 26.6 format
        for(i=0; i<n; i++) points[i].x <<= 6, points[i].y <<= 6;
        outline.points = points;
        outline.tags = tags;
        outline.contours = &endpoint;
    }

    // init target bitmap
    {
        enum{ a = 1 }; // width and height of target bitmap in pixels
        char buffer[a*a] = {0}; // buffer for target bitmap

        bitmap.rows = bitmap.width = bitmap.pitch = a;
        bitmap.buffer = buffer;
        bitmap.pixel_mode = FT_PIXEL_MODE_GRAY; // ignored
        bitmap.num_grays = 256; // ignored
    }

    // init raster params
    params.source = &outline;
    params.target = &bitmap;
    params.flags = FT_RASTER_FLAG_AA;
    //params.clip_box; // used only if both FT_RASTER_FLAG_DIRECT and FT_RASTER_FLAG_CLIP are set

    //////////////////////////////////////////////////////////////////////////

    raster_new( 0, &raster );
    raster_reset( raster, pool, sizeof pool );
    raster_render( raster, &params );

    // 213 == (5/6)*255 (with math rounding)
    if( bitmap.buffer[0] != 213 ) __asm int 3;
}