C# Textrenderer - Measuring smaller fontsize results in larger size

Question

I am trying to measure the size of a string given a certain font using the TextRenderer class. Despite the fact that i tried measuring it with 3 different approaches (Graphics.MeasureCharacterRanges, Graphics.MeasureString, TextRenderer.MeasureText) and they all give me different results without being accurate, i've stumbled across something else.
Measuring the same string START with the same font using a fontsize of 7 and 8, the fontsize 7 measurement turns out to be wider than the the fontsize 8 measurement.

Here's the code i use:

Font f1 = new Font("Arial", 7, FontStyle.Regular);
Font f2 = new Font("Arial", 8, FontStyle.Regular);
Size s1 = TextRenderer.MeasureText("START", f1);
Size s2 = TextRenderer.MeasureText("START", f2);

The result is s1 having a width of 41 and a height of 13 while s2 having a width of 40 and a height of 14.

Why would a smaller font result in a larger width?

Answer 1

To address specifically why it would be possible for a larger font to produce a smaller width, I put together this sample console app. It's worth noting that I adjust the 7 & 8 font sizes to 7.5 & 8.25, respectively, as this is what size TextRenderer evaluates them as internally.

using System;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;

namespace FontSizeDifference
{
    static class Program
    {
        [StructLayout(LayoutKind.Sequential)]
        struct ABCFLOAT
        {
            public float abcfA;
            public float abcfB;
            public float abcfC;
        }

        [DllImport("gdi32.dll")]
        static extern bool GetCharABCWidthsFloat(IntPtr hdc, int iFirstChar, int iLastChar, [Out] ABCFLOAT[] lpABCF);

        [DllImport("gdi32.dll", CharSet = CharSet.Auto, EntryPoint = "SelectObject", SetLastError = true)]
        static extern IntPtr SelectObject(IntPtr hdc, IntPtr obj);

        [DllImport("gdi32.dll", EntryPoint = "DeleteObject")]
        static extern bool DeleteObject([In] IntPtr hObject);

        [StructLayout(LayoutKind.Sequential)]
        struct KERNINGPAIR
        {
            public ushort wFirst;
            public ushort wSecond;
            public int iKernAmount;
        }

        [DllImport("gdi32.dll")]
        static extern int GetKerningPairs(IntPtr hdc, int nNumPairs, [Out] KERNINGPAIR[] lpkrnpair);

        [STAThread]
        static void Main()
        {
            var fonts = new[] {
                new Font("Arial", 7.5f, FontStyle.Regular),
                new Font("Arial", 8.25f, FontStyle.Regular)
            };
            string textToMeasure = "START";

            using (Graphics g = Graphics.FromHwnd(IntPtr.Zero))
            {
                IntPtr hDC = g.GetHdc();

                foreach (Font font in fonts)
                {
                    float totalWidth = 0F;
                    IntPtr hFont = font.ToHfont();

                    // Apply the font to dc
                    SelectObject(hDC, hFont);

                    int pairCount = GetKerningPairs(hDC, short.MaxValue, null);
                    var lpkrnpair = new KERNINGPAIR[pairCount];
                    GetKerningPairs(hDC, pairCount, lpkrnpair);

                    Console.WriteLine("\r\n" + font.ToString());

                    for (int ubound = textToMeasure.Length - 1, i = 0; i <= ubound; ++i)
                    {
                        char c = textToMeasure[i];
                        ABCFLOAT characterWidths = GetCharacterWidths(hDC, c);
                        float charWidth = (characterWidths.abcfA + characterWidths.abcfB + characterWidths.abcfC);
                        totalWidth += charWidth;

                        int kerning = 0;
                        if (i < ubound)
                        {
                            kerning = GetKerningBetweenCharacters(lpkrnpair, c, textToMeasure[i + 1]).iKernAmount;
                            totalWidth += kerning;
                        }

                        Console.WriteLine(c + ": " + (charWidth + kerning) + " (" + charWidth + " + " + kerning + ")");
                    }

                    Console.WriteLine("Total width: " + totalWidth);

                    DeleteObject(hFont);
                }

                g.ReleaseHdc(hDC);
            }
        }

        static KERNINGPAIR GetKerningBetweenCharacters(KERNINGPAIR[] lpkrnpair, char first, char second)
        {
            return lpkrnpair.Where(x => (x.wFirst == first) && (x.wSecond == second)).FirstOrDefault();
        }

        static ABCFLOAT GetCharacterWidths(IntPtr hDC, char character)
        {
            ABCFLOAT[] values = new ABCFLOAT[1];
            GetCharABCWidthsFloat(hDC, character, character, values);
            return values[0];
        }
    }
}

For each font size, it outputs the width of each character, including kerning. At 96 DPI, for me, this results in:

[Font: Name=Arial, Size=7.5, Units=3, GdiCharSet=1, GdiVerticalFont=False]
S: 7 (7 + 0)
T: 6 (7 + -1)
A: 7 (7 + 0)
R: 7 (7 + 0)
T: 7 (7 + 0)
Total width: 34

[Font: Name=Arial, Size=8.25, Units=3, GdiCharSet=1, GdiVerticalFont=False]
S: 7 (7 + 0)
T: 5 (6 + -1)
A: 8 (8 + 0)
R: 7 (7 + 0)
T: 6 (6 + 0)
Total width: 33

Though I've obviously not captured the exact formula for measurements made by TextRenderer, it does illustrate the same width-discrepancy. At font size 7, all characters are 7 in width. However, at font size 8, the character widths begin to vary, some larger, some smaller, ultimately adding up to a smaller width.