feat(video): track background and sprite mosaic state

Add vertical mosaic counters and background/sprite sampling regressions.

tests/mosaic/backgrounds.rs

@@ -0,0 +1,212 @@
+use super::*;
+
+fn bitmap(mode: u16, page: u16) -> Memory {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x400 | mode | (page * 16)).unwrap();
+    m.write16(BG2PA, 256).unwrap();
+    m.write16(BG2PD, 256).unwrap();
+    let (w, h) = if mode == 5 { (160, 128) } else { (240, 160) };
+    for y in 0..h {
+        for x in 0..w {
+            let offset = (y * w + x) as u32;
+            let index = (x * 3 + y * 7) as u16 & 255;
+            if mode == 4 {
+                if x % 2 == 0 {
+                    let next = ((x + 1) * 3 + y * 7) as u16 & 255;
+                    m.write16(VRAM + page as u32 * 0xa000 + offset, index | (next << 8))
+                        .unwrap();
+                }
+            } else {
+                let color =
+                    (x as u16 & 31) | ((y as u16 & 31) << 5) | (((x + y) as u16 & 31) << 10);
+                m.write16(VRAM + page as u32 * 0xa000 + offset * 2, color)
+                    .unwrap();
+            }
+        }
+    }
+    m
+}
+
+#[test]
+fn write_only_register_merges_bytes_and_ignores_upper_word_half() {
+    let mut m = bitmap(3, 0);
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write32(MOSAIC, 0xffff_0034).unwrap(); // BG 5x4, OBJ1x1.
+    assert_eq!(m.read32(MOSAIC).unwrap(), 0);
+    m.write8(MOSAIC + 1, 0xff).unwrap(); // Must retain BG dimensions.
+    let f = render(&m);
+    assert_eq!(at(&f, 4, 3), at(&f, 0, 0));
+    assert_ne!(at(&f, 5, 4), at(&f, 0, 0));
+    m.write8(MOSAIC, 0).unwrap();
+    assert_ne!(at(&render(&m), 1, 0), at(&f, 0, 0));
+    assert_eq!(m.read16(MOSAIC + 2).unwrap(), 0);
+    assert_eq!(
+        m.read8(MOSAIC + 0x10000),
+        Err(MemoryError::Unmapped(MOSAIC + 0x10000))
+    );
+    assert_eq!(
+        m.write16(MOSAIC + 1, 0),
+        Err(MemoryError::Unaligned(MOSAIC + 1))
+    );
+    assert_eq!(m.cycles(), 0);
+}
+
+#[test]
+fn every_bg_size_and_bitmap_page_repeats_screen_grid_samples() {
+    for mode in [3, 4, 5] {
+        for page in 0..=u16::from(mode != 3) {
+            let mut m = bitmap(mode, page);
+            let base = render(&m);
+            m.write16(BG2CNT, 0x40).unwrap();
+            // Exercise every field value, including asymmetric blocks and partial LCD blocks.
+            for w in 1..=16 {
+                let h = 17 - w;
+                m.write16(MOSAIC, ((h - 1) * 16 + w - 1) as u16).unwrap();
+                let f = render(&m);
+                for y in 0..HEIGHT {
+                    for x in 0..WIDTH {
+                        assert_eq!(
+                            at(&f, x, y),
+                            at(&base, x / w * w, y / h * h),
+                            "mode={mode} page={page} size={w}x{h} at={x},{y}"
+                        );
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn mosaic_flag_is_independent_of_dimensions_and_one_by_one_is_identity() {
+    let mut m = bitmap(3, 0);
+    let base = render(&m);
+    m.write16(MOSAIC, 0xffff).unwrap();
+    assert_eq!(render(&m).pixels(), base.pixels()); // Flag is off.
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write16(MOSAIC, 0xff00).unwrap(); // OBJ fields do not affect BG.
+    assert_eq!(render(&m).pixels(), base.pixels());
+    m.write16(MOSAIC, 0x000f).unwrap();
+    let f = render(&m);
+    assert_eq!(at(&f, 15, 1), at(&base, 0, 1));
+    assert_ne!(at(&f, 15, 1), at(&base, 0, 0));
+}
+
+#[test]
+fn text_mosaic_samples_before_scroll_and_tile_flips_in_both_depths() {
+    for eight_bit in [false, true] {
+        let mut m = memory();
+        m.write16(DISPCNT, 0x100).unwrap();
+        let control = 0x1000 | if eight_bit { 0x80 } else { 0 };
+        m.write16(BG0CNT, control).unwrap();
+        m.write16(BG0HOFS, 7).unwrap();
+        m.write16(BG0VOFS, 5).unwrap();
+        fill(&mut m, VRAM + 0x8000, 0x800, 0x0c00); // Both tile flips.
+        for y in 0..8 {
+            for x in (0..8).step_by(if eight_bit { 2 } else { 4 }) {
+                let mut value = 0;
+                for i in 0..if eight_bit { 2 } else { 4 } {
+                    let index = ((x + i + y * 3) % 15) as u16;
+                    value |= index << (i * if eight_bit { 8 } else { 4 });
+                }
+                m.write16(
+                    VRAM + (y * if eight_bit { 8 } else { 4 } + x / if eight_bit { 1 } else { 2 })
+                        as u32,
+                    value,
+                )
+                .unwrap();
+            }
+        }
+        let base = render(&m);
+        m.write16(BG0CNT, control | 0x40).unwrap();
+        m.write16(MOSAIC, 0x54).unwrap(); // 5x6 crosses tile boundaries.
+        let f = render(&m);
+        for y in 0..HEIGHT {
+            for x in 0..WIDTH {
+                assert_eq!(at(&f, x, y), at(&base, x / 5 * 5, y / 6 * 6));
+            }
+        }
+    }
+}
+
+#[test]
+fn affine_mosaic_samples_screen_positions_before_signed_transform() {
+    for mode in [1, 2, 3, 4, 5] {
+        let mut m = if mode >= 3 { bitmap(mode, 0) } else { memory() };
+        m.write16(DISPCNT, 0x400 | mode).unwrap();
+        let control = if mode <= 2 { 0x3000 } else { 0 };
+        m.write16(BG2CNT, control).unwrap();
+        if mode <= 2 {
+            for offset in (0..64).step_by(2) {
+                m.write16(VRAM + offset, (offset as u16 * 0x101).wrapping_add(0x100))
+                    .unwrap();
+            }
+        }
+        for (reg, value) in [
+            (BG2PA, 181),
+            (BG2PB, 91),
+            (BG2PC, (-181i16) as u16),
+            (BG2PD, 256),
+        ] {
+            m.write16(reg, value).unwrap();
+        }
+        m.write32(BG2X, (-769i32) as u32).unwrap();
+        m.write32(BG2Y, 31 * 256 + 127).unwrap();
+        let base = render(&m);
+        m.write16(BG2CNT, control | 0x40).unwrap();
+        m.write16(MOSAIC, 0x76).unwrap();
+        let f = render(&m);
+        for y in 0..HEIGHT {
+            for x in 0..WIDTH {
+                assert_eq!(
+                    at(&f, x, y),
+                    at(&base, x / 7 * 7, y / 8 * 8),
+                    "mode={mode} pixel={x},{y}"
+                );
+            }
+        }
+    }
+}
+
+#[test]
+fn independent_background_flags_preserve_transparency_and_priority() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x300).unwrap();
+    m.write16(BG0CNT, 0x1040).unwrap();
+    m.write16(BG1CNT, 0x1105).unwrap();
+    m.write16(PAL + 2, 31).unwrap();
+    m.write16(PAL + 4, 0x3e0).unwrap();
+    fill(&mut m, VRAM, 32, 0x1010);
+    fill(&mut m, VRAM + 0x4000, 32, 0x2222);
+    m.write16(MOSAIC, 2).unwrap();
+    let f = render(&m);
+    for x in 0..18 {
+        assert_eq!(at(&f, x, 0), rgb(if x / 3 % 2 == 0 { 0x3e0 } else { 31 }));
+    }
+    m.write16(BG0CNT, 0x1000).unwrap();
+    m.write16(BG1CNT, 0x1145).unwrap();
+    let f = render(&m);
+    for x in 0..18 {
+        assert_eq!(at(&f, x, 0), rgb(if x % 2 == 0 { 0x3e0 } else { 31 }));
+    }
+}
+
+#[test]
+fn windows_and_color_effects_use_output_coordinate_not_mosaic_sample() {
+    let mut m = bitmap(3, 0);
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write16(MOSAIC, 0x77).unwrap();
+    m.write16(DISPCNT, 0x2403).unwrap();
+    m.write16(WIN0H, 0x0306).unwrap();
+    m.write16(WIN0V, 0x0008).unwrap();
+    m.write16(WININ, 4).unwrap();
+    m.write16(WINOUT, 0x24).unwrap();
+    m.write16(BLDCNT, 0x84).unwrap();
+    m.write16(BLDY, 16).unwrap();
+    let f = render(&m);
+    for x in 0..8 {
+        assert_eq!(at(&f, x, 0), if (3..6).contains(&x) { 0 } else { 0xffffff });
+    }
+    m.write16(DISPCNT, 0xffff).unwrap();
+    assert!(render(&m).pixels().iter().all(|&p| p == 0xffffff));
+}

tests/mosaic/demo.rs

@@ -0,0 +1,100 @@
+use super::*;
+use gba_rust::{
+    input::{Button, Buttons},
+    mosaic_demo::{MosaicDemo, MOSAIC_STATE},
+    tile_demo::TileDemo,
+};
+
+#[test]
+fn cpu_demo_cycles_all_sizes_and_wraps_without_host_video_writes() {
+    let mut demo = MosaicDemo::new().unwrap();
+    let mut frame = Framebuffer::default();
+    assert_eq!(demo.machine().memory().read16(DISPCNT).unwrap(), 0);
+    assert_eq!(demo.machine().memory().read16(VRAM).unwrap(), 0);
+    for update in 1..=129 {
+        demo.frame(Buttons::default(), &mut frame).unwrap();
+        let m = demo.machine().memory();
+        assert_eq!(m.read32(MOSAIC_STATE).unwrap(), update);
+        assert_eq!(m.read16(DISPCNT).unwrap(), 0x1340);
+        assert_eq!(m.read16(BG0CNT).unwrap(), 0x1844);
+        assert_eq!(m.read16(BG1CNT).unwrap(), 0xd0c1);
+        assert_eq!(m.read16(OAM).unwrap(), 0x1048);
+        assert_eq!(m.read32(MOSAIC).unwrap(), 0);
+        assert_eq!(m.display_position().scanline, 160);
+        let size = (1 + (update / 8) % 16) as usize;
+        // Check terrain and foreground away from the sprite for every size.
+        for y in 0..48 {
+            for x in 0..WIDTH {
+                let sx = x / size * size;
+                let sy = y / size * size;
+                let cross = sx / 8 % 4 == 1 && sy / 8 % 4 == 1 && (sx % 8 == 3 || sy % 8 == 3);
+                let color = if cross {
+                    0x3ff
+                } else {
+                    [0x260, 0x3a0, 0x7d20, 0x7e80]
+                        [((sx / 32 + sy / 32) % 2) * 2 + (sx / 2 + sy / 2) % 2]
+                };
+                assert_eq!(at(&frame, x, y), rgb(color), "update={update} at={x},{y}");
+            }
+        }
+    }
+}
+
+#[test]
+fn both_bypasses_restore_original_scene_with_scroll_rotation_zoom_and_reset() {
+    let mut demo = MosaicDemo::new().unwrap();
+    let mut base = TileDemo::new().unwrap();
+    let mut f = Framebuffer::default();
+    let mut reference = Framebuffer::default();
+    for i in 0..32 {
+        let buttons = Buttons::default()
+            .with(Button::Left, i < 16)
+            .with(Button::Down, true)
+            .with(Button::Start, i == 31)
+            .with(Button::L, (8..24).contains(&i))
+            .with(Button::R, (16..28).contains(&i));
+        base.frame(buttons, &mut reference).unwrap();
+        demo.frame(buttons.with(Button::A, true).with(Button::B, true), &mut f)
+            .unwrap();
+        assert_eq!(f.pixels(), reference.pixels());
+        let m = demo.machine().memory();
+        assert_ne!(m.read16(OAM).unwrap() & 0x1000, 0);
+        assert_eq!(m.read16(OAM + 2).unwrap() & 0x1000, 0); // Z no longer flips.
+        assert_eq!(m.read16(OAM + 4).unwrap() & 0xc00, 0); // X no longer lowers priority.
+        assert_eq!(
+            m.read32(MOSAIC_STATE + 4).unwrap(),
+            base.machine().memory().read32(MOSAIC_STATE + 4).unwrap()
+        );
+    }
+    assert_eq!(demo.machine().memory().read32(MOSAIC_STATE + 4).unwrap(), 0);
+    assert_eq!(demo.machine().memory().read32(MOSAIC_STATE + 8).unwrap(), 0);
+}
+
+#[test]
+fn dma_word_write_sets_bg_and_obj_fields_while_unused_halfword_ignores_writes() {
+    use gba_rust::{
+        cpu::Cpu,
+        dma::DMA_BASE,
+        machine::{Machine, StepKind},
+    };
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1140).unwrap();
+    m.write16(BG0CNT, 0x1040).unwrap();
+    fill(&mut m, VRAM, 32, 0x1212);
+    m.write16(PAL + 4, 31).unwrap();
+    m.write16(PAL + 2, 0x3e0).unwrap();
+    object(&mut m, 0, 0x1000, 0, 0);
+    fill(&mut m, VRAM + 0x10000, 32, 0x0101);
+    m.write16(PAL + 0x202, 0x7c00).unwrap();
+    m.write32(0x0200_0000, 0xffff_0302).unwrap(); // BG3x1, OBJ4x1.
+    m.write32(DMA_BASE + 36, 0x0200_0000).unwrap();
+    m.write32(DMA_BASE + 40, MOSAIC).unwrap();
+    m.write32(DMA_BASE + 44, 0x8400_0001).unwrap();
+    let mut machine = Machine::new(Cpu::new(0x0300_0000), m);
+    assert_eq!(machine.step().unwrap(), StepKind::Dma { channel: 3 });
+    let f = render(machine.memory());
+    assert_eq!(at(&f, 3, 0), rgb(0x7c00));
+    assert_eq!(at(&f, 8, 0), rgb(31)); // BG samples x6, not x8's independent phase.
+    assert_eq!(at(&f, 9, 0), rgb(0x3e0));
+    assert_eq!(machine.memory().read32(MOSAIC).unwrap(), 0);
+}

tests/mosaic/objects.rs

@@ -0,0 +1,215 @@
+use super::*;
+
+fn scene(a: u16, b: u16) -> Memory {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1040).unwrap();
+    object(&mut m, 0, a | 7, b | 5 | 0x4000, 0); // 16x16 at (5,7).
+    for y in 0..16 {
+        for x in (0..16).step_by(2) {
+            let tile = y / 8 * 2 + x / 8;
+            let address = VRAM + 0x10000 + tile * 64 + y % 8 * 8 + x % 8;
+            let first = 1 + (x + y * 5) % 254;
+            let second = 1 + (x + 1 + y * 5) % 254;
+            m.write16(address, (first | second << 8) as u16).unwrap();
+        }
+    }
+    m
+}
+
+#[test]
+fn regular_obj_uses_screen_grid_clamps_partial_first_blocks_and_preserves_flips() {
+    for flips in [0, 0x1000, 0x2000, 0x3000] {
+        let mut m = scene(0x2000, flips);
+        let base = render(&m);
+        m.write16(OAM, 0x3007).unwrap(); // 8bpp plus mosaic, y7.
+        for width in 1..=16 {
+            let height = 17 - width;
+            m.write16(MOSAIC, (((height - 1) << 12) | ((width - 1) << 8)) as u16)
+                .unwrap();
+            let f = render(&m);
+            for y in 0..32 {
+                for x in 0..32 {
+                    let expected = if (5..21).contains(&x) && (7..23).contains(&y) {
+                        at(
+                            &base,
+                            (x / width * width).max(5),
+                            (y / height * height).max(7),
+                        )
+                    } else {
+                        0
+                    };
+                    assert_eq!(
+                        at(&f, x, y),
+                        expected,
+                        "flip={flips:x} size={width}x{height} at={x},{y}"
+                    );
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn affine_and_double_size_mosaic_repeat_transformed_samples() {
+    for doubled in [false, true] {
+        let mut m = scene(0x2100 | if doubled { 0x200 } else { 0 }, 0);
+        let matrix: [i16; 4] = if doubled {
+            [64, 32, -32, 64]
+        } else {
+            [128, 64, -64, 128]
+        };
+        for (i, value) in matrix.into_iter().enumerate() {
+            m.write16(OAM + 6 + i as u32 * 8, value as u16).unwrap();
+        }
+        let base = render(&m);
+        m.write16(OAM, 0x3107 | if doubled { 0x200 } else { 0 })
+            .unwrap();
+        m.write16(MOSAIC, 0x4500).unwrap(); // OBJ6x5; BG1x1.
+        let f = render(&m);
+        let size = if doubled { 32 } else { 16 };
+        for y in 0..48 {
+            for x in 0..48 {
+                let expected = if (5..5 + size).contains(&x) && (7..7 + size).contains(&y) {
+                    at(&base, (x / 6 * 6).max(5), (y / 5 * 5).max(7))
+                } else {
+                    0
+                };
+                assert_eq!(at(&f, x, y), expected, "double={doubled} at={x},{y}");
+            }
+        }
+    }
+}
+
+#[test]
+fn obj_flag_and_high_byte_are_independent_and_vertical_sampling_wraps_at_screen_top() {
+    let mut m = scene(0x2000, 0);
+    let base = render(&m);
+    m.write16(MOSAIC, 0xffff).unwrap();
+    assert_eq!(render(&m).pixels(), base.pixels()); // Flag disabled.
+    m.write16(OAM, 0x3007).unwrap();
+    m.write8(MOSAIC + 1, 0).unwrap(); // BG15 bits do not affect OBJ.
+    assert_eq!(render(&m).pixels(), base.pixels());
+    object(&mut m, 0, 0x20fc, 0x41fc, 0); // 16x16, wraps from (-4,-4).
+    let base = render(&m);
+    m.write16(OAM, 0x30fc).unwrap();
+    m.write16(MOSAIC, 0x3400).unwrap();
+    let f = render(&m);
+    for y in 0..12 {
+        for x in 0..12 {
+            assert_eq!(at(&f, x, y), at(&base, x / 5 * 5, y / 4 * 4));
+        }
+    }
+    assert_eq!(at(&f, 12, 0), 0);
+    assert_eq!(at(&f, 0, 12), 0);
+}
+
+#[test]
+fn transparent_samples_repeat_but_non_mosaic_samples_restart_the_latch() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1040).unwrap();
+    m.write16(MOSAIC, 0x0300).unwrap(); // OBJ4x1.
+    m.write16(PAL + 0x202, 31).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x1010); // First sample transparent, second red.
+    object(&mut m, 0, 0x1000, 0, 0);
+    assert!(render(&m).pixels().iter().all(|&p| p == 0));
+    m.write16(VRAM + 0x10000, 0x0101).unwrap();
+    let f = render(&m);
+    for x in 0..4 {
+        assert_eq!(at(&f, x, 0), rgb(31));
+    }
+    // A lower-priority non-mosaic sprite fills transparent texels before mosaic.
+    // Its non-mosaic metadata makes those pixels latch immediately.
+    fill(&mut m, VRAM + 0x10020, 32, 0x2222);
+    m.write16(PAL + 0x204, 0x3e0).unwrap();
+    object(&mut m, 1, 0, 0, 0x401);
+    let f = render(&m);
+    assert_eq!(at(&f, 0, 0), rgb(31));
+    assert_eq!(at(&f, 1, 0), rgb(0x3e0));
+}
+
+#[test]
+fn horizontal_latch_restarts_for_non_mosaic_and_higher_priority_pixels() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1040).unwrap();
+    m.write16(MOSAIC, 0x0f00).unwrap();
+    for (tile, color) in [(0, 31), (1, 0x3e0), (2, 0x7c00)] {
+        fill(
+            &mut m,
+            VRAM + 0x10000 + tile * 32,
+            32,
+            (tile as u16 + 1) * 0x1111,
+        );
+        m.write16(PAL + 0x202 + tile * 2, color).unwrap();
+    }
+    object(&mut m, 0, 0x1000, 1, 0x800);
+    object(&mut m, 1, 0x1000, 3, 0x401);
+    object(&mut m, 2, 0, 5, 2);
+    let f = render(&m);
+    assert_eq!(at(&f, 0, 0), 0);
+    assert_eq!(at(&f, 1, 0), rgb(31));
+    assert_eq!(at(&f, 3, 0), rgb(0x3e0));
+    assert_eq!(at(&f, 5, 0), rgb(0x7c00));
+    assert_eq!(at(&f, 13, 0), 0); // Leaving the canvas resets the latch.
+}
+
+#[test]
+fn transparent_higher_priority_obj_changes_metadata_without_replacing_color() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1140).unwrap();
+    m.write16(BG0CNT, 0x1001).unwrap(); // BG priority1.
+    fill(&mut m, VRAM, 32, 0x3333);
+    m.write16(PAL + 6, 0x7c00).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x1111);
+    m.write16(PAL + 0x202, 31).unwrap();
+    object(&mut m, 0, 0x400, 0, 0x800); // Semi-transparent red, priority2.
+    object(&mut m, 1, 0x1000, 0, 0x401); // Transparent tile1, priority1, mosaic.
+    fill(&mut m, VRAM + 0x10040, 32, 0x2222);
+    object(&mut m, 2, 0, 0, 0x402); // Same priority as metadata; cannot replace red.
+    m.write16(MOSAIC, 0x0300).unwrap();
+    m.write16(BLDCNT, 0x100).unwrap();
+    m.write16(BLDALPHA, 0x0808).unwrap();
+    assert_eq!(at(&render(&m), 0, 0), rgb(0x3c0f)); // Alpha flag retained, priority now ties BG.
+}
+
+#[test]
+fn object_window_ignores_mosaic_and_window_edges_do_not_quantize() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x9100).unwrap();
+    m.write16(BG0CNT, 0x1000).unwrap();
+    fill(&mut m, VRAM, 32, 0x1111);
+    m.write16(PAL + 2, 31).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x1010);
+    object(&mut m, 0, 0x1800, 0, 0);
+    m.write16(MOSAIC, 0xffff).unwrap();
+    m.write16(WINOUT, 0x0100).unwrap();
+    let f = render(&m);
+    for x in 0..8 {
+        assert_eq!(at(&f, x, 0), if x % 2 == 0 { 0 } else { rgb(31) });
+    }
+    m.write16(VRAM + 0x10004, 0x0101).unwrap(); // Next source row remains distinct.
+    assert_eq!(at(&render(&m), 0, 1), rgb(31));
+}
+
+#[test]
+fn mosaic_latch_precedes_window_masking_and_semi_transparent_blending() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x3100).unwrap();
+    m.write16(BG0CNT, 0x1000).unwrap();
+    fill(&mut m, VRAM, 32, 0x2222);
+    m.write16(PAL + 4, 0x7c00).unwrap();
+    m.write16(PAL + 0x202, 31).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x0101);
+    object(&mut m, 0, 0x1400, 0, 0);
+    m.write16(MOSAIC, 0x0300).unwrap();
+    m.write16(WIN0H, 0x0001).unwrap();
+    m.write16(WIN0V, 0x0008).unwrap();
+    m.write16(WININ, 1).unwrap(); // First pixel masks OBJ but must not reset its latch.
+    m.write16(WINOUT, 0x31).unwrap();
+    m.write16(BLDCNT, 0x100).unwrap();
+    m.write16(BLDALPHA, 0x0808).unwrap();
+    let f = render(&m);
+    assert_eq!(at(&f, 0, 0), rgb(0x7c00));
+    for x in 1..4 {
+        assert_eq!(at(&f, x, 0), rgb(0x3c0f));
+    }
+}